packed_object_info: hoist delta type resolution to helper

[git] / sha1_file.c

/*
 * GIT - The information manager from hell
 *
 * Copyright (C) Linus Torvalds, 2005
 *
 * This handles basic git sha1 object files - packing, unpacking,
 * creation etc.
 */
#include "cache.h"
#include "string-list.h"
#include "delta.h"
#include "pack.h"
#include "blob.h"
#include "commit.h"
#include "run-command.h"
#include "tag.h"
#include "tree.h"
#include "tree-walk.h"
#include "refs.h"
#include "pack-revindex.h"
#include "sha1-lookup.h"
#include "bulk-checkin.h"
#include "streaming.h"
#include "dir.h"

#ifndef O_NOATIME
#if defined(__linux__) && (defined(__i386__) || defined(__PPC__))
#define O_NOATIME 01000000
#else
#define O_NOATIME 0
#endif
#endif

#define SZ_FMT PRIuMAX
static inline uintmax_t sz_fmt(size_t s) { return s; }

const unsigned char null_sha1[20];

/*
 * This is meant to hold a *small* number of objects that you would
 * want read_sha1_file() to be able to return, but yet you do not want
 * to write them into the object store (e.g. a browse-only
 * application).
 */
static struct cached_object {
        unsigned char sha1[20];
        enum object_type type;
        void *buf;
        unsigned long size;
} *cached_objects;
static int cached_object_nr, cached_object_alloc;

static struct cached_object empty_tree = {
        EMPTY_TREE_SHA1_BIN_LITERAL,
        OBJ_TREE,
        "",
        0
};

static struct packed_git *last_found_pack;

static struct cached_object *find_cached_object(const unsigned char *sha1)
{
        int i;
        struct cached_object *co = cached_objects;

        for (i = 0; i < cached_object_nr; i++, co++) {
                if (!hashcmp(co->sha1, sha1))
                        return co;
        }
        if (!hashcmp(sha1, empty_tree.sha1))
                return &empty_tree;
        return NULL;
}

int mkdir_in_gitdir(const char *path)
{
        if (mkdir(path, 0777)) {
                int saved_errno = errno;
                struct stat st;
                struct strbuf sb = STRBUF_INIT;

                if (errno != EEXIST)
                        return -1;
                /*
                 * Are we looking at a path in a symlinked worktree
                 * whose original repository does not yet have it?
                 * e.g. .git/rr-cache pointing at its original
                 * repository in which the user hasn't performed any
                 * conflict resolution yet?
                 */
                if (lstat(path, &st) || !S_ISLNK(st.st_mode) ||
                    strbuf_readlink(&sb, path, st.st_size) ||
                    !is_absolute_path(sb.buf) ||
                    mkdir(sb.buf, 0777)) {
                        strbuf_release(&sb);
                        errno = saved_errno;
                        return -1;
                }
                strbuf_release(&sb);
        }
        return adjust_shared_perm(path);
}

int safe_create_leading_directories(char *path)
{
        char *pos = path + offset_1st_component(path);
        struct stat st;

        while (pos) {
                pos = strchr(pos, '/');
                if (!pos)
                        break;
                while (*++pos == '/')
                        ;
                if (!*pos)
                        break;
                *--pos = '\0';
                if (!stat(path, &st)) {
                        /* path exists */
                        if (!S_ISDIR(st.st_mode)) {
                                *pos = '/';
                                return -3;
                        }
                }
                else if (mkdir(path, 0777)) {
                        if (errno == EEXIST &&
                            !stat(path, &st) && S_ISDIR(st.st_mode)) {
                                ; /* somebody created it since we checked */
                        } else {
                                *pos = '/';
                                return -1;
                        }
                }
                else if (adjust_shared_perm(path)) {
                        *pos = '/';
                        return -2;
                }
                *pos++ = '/';
        }
        return 0;
}

int safe_create_leading_directories_const(const char *path)
{
        /* path points to cache entries, so xstrdup before messing with it */
        char *buf = xstrdup(path);
        int result = safe_create_leading_directories(buf);
        free(buf);
        return result;
}

static void fill_sha1_path(char *pathbuf, const unsigned char *sha1)
{
        int i;
        for (i = 0; i < 20; i++) {
                static char hex[] = "0123456789abcdef";
                unsigned int val = sha1[i];
                char *pos = pathbuf + i*2 + (i > 0);
                *pos++ = hex[val >> 4];
                *pos = hex[val & 0xf];
        }
}

/*
 * NOTE! This returns a statically allocated buffer, so you have to be
 * careful about using it. Do an "xstrdup()" if you need to save the
 * filename.
 *
 * Also note that this returns the location for creating.  Reading
 * SHA1 file can happen from any alternate directory listed in the
 * DB_ENVIRONMENT environment variable if it is not found in
 * the primary object database.
 */
char *sha1_file_name(const unsigned char *sha1)
{
        static char buf[PATH_MAX];
        const char *objdir;
        int len;

        objdir = get_object_directory();
        len = strlen(objdir);

        /* '/' + sha1(2) + '/' + sha1(38) + '\0' */
        if (len + 43 > PATH_MAX)
                die("insanely long object directory %s", objdir);
        memcpy(buf, objdir, len);
        buf[len] = '/';
        buf[len+3] = '/';
        buf[len+42] = '\0';
        fill_sha1_path(buf + len + 1, sha1);
        return buf;
}

static char *sha1_get_pack_name(const unsigned char *sha1,
                                char **name, char **base, const char *which)
{
        static const char hex[] = "0123456789abcdef";
        char *buf;
        int i;

        if (!*base) {
                const char *sha1_file_directory = get_object_directory();
                int len = strlen(sha1_file_directory);
                *base = xmalloc(len + 60);
                sprintf(*base, "%s/pack/pack-1234567890123456789012345678901234567890.%s",
                        sha1_file_directory, which);
                *name = *base + len + 11;
        }

        buf = *name;

        for (i = 0; i < 20; i++) {
                unsigned int val = *sha1++;
                *buf++ = hex[val >> 4];
                *buf++ = hex[val & 0xf];
        }

        return *base;
}

char *sha1_pack_name(const unsigned char *sha1)
{
        static char *name, *base;

        return sha1_get_pack_name(sha1, &name, &base, "pack");
}

char *sha1_pack_index_name(const unsigned char *sha1)
{
        static char *name, *base;

        return sha1_get_pack_name(sha1, &name, &base, "idx");
}

struct alternate_object_database *alt_odb_list;
static struct alternate_object_database **alt_odb_tail;

static int git_open_noatime(const char *name);

/*
 * Prepare alternate object database registry.
 *
 * The variable alt_odb_list points at the list of struct
 * alternate_object_database.  The elements on this list come from
 * non-empty elements from colon separated ALTERNATE_DB_ENVIRONMENT
 * environment variable, and $GIT_OBJECT_DIRECTORY/info/alternates,
 * whose contents is similar to that environment variable but can be
 * LF separated.  Its base points at a statically allocated buffer that
 * contains "/the/directory/corresponding/to/.git/objects/...", while
 * its name points just after the slash at the end of ".git/objects/"
 * in the example above, and has enough space to hold 40-byte hex
 * SHA1, an extra slash for the first level indirection, and the
 * terminating NUL.
 */
static int link_alt_odb_entry(const char *entry, const char *relative_base, int depth)
{
        const char *objdir = get_object_directory();
        struct alternate_object_database *ent;
        struct alternate_object_database *alt;
        int pfxlen, entlen;
        struct strbuf pathbuf = STRBUF_INIT;

        if (!is_absolute_path(entry) && relative_base) {
                strbuf_addstr(&pathbuf, real_path(relative_base));
                strbuf_addch(&pathbuf, '/');
        }
        strbuf_addstr(&pathbuf, entry);

        normalize_path_copy(pathbuf.buf, pathbuf.buf);

        pfxlen = strlen(pathbuf.buf);

        /*
         * The trailing slash after the directory name is given by
         * this function at the end. Remove duplicates.
         */
        while (pfxlen && pathbuf.buf[pfxlen-1] == '/')
                pfxlen -= 1;

        entlen = pfxlen + 43; /* '/' + 2 hex + '/' + 38 hex + NUL */
        ent = xmalloc(sizeof(*ent) + entlen);
        memcpy(ent->base, pathbuf.buf, pfxlen);
        strbuf_release(&pathbuf);

        ent->name = ent->base + pfxlen + 1;
        ent->base[pfxlen + 3] = '/';
        ent->base[pfxlen] = ent->base[entlen-1] = 0;

        /* Detect cases where alternate disappeared */
        if (!is_directory(ent->base)) {
                error("object directory %s does not exist; "
                      "check .git/objects/info/alternates.",
                      ent->base);
                free(ent);
                return -1;
        }

        /* Prevent the common mistake of listing the same
         * thing twice, or object directory itself.
         */
        for (alt = alt_odb_list; alt; alt = alt->next) {
                if (!memcmp(ent->base, alt->base, pfxlen)) {
                        free(ent);
                        return -1;
                }
        }
        if (!strcmp(ent->base, objdir)) {
                free(ent);
                return -1;
        }

        /* add the alternate entry */
        *alt_odb_tail = ent;
        alt_odb_tail = &(ent->next);
        ent->next = NULL;

        /* recursively add alternates */
        read_info_alternates(ent->base, depth + 1);

        ent->base[pfxlen] = '/';

        return 0;
}

static void link_alt_odb_entries(const char *alt, int len, int sep,
                                 const char *relative_base, int depth)
{
        struct string_list entries = STRING_LIST_INIT_NODUP;
        char *alt_copy;
        int i;

        if (depth > 5) {
                error("%s: ignoring alternate object stores, nesting too deep.",
                                relative_base);
                return;
        }

        alt_copy = xmemdupz(alt, len);
        string_list_split_in_place(&entries, alt_copy, sep, -1);
        for (i = 0; i < entries.nr; i++) {
                const char *entry = entries.items[i].string;
                if (entry[0] == '\0' || entry[0] == '#')
                        continue;
                if (!is_absolute_path(entry) && depth) {
                        error("%s: ignoring relative alternate object store %s",
                                        relative_base, entry);
                } else {
                        link_alt_odb_entry(entry, relative_base, depth);
                }
        }
        string_list_clear(&entries, 0);
        free(alt_copy);
}

void read_info_alternates(const char * relative_base, int depth)
{
        char *map;
        size_t mapsz;
        struct stat st;
        const char alt_file_name[] = "info/alternates";
        /* Given that relative_base is no longer than PATH_MAX,
           ensure that "path" has enough space to append "/", the
           file name, "info/alternates", and a trailing NUL.  */
        char path[PATH_MAX + 1 + sizeof alt_file_name];
        int fd;

        sprintf(path, "%s/%s", relative_base, alt_file_name);
        fd = git_open_noatime(path);
        if (fd < 0)
                return;
        if (fstat(fd, &st) || (st.st_size == 0)) {
                close(fd);
                return;
        }
        mapsz = xsize_t(st.st_size);
        map = xmmap(NULL, mapsz, PROT_READ, MAP_PRIVATE, fd, 0);
        close(fd);

        link_alt_odb_entries(map, mapsz, '\n', relative_base, depth);

        munmap(map, mapsz);
}

void add_to_alternates_file(const char *reference)
{
        struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));
        int fd = hold_lock_file_for_append(lock, git_path("objects/info/alternates"), LOCK_DIE_ON_ERROR);
        char *alt = mkpath("%s\n", reference);
        write_or_die(fd, alt, strlen(alt));
        if (commit_lock_file(lock))
                die("could not close alternates file");
        if (alt_odb_tail)
                link_alt_odb_entries(alt, strlen(alt), '\n', NULL, 0);
}

void foreach_alt_odb(alt_odb_fn fn, void *cb)
{
        struct alternate_object_database *ent;

        prepare_alt_odb();
        for (ent = alt_odb_list; ent; ent = ent->next)
                if (fn(ent, cb))
                        return;
}

void prepare_alt_odb(void)
{
        const char *alt;

        if (alt_odb_tail)
                return;

        alt = getenv(ALTERNATE_DB_ENVIRONMENT);
        if (!alt) alt = "";

        alt_odb_tail = &alt_odb_list;
        link_alt_odb_entries(alt, strlen(alt), PATH_SEP, NULL, 0);

        read_info_alternates(get_object_directory(), 0);
}

static int has_loose_object_local(const unsigned char *sha1)
{
        char *name = sha1_file_name(sha1);
        return !access(name, F_OK);
}

int has_loose_object_nonlocal(const unsigned char *sha1)
{
        struct alternate_object_database *alt;
        prepare_alt_odb();
        for (alt = alt_odb_list; alt; alt = alt->next) {
                fill_sha1_path(alt->name, sha1);
                if (!access(alt->base, F_OK))
                        return 1;
        }
        return 0;
}

static int has_loose_object(const unsigned char *sha1)
{
        return has_loose_object_local(sha1) ||
               has_loose_object_nonlocal(sha1);
}

static unsigned int pack_used_ctr;
static unsigned int pack_mmap_calls;
static unsigned int peak_pack_open_windows;
static unsigned int pack_open_windows;
static unsigned int pack_open_fds;
static unsigned int pack_max_fds;
static size_t peak_pack_mapped;
static size_t pack_mapped;
struct packed_git *packed_git;

void pack_report(void)
{
        fprintf(stderr,
                "pack_report: getpagesize()            = %10" SZ_FMT "\n"
                "pack_report: core.packedGitWindowSize = %10" SZ_FMT "\n"
                "pack_report: core.packedGitLimit      = %10" SZ_FMT "\n",
                sz_fmt(getpagesize()),
                sz_fmt(packed_git_window_size),
                sz_fmt(packed_git_limit));
        fprintf(stderr,
                "pack_report: pack_used_ctr            = %10u\n"
                "pack_report: pack_mmap_calls          = %10u\n"
                "pack_report: pack_open_windows        = %10u / %10u\n"
                "pack_report: pack_mapped              = "
                        "%10" SZ_FMT " / %10" SZ_FMT "\n",
                pack_used_ctr,
                pack_mmap_calls,
                pack_open_windows, peak_pack_open_windows,
                sz_fmt(pack_mapped), sz_fmt(peak_pack_mapped));
}

static int check_packed_git_idx(const char *path,  struct packed_git *p)
{
        void *idx_map;
        struct pack_idx_header *hdr;
        size_t idx_size;
        uint32_t version, nr, i, *index;
        int fd = git_open_noatime(path);
        struct stat st;

        if (fd < 0)
                return -1;
        if (fstat(fd, &st)) {
                close(fd);
                return -1;
        }
        idx_size = xsize_t(st.st_size);
        if (idx_size < 4 * 256 + 20 + 20) {
                close(fd);
                return error("index file %s is too small", path);
        }
        idx_map = xmmap(NULL, idx_size, PROT_READ, MAP_PRIVATE, fd, 0);
        close(fd);

        hdr = idx_map;
        if (hdr->idx_signature == htonl(PACK_IDX_SIGNATURE)) {
                version = ntohl(hdr->idx_version);
                if (version < 2 || version > 2) {
                        munmap(idx_map, idx_size);
                        return error("index file %s is version %"PRIu32
                                     " and is not supported by this binary"
                                     " (try upgrading GIT to a newer version)",
                                     path, version);
                }
        } else
                version = 1;

        nr = 0;
        index = idx_map;
        if (version > 1)
                index += 2;  /* skip index header */
        for (i = 0; i < 256; i++) {
                uint32_t n = ntohl(index[i]);
                if (n < nr) {
                        munmap(idx_map, idx_size);
                        return error("non-monotonic index %s", path);
                }
                nr = n;
        }

        if (version == 1) {
                /*
                 * Total size:
                 *  - 256 index entries 4 bytes each
                 *  - 24-byte entries * nr (20-byte sha1 + 4-byte offset)
                 *  - 20-byte SHA1 of the packfile
                 *  - 20-byte SHA1 file checksum
                 */
                if (idx_size != 4*256 + nr * 24 + 20 + 20) {
                        munmap(idx_map, idx_size);
                        return error("wrong index v1 file size in %s", path);
                }
        } else if (version == 2) {
                /*
                 * Minimum size:
                 *  - 8 bytes of header
                 *  - 256 index entries 4 bytes each
                 *  - 20-byte sha1 entry * nr
                 *  - 4-byte crc entry * nr
                 *  - 4-byte offset entry * nr
                 *  - 20-byte SHA1 of the packfile
                 *  - 20-byte SHA1 file checksum
                 * And after the 4-byte offset table might be a
                 * variable sized table containing 8-byte entries
                 * for offsets larger than 2^31.
                 */
                unsigned long min_size = 8 + 4*256 + nr*(20 + 4 + 4) + 20 + 20;
                unsigned long max_size = min_size;
                if (nr)
                        max_size += (nr - 1)*8;
                if (idx_size < min_size || idx_size > max_size) {
                        munmap(idx_map, idx_size);
                        return error("wrong index v2 file size in %s", path);
                }
                if (idx_size != min_size &&
                    /*
                     * make sure we can deal with large pack offsets.
                     * 31-bit signed offset won't be enough, neither
                     * 32-bit unsigned one will be.
                     */
                    (sizeof(off_t) <= 4)) {
                        munmap(idx_map, idx_size);
                        return error("pack too large for current definition of off_t in %s", path);
                }
        }

        p->index_version = version;
        p->index_data = idx_map;
        p->index_size = idx_size;
        p->num_objects = nr;
        return 0;
}

int open_pack_index(struct packed_git *p)
{
        char *idx_name;
        int ret;

        if (p->index_data)
                return 0;

        idx_name = xstrdup(p->pack_name);
        strcpy(idx_name + strlen(idx_name) - strlen(".pack"), ".idx");
        ret = check_packed_git_idx(idx_name, p);
        free(idx_name);
        return ret;
}

static void scan_windows(struct packed_git *p,
        struct packed_git **lru_p,
        struct pack_window **lru_w,
        struct pack_window **lru_l)
{
        struct pack_window *w, *w_l;

        for (w_l = NULL, w = p->windows; w; w = w->next) {
                if (!w->inuse_cnt) {
                        if (!*lru_w || w->last_used < (*lru_w)->last_used) {
                                *lru_p = p;
                                *lru_w = w;
                                *lru_l = w_l;
                        }
                }
                w_l = w;
        }
}

static int unuse_one_window(struct packed_git *current, int keep_fd)
{
        struct packed_git *p, *lru_p = NULL;
        struct pack_window *lru_w = NULL, *lru_l = NULL;

        if (current)
                scan_windows(current, &lru_p, &lru_w, &lru_l);
        for (p = packed_git; p; p = p->next)
                scan_windows(p, &lru_p, &lru_w, &lru_l);
        if (lru_p) {
                munmap(lru_w->base, lru_w->len);
                pack_mapped -= lru_w->len;
                if (lru_l)
                        lru_l->next = lru_w->next;
                else {
                        lru_p->windows = lru_w->next;
                        if (!lru_p->windows && lru_p->pack_fd != -1
                                && lru_p->pack_fd != keep_fd) {
                                close(lru_p->pack_fd);
                                pack_open_fds--;
                                lru_p->pack_fd = -1;
                        }
                }
                free(lru_w);
                pack_open_windows--;
                return 1;
        }
        return 0;
}

void release_pack_memory(size_t need, int fd)
{
        size_t cur = pack_mapped;
        while (need >= (cur - pack_mapped) && unuse_one_window(NULL, fd))
                ; /* nothing */
}

void *xmmap(void *start, size_t length,
        int prot, int flags, int fd, off_t offset)
{
        void *ret = mmap(start, length, prot, flags, fd, offset);
        if (ret == MAP_FAILED) {
                if (!length)
                        return NULL;
                release_pack_memory(length, fd);
                ret = mmap(start, length, prot, flags, fd, offset);
                if (ret == MAP_FAILED)
                        die_errno("Out of memory? mmap failed");
        }
        return ret;
}

void close_pack_windows(struct packed_git *p)
{
        while (p->windows) {
                struct pack_window *w = p->windows;

                if (w->inuse_cnt)
                        die("pack '%s' still has open windows to it",
                            p->pack_name);
                munmap(w->base, w->len);
                pack_mapped -= w->len;
                pack_open_windows--;
                p->windows = w->next;
                free(w);
        }
}

void unuse_pack(struct pack_window **w_cursor)
{
        struct pack_window *w = *w_cursor;
        if (w) {
                w->inuse_cnt--;
                *w_cursor = NULL;
        }
}

void close_pack_index(struct packed_git *p)
{
        if (p->index_data) {
                munmap((void *)p->index_data, p->index_size);
                p->index_data = NULL;
        }
}

/*
 * This is used by git-repack in case a newly created pack happens to
 * contain the same set of objects as an existing one.  In that case
 * the resulting file might be different even if its name would be the
 * same.  It is best to close any reference to the old pack before it is
 * replaced on disk.  Of course no index pointers nor windows for given pack
 * must subsist at this point.  If ever objects from this pack are requested
 * again, the new version of the pack will be reinitialized through
 * reprepare_packed_git().
 */
void free_pack_by_name(const char *pack_name)
{
        struct packed_git *p, **pp = &packed_git;

        while (*pp) {
                p = *pp;
                if (strcmp(pack_name, p->pack_name) == 0) {
                        clear_delta_base_cache();
                        close_pack_windows(p);
                        if (p->pack_fd != -1) {
                                close(p->pack_fd);
                                pack_open_fds--;
                        }
                        close_pack_index(p);
                        free(p->bad_object_sha1);
                        *pp = p->next;
                        if (last_found_pack == p)
                                last_found_pack = NULL;
                        free(p);
                        return;
                }
                pp = &p->next;
        }
}

static unsigned int get_max_fd_limit(void)
{
#ifdef RLIMIT_NOFILE
        struct rlimit lim;

        if (getrlimit(RLIMIT_NOFILE, &lim))
                die_errno("cannot get RLIMIT_NOFILE");

        return lim.rlim_cur;
#elif defined(_SC_OPEN_MAX)
        return sysconf(_SC_OPEN_MAX);
#elif defined(OPEN_MAX)
        return OPEN_MAX;
#else
        return 1; /* see the caller ;-) */
#endif
}

/*
 * Do not call this directly as this leaks p->pack_fd on error return;
 * call open_packed_git() instead.
 */
static int open_packed_git_1(struct packed_git *p)
{
        struct stat st;
        struct pack_header hdr;
        unsigned char sha1[20];
        unsigned char *idx_sha1;
        long fd_flag;

        if (!p->index_data && open_pack_index(p))
                return error("packfile %s index unavailable", p->pack_name);

        if (!pack_max_fds) {
                unsigned int max_fds = get_max_fd_limit();

                /* Save 3 for stdin/stdout/stderr, 22 for work */
                if (25 < max_fds)
                        pack_max_fds = max_fds - 25;
                else
                        pack_max_fds = 1;
        }

        while (pack_max_fds <= pack_open_fds && unuse_one_window(NULL, -1))
                ; /* nothing */

        p->pack_fd = git_open_noatime(p->pack_name);
        if (p->pack_fd < 0 || fstat(p->pack_fd, &st))
                return -1;
        pack_open_fds++;

        /* If we created the struct before we had the pack we lack size. */
        if (!p->pack_size) {
                if (!S_ISREG(st.st_mode))
                        return error("packfile %s not a regular file", p->pack_name);
                p->pack_size = st.st_size;
        } else if (p->pack_size != st.st_size)
                return error("packfile %s size changed", p->pack_name);

        /* We leave these file descriptors open with sliding mmap;
         * there is no point keeping them open across exec(), though.
         */
        fd_flag = fcntl(p->pack_fd, F_GETFD, 0);
        if (fd_flag < 0)
                return error("cannot determine file descriptor flags");
        fd_flag |= FD_CLOEXEC;
        if (fcntl(p->pack_fd, F_SETFD, fd_flag) == -1)
                return error("cannot set FD_CLOEXEC");

        /* Verify we recognize this pack file format. */
        if (read_in_full(p->pack_fd, &hdr, sizeof(hdr)) != sizeof(hdr))
                return error("file %s is far too short to be a packfile", p->pack_name);
        if (hdr.hdr_signature != htonl(PACK_SIGNATURE))
                return error("file %s is not a GIT packfile", p->pack_name);
        if (!pack_version_ok(hdr.hdr_version))
                return error("packfile %s is version %"PRIu32" and not"
                        " supported (try upgrading GIT to a newer version)",
                        p->pack_name, ntohl(hdr.hdr_version));

        /* Verify the pack matches its index. */
        if (p->num_objects != ntohl(hdr.hdr_entries))
                return error("packfile %s claims to have %"PRIu32" objects"
                             " while index indicates %"PRIu32" objects",
                             p->pack_name, ntohl(hdr.hdr_entries),
                             p->num_objects);
        if (lseek(p->pack_fd, p->pack_size - sizeof(sha1), SEEK_SET) == -1)
                return error("end of packfile %s is unavailable", p->pack_name);
        if (read_in_full(p->pack_fd, sha1, sizeof(sha1)) != sizeof(sha1))
                return error("packfile %s signature is unavailable", p->pack_name);
        idx_sha1 = ((unsigned char *)p->index_data) + p->index_size - 40;
        if (hashcmp(sha1, idx_sha1))
                return error("packfile %s does not match index", p->pack_name);
        return 0;
}

static int open_packed_git(struct packed_git *p)
{
        if (!open_packed_git_1(p))
                return 0;
        if (p->pack_fd != -1) {
                close(p->pack_fd);
                pack_open_fds--;
                p->pack_fd = -1;
        }
        return -1;
}

static int in_window(struct pack_window *win, off_t offset)
{
        /* We must promise at least 20 bytes (one hash) after the
         * offset is available from this window, otherwise the offset
         * is not actually in this window and a different window (which
         * has that one hash excess) must be used.  This is to support
         * the object header and delta base parsing routines below.
         */
        off_t win_off = win->offset;
        return win_off <= offset
                && (offset + 20) <= (win_off + win->len);
}

unsigned char *use_pack(struct packed_git *p,
                struct pack_window **w_cursor,
                off_t offset,
                unsigned long *left)
{
        struct pack_window *win = *w_cursor;

        /* Since packfiles end in a hash of their content and it's
         * pointless to ask for an offset into the middle of that
         * hash, and the in_window function above wouldn't match
         * don't allow an offset too close to the end of the file.
         */
        if (!p->pack_size && p->pack_fd == -1 && open_packed_git(p))
                die("packfile %s cannot be accessed", p->pack_name);
        if (offset > (p->pack_size - 20))
                die("offset beyond end of packfile (truncated pack?)");

        if (!win || !in_window(win, offset)) {
                if (win)
                        win->inuse_cnt--;
                for (win = p->windows; win; win = win->next) {
                        if (in_window(win, offset))
                                break;
                }
                if (!win) {
                        size_t window_align = packed_git_window_size / 2;
                        off_t len;

                        if (p->pack_fd == -1 && open_packed_git(p))
                                die("packfile %s cannot be accessed", p->pack_name);

                        win = xcalloc(1, sizeof(*win));
                        win->offset = (offset / window_align) * window_align;
                        len = p->pack_size - win->offset;
                        if (len > packed_git_window_size)
                                len = packed_git_window_size;
                        win->len = (size_t)len;
                        pack_mapped += win->len;
                        while (packed_git_limit < pack_mapped
                                && unuse_one_window(p, p->pack_fd))
                                ; /* nothing */
                        win->base = xmmap(NULL, win->len,
                                PROT_READ, MAP_PRIVATE,
                                p->pack_fd, win->offset);
                        if (win->base == MAP_FAILED)
                                die("packfile %s cannot be mapped: %s",
                                        p->pack_name,
                                        strerror(errno));
                        if (!win->offset && win->len == p->pack_size
                                && !p->do_not_close) {
                                close(p->pack_fd);
                                pack_open_fds--;
                                p->pack_fd = -1;
                        }
                        pack_mmap_calls++;
                        pack_open_windows++;
                        if (pack_mapped > peak_pack_mapped)
                                peak_pack_mapped = pack_mapped;
                        if (pack_open_windows > peak_pack_open_windows)
                                peak_pack_open_windows = pack_open_windows;
                        win->next = p->windows;
                        p->windows = win;
                }
        }
        if (win != *w_cursor) {
                win->last_used = pack_used_ctr++;
                win->inuse_cnt++;
                *w_cursor = win;
        }
        offset -= win->offset;
        if (left)
                *left = win->len - xsize_t(offset);
        return win->base + offset;
}

static struct packed_git *alloc_packed_git(int extra)
{
        struct packed_git *p = xmalloc(sizeof(*p) + extra);
        memset(p, 0, sizeof(*p));
        p->pack_fd = -1;
        return p;
}

static void try_to_free_pack_memory(size_t size)
{
        release_pack_memory(size, -1);
}

struct packed_git *add_packed_git(const char *path, int path_len, int local)
{
        static int have_set_try_to_free_routine;
        struct stat st;
        struct packed_git *p = alloc_packed_git(path_len + 2);

        if (!have_set_try_to_free_routine) {
                have_set_try_to_free_routine = 1;
                set_try_to_free_routine(try_to_free_pack_memory);
        }

        /*
         * Make sure a corresponding .pack file exists and that
         * the index looks sane.
         */
        path_len -= strlen(".idx");
        if (path_len < 1) {
                free(p);
                return NULL;
        }
        memcpy(p->pack_name, path, path_len);

        strcpy(p->pack_name + path_len, ".keep");
        if (!access(p->pack_name, F_OK))
                p->pack_keep = 1;

        strcpy(p->pack_name + path_len, ".pack");
        if (stat(p->pack_name, &st) || !S_ISREG(st.st_mode)) {
                free(p);
                return NULL;
        }

        /* ok, it looks sane as far as we can check without
         * actually mapping the pack file.
         */
        p->pack_size = st.st_size;
        p->pack_local = local;
        p->mtime = st.st_mtime;
        if (path_len < 40 || get_sha1_hex(path + path_len - 40, p->sha1))
                hashclr(p->sha1);
        return p;
}

struct packed_git *parse_pack_index(unsigned char *sha1, const char *idx_path)
{
        const char *path = sha1_pack_name(sha1);
        struct packed_git *p = alloc_packed_git(strlen(path) + 1);

        strcpy(p->pack_name, path);
        hashcpy(p->sha1, sha1);
        if (check_packed_git_idx(idx_path, p)) {
                free(p);
                return NULL;
        }

        return p;
}

void install_packed_git(struct packed_git *pack)
{
        if (pack->pack_fd != -1)
                pack_open_fds++;

        pack->next = packed_git;
        packed_git = pack;
}

void (*report_garbage)(const char *desc, const char *path);

static void report_helper(const struct string_list *list,
                          int seen_bits, int first, int last)
{
        const char *msg;
        switch (seen_bits) {
        case 0:
                msg = "no corresponding .idx nor .pack";
                break;
        case 1:
                msg = "no corresponding .idx";
                break;
        case 2:
                msg = "no corresponding .pack";
                break;
        default:
                return;
        }
        for (; first < last; first++)
                report_garbage(msg, list->items[first].string);
}

static void report_pack_garbage(struct string_list *list)
{
        int i, baselen = -1, first = 0, seen_bits = 0;

        if (!report_garbage)
                return;

        sort_string_list(list);

        for (i = 0; i < list->nr; i++) {
                const char *path = list->items[i].string;
                if (baselen != -1 &&
                    strncmp(path, list->items[first].string, baselen)) {
                        report_helper(list, seen_bits, first, i);
                        baselen = -1;
                        seen_bits = 0;
                }
                if (baselen == -1) {
                        const char *dot = strrchr(path, '.');
                        if (!dot) {
                                report_garbage("garbage found", path);
                                continue;
                        }
                        baselen = dot - path + 1;
                        first = i;
                }
                if (!strcmp(path + baselen, "pack"))
                        seen_bits |= 1;
                else if (!strcmp(path + baselen, "idx"))
                        seen_bits |= 2;
        }
        report_helper(list, seen_bits, first, list->nr);
}

static void prepare_packed_git_one(char *objdir, int local)
{
        /* Ensure that this buffer is large enough so that we can
           append "/pack/" without clobbering the stack even if
           strlen(objdir) were PATH_MAX.  */
        char path[PATH_MAX + 1 + 4 + 1 + 1];
        int len;
        DIR *dir;
        struct dirent *de;
        struct string_list garbage = STRING_LIST_INIT_DUP;

        sprintf(path, "%s/pack", objdir);
        len = strlen(path);
        dir = opendir(path);
        if (!dir) {
                if (errno != ENOENT)
                        error("unable to open object pack directory: %s: %s",
                              path, strerror(errno));
                return;
        }
        path[len++] = '/';
        while ((de = readdir(dir)) != NULL) {
                int namelen = strlen(de->d_name);
                struct packed_git *p;

                if (len + namelen + 1 > sizeof(path)) {
                        if (report_garbage) {
                                struct strbuf sb = STRBUF_INIT;
                                strbuf_addf(&sb, "%.*s/%s", len - 1, path, de->d_name);
                                report_garbage("path too long", sb.buf);
                                strbuf_release(&sb);
                        }
                        continue;
                }

                if (is_dot_or_dotdot(de->d_name))
                        continue;

                strcpy(path + len, de->d_name);

                if (has_extension(de->d_name, ".idx")) {
                        /* Don't reopen a pack we already have. */
                        for (p = packed_git; p; p = p->next) {
                                if (!memcmp(path, p->pack_name, len + namelen - 4))
                                        break;
                        }
                        if (p == NULL &&
                            /*
                             * See if it really is a valid .idx file with
                             * corresponding .pack file that we can map.
                             */
                            (p = add_packed_git(path, len + namelen, local)) != NULL)
                                install_packed_git(p);
                }

                if (!report_garbage)
                        continue;

                if (has_extension(de->d_name, ".idx") ||
                    has_extension(de->d_name, ".pack") ||
                    has_extension(de->d_name, ".keep"))
                        string_list_append(&garbage, path);
                else
                        report_garbage("garbage found", path);
        }
        closedir(dir);
        report_pack_garbage(&garbage);
        string_list_clear(&garbage, 0);
}

static int sort_pack(const void *a_, const void *b_)
{
        struct packed_git *a = *((struct packed_git **)a_);
        struct packed_git *b = *((struct packed_git **)b_);
        int st;

        /*
         * Local packs tend to contain objects specific to our
         * variant of the project than remote ones.  In addition,
         * remote ones could be on a network mounted filesystem.
         * Favor local ones for these reasons.
         */
        st = a->pack_local - b->pack_local;
        if (st)
                return -st;

        /*
         * Younger packs tend to contain more recent objects,
         * and more recent objects tend to get accessed more
         * often.
         */
        if (a->mtime < b->mtime)
                return 1;
        else if (a->mtime == b->mtime)
                return 0;
        return -1;
}

static void rearrange_packed_git(void)
{
        struct packed_git **ary, *p;
        int i, n;

        for (n = 0, p = packed_git; p; p = p->next)
                n++;
        if (n < 2)
                return;

        /* prepare an array of packed_git for easier sorting */
        ary = xcalloc(n, sizeof(struct packed_git *));
        for (n = 0, p = packed_git; p; p = p->next)
                ary[n++] = p;

        qsort(ary, n, sizeof(struct packed_git *), sort_pack);

        /* link them back again */
        for (i = 0; i < n - 1; i++)
                ary[i]->next = ary[i + 1];
        ary[n - 1]->next = NULL;
        packed_git = ary[0];

        free(ary);
}

static int prepare_packed_git_run_once = 0;
void prepare_packed_git(void)
{
        struct alternate_object_database *alt;

        if (prepare_packed_git_run_once)
                return;
        prepare_packed_git_one(get_object_directory(), 1);
        prepare_alt_odb();
        for (alt = alt_odb_list; alt; alt = alt->next) {
                alt->name[-1] = 0;
                prepare_packed_git_one(alt->base, 0);
                alt->name[-1] = '/';
        }
        rearrange_packed_git();
        prepare_packed_git_run_once = 1;
}

void reprepare_packed_git(void)
{
        discard_revindex();
        prepare_packed_git_run_once = 0;
        prepare_packed_git();
}

static void mark_bad_packed_object(struct packed_git *p,
                                   const unsigned char *sha1)
{
        unsigned i;
        for (i = 0; i < p->num_bad_objects; i++)
                if (!hashcmp(sha1, p->bad_object_sha1 + 20 * i))
                        return;
        p->bad_object_sha1 = xrealloc(p->bad_object_sha1, 20 * (p->num_bad_objects + 1));
        hashcpy(p->bad_object_sha1 + 20 * p->num_bad_objects, sha1);
        p->num_bad_objects++;
}

static const struct packed_git *has_packed_and_bad(const unsigned char *sha1)
{
        struct packed_git *p;
        unsigned i;

        for (p = packed_git; p; p = p->next)
                for (i = 0; i < p->num_bad_objects; i++)
                        if (!hashcmp(sha1, p->bad_object_sha1 + 20 * i))
                                return p;
        return NULL;
}

/*
 * With an in-core object data in "map", rehash it to make sure the
 * object name actually matches "sha1" to detect object corruption.
 * With "map" == NULL, try reading the object named with "sha1" using
 * the streaming interface and rehash it to do the same.
 */
int check_sha1_signature(const unsigned char *sha1, void *map,
                         unsigned long size, const char *type)
{
        unsigned char real_sha1[20];
        enum object_type obj_type;
        struct git_istream *st;
        git_SHA_CTX c;
        char hdr[32];
        int hdrlen;

        if (map) {
                hash_sha1_file(map, size, type, real_sha1);
                return hashcmp(sha1, real_sha1) ? -1 : 0;
        }

        st = open_istream(sha1, &obj_type, &size, NULL);
        if (!st)
                return -1;

        /* Generate the header */
        hdrlen = sprintf(hdr, "%s %lu", typename(obj_type), size) + 1;

        /* Sha1.. */
        git_SHA1_Init(&c);
        git_SHA1_Update(&c, hdr, hdrlen);
        for (;;) {
                char buf[1024 * 16];
                ssize_t readlen = read_istream(st, buf, sizeof(buf));

                if (readlen < 0) {
                        close_istream(st);
                        return -1;
                }
                if (!readlen)
                        break;
                git_SHA1_Update(&c, buf, readlen);
        }
        git_SHA1_Final(real_sha1, &c);
        close_istream(st);
        return hashcmp(sha1, real_sha1) ? -1 : 0;
}

static int git_open_noatime(const char *name)
{
        static int sha1_file_open_flag = O_NOATIME;

        for (;;) {
                int fd = open(name, O_RDONLY | sha1_file_open_flag);
                if (fd >= 0)
                        return fd;

                /* Might the failure be due to O_NOATIME? */
                if (errno != ENOENT && sha1_file_open_flag) {
                        sha1_file_open_flag = 0;
                        continue;
                }

                return -1;
        }
}

static int stat_sha1_file(const unsigned char *sha1, struct stat *st)
{
        char *name = sha1_file_name(sha1);
        struct alternate_object_database *alt;

        if (!lstat(name, st))
                return 0;

        prepare_alt_odb();
        errno = ENOENT;
        for (alt = alt_odb_list; alt; alt = alt->next) {
                name = alt->name;
                fill_sha1_path(name, sha1);
                if (!lstat(alt->base, st))
                        return 0;
        }

        return -1;
}

static int open_sha1_file(const unsigned char *sha1)
{
        int fd;
        char *name = sha1_file_name(sha1);
        struct alternate_object_database *alt;

        fd = git_open_noatime(name);
        if (fd >= 0)
                return fd;

        prepare_alt_odb();
        errno = ENOENT;
        for (alt = alt_odb_list; alt; alt = alt->next) {
                name = alt->name;
                fill_sha1_path(name, sha1);
                fd = git_open_noatime(alt->base);
                if (fd >= 0)
                        return fd;
        }
        return -1;
}

void *map_sha1_file(const unsigned char *sha1, unsigned long *size)
{
        void *map;
        int fd;

        fd = open_sha1_file(sha1);
        map = NULL;
        if (fd >= 0) {
                struct stat st;

                if (!fstat(fd, &st)) {
                        *size = xsize_t(st.st_size);
                        if (!*size) {
                                /* mmap() is forbidden on empty files */
                                error("object file %s is empty", sha1_file_name(sha1));
                                return NULL;
                        }
                        map = xmmap(NULL, *size, PROT_READ, MAP_PRIVATE, fd, 0);
                }
                close(fd);
        }
        return map;
}

/*
 * There used to be a second loose object header format which
 * was meant to mimic the in-pack format, allowing for direct
 * copy of the object data.  This format turned up not to be
 * really worth it and we no longer write loose objects in that
 * format.
 */
static int experimental_loose_object(unsigned char *map)
{
        unsigned int word;

        /*
         * We must determine if the buffer contains the standard
         * zlib-deflated stream or the experimental format based
         * on the in-pack object format. Compare the header byte
         * for each format:
         *
         * RFC1950 zlib w/ deflate : 0www1000 : 0 <= www <= 7
         * Experimental pack-based : Stttssss : ttt = 1,2,3,4
         *
         * If bit 7 is clear and bits 0-3 equal 8, the buffer MUST be
         * in standard loose-object format, UNLESS it is a Git-pack
         * format object *exactly* 8 bytes in size when inflated.
         *
         * However, RFC1950 also specifies that the 1st 16-bit word
         * must be divisible by 31 - this checksum tells us our buffer
         * is in the standard format, giving a false positive only if
         * the 1st word of the Git-pack format object happens to be
         * divisible by 31, ie:
         *      ((byte0 * 256) + byte1) % 31 = 0
         *   =>        0ttt10000www1000 % 31 = 0
         *
         * As it happens, this case can only arise for www=3 & ttt=1
         * - ie, a Commit object, which would have to be 8 bytes in
         * size. As no Commit can be that small, we find that the
         * combination of these two criteria (bitmask & checksum)
         * can always correctly determine the buffer format.
         */
        word = (map[0] << 8) + map[1];
        if ((map[0] & 0x8F) == 0x08 && !(word % 31))
                return 0;
        else
                return 1;
}

unsigned long unpack_object_header_buffer(const unsigned char *buf,
                unsigned long len, enum object_type *type, unsigned long *sizep)
{
        unsigned shift;
        unsigned long size, c;
        unsigned long used = 0;

        c = buf[used++];
        *type = (c >> 4) & 7;
        size = c & 15;
        shift = 4;
        while (c & 0x80) {
                if (len <= used || bitsizeof(long) <= shift) {
                        error("bad object header");
                        size = used = 0;
                        break;
                }
                c = buf[used++];
                size += (c & 0x7f) << shift;
                shift += 7;
        }
        *sizep = size;
        return used;
}

int unpack_sha1_header(git_zstream *stream, unsigned char *map, unsigned long mapsize, void *buffer, unsigned long bufsiz)
{
        unsigned long size, used;
        static const char valid_loose_object_type[8] = {
                0, /* OBJ_EXT */
                1, 1, 1, 1, /* "commit", "tree", "blob", "tag" */
                0, /* "delta" and others are invalid in a loose object */
        };
        enum object_type type;

        /* Get the data stream */
        memset(stream, 0, sizeof(*stream));
        stream->next_in = map;
        stream->avail_in = mapsize;
        stream->next_out = buffer;
        stream->avail_out = bufsiz;

        if (experimental_loose_object(map)) {
                /*
                 * The old experimental format we no longer produce;
                 * we can still read it.
                 */
                used = unpack_object_header_buffer(map, mapsize, &type, &size);
                if (!used || !valid_loose_object_type[type])
                        return -1;
                map += used;
                mapsize -= used;

                /* Set up the stream for the rest.. */
                stream->next_in = map;
                stream->avail_in = mapsize;
                git_inflate_init(stream);

                /* And generate the fake traditional header */
                stream->total_out = 1 + snprintf(buffer, bufsiz, "%s %lu",
                                                 typename(type), size);
                return 0;
        }
        git_inflate_init(stream);
        return git_inflate(stream, 0);
}

static void *unpack_sha1_rest(git_zstream *stream, void *buffer, unsigned long size, const unsigned char *sha1)
{
        int bytes = strlen(buffer) + 1;
        unsigned char *buf = xmallocz(size);
        unsigned long n;
        int status = Z_OK;

        n = stream->total_out - bytes;
        if (n > size)
                n = size;
        memcpy(buf, (char *) buffer + bytes, n);
        bytes = n;
        if (bytes <= size) {
                /*
                 * The above condition must be (bytes <= size), not
                 * (bytes < size).  In other words, even though we
                 * expect no more output and set avail_out to zero,
                 * the input zlib stream may have bytes that express
                 * "this concludes the stream", and we *do* want to
                 * eat that input.
                 *
                 * Otherwise we would not be able to test that we
                 * consumed all the input to reach the expected size;
                 * we also want to check that zlib tells us that all
                 * went well with status == Z_STREAM_END at the end.
                 */
                stream->next_out = buf + bytes;
                stream->avail_out = size - bytes;
                while (status == Z_OK)
                        status = git_inflate(stream, Z_FINISH);
        }
        if (status == Z_STREAM_END && !stream->avail_in) {
                git_inflate_end(stream);
                return buf;
        }

        if (status < 0)
                error("corrupt loose object '%s'", sha1_to_hex(sha1));
        else if (stream->avail_in)
                error("garbage at end of loose object '%s'",
                      sha1_to_hex(sha1));
        free(buf);
        return NULL;
}

/*
 * We used to just use "sscanf()", but that's actually way
 * too permissive for what we want to check. So do an anal
 * object header parse by hand.
 */
int parse_sha1_header(const char *hdr, unsigned long *sizep)
{
        char type[10];
        int i;
        unsigned long size;

        /*
         * The type can be at most ten bytes (including the
         * terminating '\0' that we add), and is followed by
         * a space.
         */
        i = 0;
        for (;;) {
                char c = *hdr++;
                if (c == ' ')
                        break;
                type[i++] = c;
                if (i >= sizeof(type))
                        return -1;
        }
        type[i] = 0;

        /*
         * The length must follow immediately, and be in canonical
         * decimal format (ie "010" is not valid).
         */
        size = *hdr++ - '0';
        if (size > 9)
                return -1;
        if (size) {
                for (;;) {
                        unsigned long c = *hdr - '0';
                        if (c > 9)
                                break;
                        hdr++;
                        size = size * 10 + c;
                }
        }
        *sizep = size;

        /*
         * The length must be followed by a zero byte
         */
        return *hdr ? -1 : type_from_string(type);
}

static void *unpack_sha1_file(void *map, unsigned long mapsize, enum object_type *type, unsigned long *size, const unsigned char *sha1)
{
        int ret;
        git_zstream stream;
        char hdr[8192];

        ret = unpack_sha1_header(&stream, map, mapsize, hdr, sizeof(hdr));
        if (ret < Z_OK || (*type = parse_sha1_header(hdr, size)) < 0)
                return NULL;

        return unpack_sha1_rest(&stream, hdr, *size, sha1);
}

unsigned long get_size_from_delta(struct packed_git *p,
                                  struct pack_window **w_curs,
                                  off_t curpos)
{
        const unsigned char *data;
        unsigned char delta_head[20], *in;
        git_zstream stream;
        int st;

        memset(&stream, 0, sizeof(stream));
        stream.next_out = delta_head;
        stream.avail_out = sizeof(delta_head);

        git_inflate_init(&stream);
        do {
                in = use_pack(p, w_curs, curpos, &stream.avail_in);
                stream.next_in = in;
                st = git_inflate(&stream, Z_FINISH);
                curpos += stream.next_in - in;
        } while ((st == Z_OK || st == Z_BUF_ERROR) &&
                 stream.total_out < sizeof(delta_head));
        git_inflate_end(&stream);
        if ((st != Z_STREAM_END) && stream.total_out != sizeof(delta_head)) {
                error("delta data unpack-initial failed");
                return 0;
        }

        /* Examine the initial part of the delta to figure out
         * the result size.
         */
        data = delta_head;

        /* ignore base size */
        get_delta_hdr_size(&data, delta_head+sizeof(delta_head));

        /* Read the result size */
        return get_delta_hdr_size(&data, delta_head+sizeof(delta_head));
}

static off_t get_delta_base(struct packed_git *p,
                                    struct pack_window **w_curs,
                                    off_t *curpos,
                                    enum object_type type,
                                    off_t delta_obj_offset)
{
        unsigned char *base_info = use_pack(p, w_curs, *curpos, NULL);
        off_t base_offset;

        /* use_pack() assured us we have [base_info, base_info + 20)
         * as a range that we can look at without walking off the
         * end of the mapped window.  Its actually the hash size
         * that is assured.  An OFS_DELTA longer than the hash size
         * is stupid, as then a REF_DELTA would be smaller to store.
         */
        if (type == OBJ_OFS_DELTA) {
                unsigned used = 0;
                unsigned char c = base_info[used++];
                base_offset = c & 127;
                while (c & 128) {
                        base_offset += 1;
                        if (!base_offset || MSB(base_offset, 7))
                                return 0;  /* overflow */
                        c = base_info[used++];
                        base_offset = (base_offset << 7) + (c & 127);
                }
                base_offset = delta_obj_offset - base_offset;
                if (base_offset <= 0 || base_offset >= delta_obj_offset)
                        return 0;  /* out of bound */
                *curpos += used;
        } else if (type == OBJ_REF_DELTA) {
                /* The base entry _must_ be in the same pack */
                base_offset = find_pack_entry_one(base_info, p);
                *curpos += 20;
        } else
                die("I am totally screwed");
        return base_offset;
}

int unpack_object_header(struct packed_git *p,
                         struct pack_window **w_curs,
                         off_t *curpos,
                         unsigned long *sizep)
{
        unsigned char *base;
        unsigned long left;
        unsigned long used;
        enum object_type type;

        /* use_pack() assures us we have [base, base + 20) available
         * as a range that we can look at.  (Its actually the hash
         * size that is assured.)  With our object header encoding
         * the maximum deflated object size is 2^137, which is just
         * insane, so we know won't exceed what we have been given.
         */
        base = use_pack(p, w_curs, *curpos, &left);
        used = unpack_object_header_buffer(base, left, &type, sizep);
        if (!used) {
                type = OBJ_BAD;
        } else
                *curpos += used;

        return type;
}

static int retry_bad_packed_offset(struct packed_git *p, off_t obj_offset)
{
        int type;
        struct revindex_entry *revidx;
        const unsigned char *sha1;
        revidx = find_pack_revindex(p, obj_offset);
        if (!revidx)
                return OBJ_BAD;
        sha1 = nth_packed_object_sha1(p, revidx->nr);
        mark_bad_packed_object(p, sha1);
        type = sha1_object_info(sha1, NULL);
        if (type <= OBJ_NONE)
                return OBJ_BAD;
        return type;
}

#define POI_STACK_PREALLOC 64

static enum object_type packed_to_object_type(struct packed_git *p,
                                              off_t obj_offset,
                                              enum object_type type,
                                              struct pack_window **w_curs,
                                              off_t curpos)
{
        off_t small_poi_stack[POI_STACK_PREALLOC];
        off_t *poi_stack = small_poi_stack;
        int poi_stack_nr = 0, poi_stack_alloc = POI_STACK_PREALLOC;

        while (type == OBJ_OFS_DELTA || type == OBJ_REF_DELTA) {
                off_t base_offset;
                unsigned long size;
                /* Push the object we're going to leave behind */
                if (poi_stack_nr >= poi_stack_alloc && poi_stack == small_poi_stack) {
                        poi_stack_alloc = alloc_nr(poi_stack_nr);
                        poi_stack = xmalloc(sizeof(off_t)*poi_stack_alloc);
                        memcpy(poi_stack, small_poi_stack, sizeof(off_t)*poi_stack_nr);
                } else {
                        ALLOC_GROW(poi_stack, poi_stack_nr+1, poi_stack_alloc);
                }
                poi_stack[poi_stack_nr++] = obj_offset;
                /* If parsing the base offset fails, just unwind */
                base_offset = get_delta_base(p, w_curs, &curpos, type, obj_offset);
                if (!base_offset)
                        goto unwind;
                curpos = obj_offset = base_offset;
                type = unpack_object_header(p, w_curs, &curpos, &size);
                if (type <= OBJ_NONE) {
                        /* If getting the base itself fails, we first
                         * retry the base, otherwise unwind */
                        type = retry_bad_packed_offset(p, base_offset);
                        if (type > OBJ_NONE)
                                goto out;
                        goto unwind;
                }
        }

        switch (type) {
        case OBJ_BAD:
        case OBJ_COMMIT:
        case OBJ_TREE:
        case OBJ_BLOB:
        case OBJ_TAG:
                break;
        default:
                error("unknown object type %i at offset %"PRIuMAX" in %s",
                      type, (uintmax_t)obj_offset, p->pack_name);
                type = OBJ_BAD;
        }

out:
        if (poi_stack != small_poi_stack)
                free(poi_stack);
        return type;

unwind:
        while (poi_stack_nr) {
                obj_offset = poi_stack[--poi_stack_nr];
                type = retry_bad_packed_offset(p, obj_offset);
                if (type > OBJ_NONE)
                        goto out;
        }
        type = OBJ_BAD;
        goto out;
}

static int packed_object_info(struct packed_git *p, off_t obj_offset,
                              unsigned long *sizep, int *rtype,
                              unsigned long *disk_sizep)
{
        struct pack_window *w_curs = NULL;
        unsigned long size;
        off_t curpos = obj_offset;
        enum object_type type;

        type = unpack_object_header(p, &w_curs, &curpos, &size);

        if (rtype)
                *rtype = type; /* representation type */

        if (sizep) {
                if (type == OBJ_OFS_DELTA || type == OBJ_REF_DELTA) {
                        off_t tmp_pos = curpos;
                        off_t base_offset = get_delta_base(p, &w_curs, &tmp_pos,
                                                           type, obj_offset);
                        if (!base_offset) {
                                type = OBJ_BAD;
                                goto out;
                        }
                        *sizep = get_size_from_delta(p, &w_curs, tmp_pos);
                        if (*sizep == 0) {
                                type = OBJ_BAD;
                                goto out;
                        }
                } else {
                        *sizep = size;
                }
        }

        if (disk_sizep) {
                struct revindex_entry *revidx = find_pack_revindex(p, obj_offset);
                *disk_sizep = revidx[1].offset - obj_offset;
        }

        type = packed_to_object_type(p, obj_offset, type, &w_curs, curpos);

out:
        unuse_pack(&w_curs);
        return type;
}

static void *unpack_compressed_entry(struct packed_git *p,
                                    struct pack_window **w_curs,
                                    off_t curpos,
                                    unsigned long size)
{
        int st;
        git_zstream stream;
        unsigned char *buffer, *in;

        buffer = xmallocz(size);
        memset(&stream, 0, sizeof(stream));
        stream.next_out = buffer;
        stream.avail_out = size + 1;

        git_inflate_init(&stream);
        do {
                in = use_pack(p, w_curs, curpos, &stream.avail_in);
                stream.next_in = in;
                st = git_inflate(&stream, Z_FINISH);
                if (!stream.avail_out)
                        break; /* the payload is larger than it should be */
                curpos += stream.next_in - in;
        } while (st == Z_OK || st == Z_BUF_ERROR);
        git_inflate_end(&stream);
        if ((st != Z_STREAM_END) || stream.total_out != size) {
                free(buffer);
                return NULL;
        }

        return buffer;
}

#define MAX_DELTA_CACHE (256)

static size_t delta_base_cached;

static struct delta_base_cache_lru_list {
        struct delta_base_cache_lru_list *prev;
        struct delta_base_cache_lru_list *next;
} delta_base_cache_lru = { &delta_base_cache_lru, &delta_base_cache_lru };

static struct delta_base_cache_entry {
        struct delta_base_cache_lru_list lru;
        void *data;
        struct packed_git *p;
        off_t base_offset;
        unsigned long size;
        enum object_type type;
} delta_base_cache[MAX_DELTA_CACHE];

static unsigned long pack_entry_hash(struct packed_git *p, off_t base_offset)
{
        unsigned long hash;

        hash = (unsigned long)p + (unsigned long)base_offset;
        hash += (hash >> 8) + (hash >> 16);
        return hash % MAX_DELTA_CACHE;
}

static struct delta_base_cache_entry *
get_delta_base_cache_entry(struct packed_git *p, off_t base_offset)
{
        unsigned long hash = pack_entry_hash(p, base_offset);
        return delta_base_cache + hash;
}

static int eq_delta_base_cache_entry(struct delta_base_cache_entry *ent,
                                     struct packed_git *p, off_t base_offset)
{
        return (ent->data && ent->p == p && ent->base_offset == base_offset);
}

static int in_delta_base_cache(struct packed_git *p, off_t base_offset)
{
        struct delta_base_cache_entry *ent;
        ent = get_delta_base_cache_entry(p, base_offset);
        return eq_delta_base_cache_entry(ent, p, base_offset);
}

static void clear_delta_base_cache_entry(struct delta_base_cache_entry *ent)
{
        ent->data = NULL;
        ent->lru.next->prev = ent->lru.prev;
        ent->lru.prev->next = ent->lru.next;
        delta_base_cached -= ent->size;
}

static void *cache_or_unpack_entry(struct packed_git *p, off_t base_offset,
        unsigned long *base_size, enum object_type *type, int keep_cache)
{
        struct delta_base_cache_entry *ent;
        void *ret;

        ent = get_delta_base_cache_entry(p, base_offset);

        if (!eq_delta_base_cache_entry(ent, p, base_offset))
                return unpack_entry(p, base_offset, type, base_size);

        ret = ent->data;

        if (!keep_cache)
                clear_delta_base_cache_entry(ent);
        else
                ret = xmemdupz(ent->data, ent->size);
        *type = ent->type;
        *base_size = ent->size;
        return ret;
}

static inline void release_delta_base_cache(struct delta_base_cache_entry *ent)
{
        if (ent->data) {
                free(ent->data);
                ent->data = NULL;
                ent->lru.next->prev = ent->lru.prev;
                ent->lru.prev->next = ent->lru.next;
                delta_base_cached -= ent->size;
        }
}

void clear_delta_base_cache(void)
{
        unsigned long p;
        for (p = 0; p < MAX_DELTA_CACHE; p++)
                release_delta_base_cache(&delta_base_cache[p]);
}

static void add_delta_base_cache(struct packed_git *p, off_t base_offset,
        void *base, unsigned long base_size, enum object_type type)
{
        unsigned long hash = pack_entry_hash(p, base_offset);
        struct delta_base_cache_entry *ent = delta_base_cache + hash;
        struct delta_base_cache_lru_list *lru;

        release_delta_base_cache(ent);
        delta_base_cached += base_size;

        for (lru = delta_base_cache_lru.next;
             delta_base_cached > delta_base_cache_limit
             && lru != &delta_base_cache_lru;
             lru = lru->next) {
                struct delta_base_cache_entry *f = (void *)lru;
                if (f->type == OBJ_BLOB)
                        release_delta_base_cache(f);
        }
        for (lru = delta_base_cache_lru.next;
             delta_base_cached > delta_base_cache_limit
             && lru != &delta_base_cache_lru;
             lru = lru->next) {
                struct delta_base_cache_entry *f = (void *)lru;
                release_delta_base_cache(f);
        }

        ent->p = p;
        ent->base_offset = base_offset;
        ent->type = type;
        ent->data = base;
        ent->size = base_size;
        ent->lru.next = &delta_base_cache_lru;
        ent->lru.prev = delta_base_cache_lru.prev;
        delta_base_cache_lru.prev->next = &ent->lru;
        delta_base_cache_lru.prev = &ent->lru;
}

static void *read_object(const unsigned char *sha1, enum object_type *type,
                         unsigned long *size);

static void write_pack_access_log(struct packed_git *p, off_t obj_offset)
{
        static FILE *log_file;

        if (!log_file) {
                log_file = fopen(log_pack_access, "w");
                if (!log_file) {
                        error("cannot open pack access log '%s' for writing: %s",
                              log_pack_access, strerror(errno));
                        log_pack_access = NULL;
                        return;
                }
        }
        fprintf(log_file, "%s %"PRIuMAX"\n",
                p->pack_name, (uintmax_t)obj_offset);
        fflush(log_file);
}

int do_check_packed_object_crc;

#define UNPACK_ENTRY_STACK_PREALLOC 64
struct unpack_entry_stack_ent {
        off_t obj_offset;
        off_t curpos;
        unsigned long size;
};

void *unpack_entry(struct packed_git *p, off_t obj_offset,
                   enum object_type *final_type, unsigned long *final_size)
{
        struct pack_window *w_curs = NULL;
        off_t curpos = obj_offset;
        void *data = NULL;
        unsigned long size;
        enum object_type type;
        struct unpack_entry_stack_ent small_delta_stack[UNPACK_ENTRY_STACK_PREALLOC];
        struct unpack_entry_stack_ent *delta_stack = small_delta_stack;
        int delta_stack_nr = 0, delta_stack_alloc = UNPACK_ENTRY_STACK_PREALLOC;
        int base_from_cache = 0;

        if (log_pack_access)
                write_pack_access_log(p, obj_offset);

        /* PHASE 1: drill down to the innermost base object */
        for (;;) {
                off_t base_offset;
                int i;
                struct delta_base_cache_entry *ent;

                if (do_check_packed_object_crc && p->index_version > 1) {
                        struct revindex_entry *revidx = find_pack_revindex(p, obj_offset);
                        unsigned long len = revidx[1].offset - obj_offset;
                        if (check_pack_crc(p, &w_curs, obj_offset, len, revidx->nr)) {
                                const unsigned char *sha1 =
                                        nth_packed_object_sha1(p, revidx->nr);
                                error("bad packed object CRC for %s",
                                      sha1_to_hex(sha1));
                                mark_bad_packed_object(p, sha1);
                                unuse_pack(&w_curs);
                                return NULL;
                        }
                }

                ent = get_delta_base_cache_entry(p, curpos);
                if (eq_delta_base_cache_entry(ent, p, curpos)) {
                        type = ent->type;
                        data = ent->data;
                        size = ent->size;
                        clear_delta_base_cache_entry(ent);
                        base_from_cache = 1;
                        break;
                }

                type = unpack_object_header(p, &w_curs, &curpos, &size);
                if (type != OBJ_OFS_DELTA && type != OBJ_REF_DELTA)
                        break;

                base_offset = get_delta_base(p, &w_curs, &curpos, type, obj_offset);
                if (!base_offset) {
                        error("failed to validate delta base reference "
                              "at offset %"PRIuMAX" from %s",
                              (uintmax_t)curpos, p->pack_name);
                        /* bail to phase 2, in hopes of recovery */
                        data = NULL;
                        break;
                }

                /* push object, proceed to base */
                if (delta_stack_nr >= delta_stack_alloc
                    && delta_stack == small_delta_stack) {
                        delta_stack_alloc = alloc_nr(delta_stack_nr);
                        delta_stack = xmalloc(sizeof(*delta_stack)*delta_stack_alloc);
                        memcpy(delta_stack, small_delta_stack,
                               sizeof(*delta_stack)*delta_stack_nr);
                } else {
                        ALLOC_GROW(delta_stack, delta_stack_nr+1, delta_stack_alloc);
                }
                i = delta_stack_nr++;
                delta_stack[i].obj_offset = obj_offset;
                delta_stack[i].curpos = curpos;
                delta_stack[i].size = size;

                curpos = obj_offset = base_offset;
        }

        /* PHASE 2: handle the base */
        switch (type) {
        case OBJ_OFS_DELTA:
        case OBJ_REF_DELTA:
                if (data)
                        die("BUG in unpack_entry: left loop at a valid delta");
                break;
        case OBJ_COMMIT:
        case OBJ_TREE:
        case OBJ_BLOB:
        case OBJ_TAG:
                if (!base_from_cache)
                        data = unpack_compressed_entry(p, &w_curs, curpos, size);
                break;
        default:
                data = NULL;
                error("unknown object type %i at offset %"PRIuMAX" in %s",
                      type, (uintmax_t)obj_offset, p->pack_name);
        }

        /* PHASE 3: apply deltas in order */

        /* invariants:
         *   'data' holds the base data, or NULL if there was corruption
         */
        while (delta_stack_nr) {
                void *delta_data;
                void *base = data;
                unsigned long delta_size, base_size = size;
                int i;

                data = NULL;

                if (base)
                        add_delta_base_cache(p, obj_offset, base, base_size, type);

                if (!base) {
                        /*
                         * We're probably in deep shit, but let's try to fetch
                         * the required base anyway from another pack or loose.
                         * This is costly but should happen only in the presence
                         * of a corrupted pack, and is better than failing outright.
                         */
                        struct revindex_entry *revidx;
                        const unsigned char *base_sha1;
                        revidx = find_pack_revindex(p, obj_offset);
                        if (revidx) {
                                base_sha1 = nth_packed_object_sha1(p, revidx->nr);
                                error("failed to read delta base object %s"
                                      " at offset %"PRIuMAX" from %s",
                                      sha1_to_hex(base_sha1), (uintmax_t)obj_offset,
                                      p->pack_name);
                                mark_bad_packed_object(p, base_sha1);
                                base = read_object(base_sha1, &type, &base_size);
                        }
                }

                i = --delta_stack_nr;
                obj_offset = delta_stack[i].obj_offset;
                curpos = delta_stack[i].curpos;
                delta_size = delta_stack[i].size;

                if (!base)
                        continue;

                delta_data = unpack_compressed_entry(p, &w_curs, curpos, delta_size);

                if (!delta_data) {
                        error("failed to unpack compressed delta "
                              "at offset %"PRIuMAX" from %s",
                              (uintmax_t)curpos, p->pack_name);
                        data = NULL;
                        continue;
                }

                data = patch_delta(base, base_size,
                                   delta_data, delta_size,
                                   &size);
                if (!data)
                        die("failed to apply delta");

                free (delta_data);
        }

        *final_type = type;
        *final_size = size;

        unuse_pack(&w_curs);
        return data;
}

const unsigned char *nth_packed_object_sha1(struct packed_git *p,
                                            uint32_t n)
{
        const unsigned char *index = p->index_data;
        if (!index) {
                if (open_pack_index(p))
                        return NULL;
                index = p->index_data;
        }
        if (n >= p->num_objects)
                return NULL;
        index += 4 * 256;
        if (p->index_version == 1) {
                return index + 24 * n + 4;
        } else {
                index += 8;
                return index + 20 * n;
        }
}

off_t nth_packed_object_offset(const struct packed_git *p, uint32_t n)
{
        const unsigned char *index = p->index_data;
        index += 4 * 256;
        if (p->index_version == 1) {
                return ntohl(*((uint32_t *)(index + 24 * n)));
        } else {
                uint32_t off;
                index += 8 + p->num_objects * (20 + 4);
                off = ntohl(*((uint32_t *)(index + 4 * n)));
                if (!(off & 0x80000000))
                        return off;
                index += p->num_objects * 4 + (off & 0x7fffffff) * 8;
                return (((uint64_t)ntohl(*((uint32_t *)(index + 0)))) << 32) |
                                   ntohl(*((uint32_t *)(index + 4)));
        }
}

off_t find_pack_entry_one(const unsigned char *sha1,
                                  struct packed_git *p)
{
        const uint32_t *level1_ofs = p->index_data;
        const unsigned char *index = p->index_data;
        unsigned hi, lo, stride;
        static int use_lookup = -1;
        static int debug_lookup = -1;

        if (debug_lookup < 0)
                debug_lookup = !!getenv("GIT_DEBUG_LOOKUP");

        if (!index) {
                if (open_pack_index(p))
                        return 0;
                level1_ofs = p->index_data;
                index = p->index_data;
        }
        if (p->index_version > 1) {
                level1_ofs += 2;
                index += 8;
        }
        index += 4 * 256;
        hi = ntohl(level1_ofs[*sha1]);
        lo = ((*sha1 == 0x0) ? 0 : ntohl(level1_ofs[*sha1 - 1]));
        if (p->index_version > 1) {
                stride = 20;
        } else {
                stride = 24;
                index += 4;
        }

        if (debug_lookup)
                printf("%02x%02x%02x... lo %u hi %u nr %"PRIu32"\n",
                       sha1[0], sha1[1], sha1[2], lo, hi, p->num_objects);

        if (use_lookup < 0)
                use_lookup = !!getenv("GIT_USE_LOOKUP");
        if (use_lookup) {
                int pos = sha1_entry_pos(index, stride, 0,
                                         lo, hi, p->num_objects, sha1);
                if (pos < 0)
                        return 0;
                return nth_packed_object_offset(p, pos);
        }

        do {
                unsigned mi = (lo + hi) / 2;
                int cmp = hashcmp(index + mi * stride, sha1);

                if (debug_lookup)
                        printf("lo %u hi %u rg %u mi %u\n",
                               lo, hi, hi - lo, mi);
                if (!cmp)
                        return nth_packed_object_offset(p, mi);
                if (cmp > 0)
                        hi = mi;
                else
                        lo = mi+1;
        } while (lo < hi);
        return 0;
}

int is_pack_valid(struct packed_git *p)
{
        /* An already open pack is known to be valid. */
        if (p->pack_fd != -1)
                return 1;

        /* If the pack has one window completely covering the
         * file size, the pack is known to be valid even if
         * the descriptor is not currently open.
         */
        if (p->windows) {
                struct pack_window *w = p->windows;

                if (!w->offset && w->len == p->pack_size)
                        return 1;
        }

        /* Force the pack to open to prove its valid. */
        return !open_packed_git(p);
}

static int fill_pack_entry(const unsigned char *sha1,
                           struct pack_entry *e,
                           struct packed_git *p)
{
        off_t offset;

        if (p->num_bad_objects) {
                unsigned i;
                for (i = 0; i < p->num_bad_objects; i++)
                        if (!hashcmp(sha1, p->bad_object_sha1 + 20 * i))
                                return 0;
        }

        offset = find_pack_entry_one(sha1, p);
        if (!offset)
                return 0;

        /*
         * We are about to tell the caller where they can locate the
         * requested object.  We better make sure the packfile is
         * still here and can be accessed before supplying that
         * answer, as it may have been deleted since the index was
         * loaded!
         */
        if (!is_pack_valid(p)) {
                warning("packfile %s cannot be accessed", p->pack_name);
                return 0;
        }
        e->offset = offset;
        e->p = p;
        hashcpy(e->sha1, sha1);
        return 1;
}

static int find_pack_entry(const unsigned char *sha1, struct pack_entry *e)
{
        struct packed_git *p;

        prepare_packed_git();
        if (!packed_git)
                return 0;

        if (last_found_pack && fill_pack_entry(sha1, e, last_found_pack))
                return 1;

        for (p = packed_git; p; p = p->next) {
                if (p == last_found_pack || !fill_pack_entry(sha1, e, p))
                        continue;

                last_found_pack = p;
                return 1;
        }
        return 0;
}

struct packed_git *find_sha1_pack(const unsigned char *sha1,
                                  struct packed_git *packs)
{
        struct packed_git *p;

        for (p = packs; p; p = p->next) {
                if (find_pack_entry_one(sha1, p))
                        return p;
        }
        return NULL;

}

static int sha1_loose_object_info(const unsigned char *sha1,
                                  enum object_type *typep,
                                  unsigned long *sizep,
                                  unsigned long *disk_sizep)
{
        int status;
        unsigned long mapsize, size;
        void *map;
        git_zstream stream;
        char hdr[32];

        /*
         * If we don't care about type or size, then we don't
         * need to look inside the object at all.
         */
        if (!typep && !sizep) {
                if (disk_sizep) {
                        struct stat st;
                        if (stat_sha1_file(sha1, &st) < 0)
                                return -1;
                        *disk_sizep = st.st_size;
                }
                return 0;
        }

        map = map_sha1_file(sha1, &mapsize);
        if (!map)
                return error("unable to find %s", sha1_to_hex(sha1));
        if (disk_sizep)
                *disk_sizep = mapsize;
        if (unpack_sha1_header(&stream, map, mapsize, hdr, sizeof(hdr)) < 0)
                status = error("unable to unpack %s header",
                               sha1_to_hex(sha1));
        else if ((status = parse_sha1_header(hdr, &size)) < 0)
                status = error("unable to parse %s header", sha1_to_hex(sha1));
        else if (sizep)
                *sizep = size;
        git_inflate_end(&stream);
        munmap(map, mapsize);
        if (typep)
                *typep = status;
        return 0;
}

/* returns enum object_type or negative */
int sha1_object_info_extended(const unsigned char *sha1, struct object_info *oi)
{
        struct cached_object *co;
        struct pack_entry e;
        int type, rtype;

        co = find_cached_object(sha1);
        if (co) {
                if (oi->sizep)
                        *(oi->sizep) = co->size;
                if (oi->disk_sizep)
                        *(oi->disk_sizep) = 0;
                oi->whence = OI_CACHED;
                return co->type;
        }

        if (!find_pack_entry(sha1, &e)) {
                /* Most likely it's a loose object. */
                if (!sha1_loose_object_info(sha1, &type,
                                            oi->sizep, oi->disk_sizep)) {
                        oi->whence = OI_LOOSE;
                        return type;
                }

                /* Not a loose object; someone else may have just packed it. */
                reprepare_packed_git();
                if (!find_pack_entry(sha1, &e))
                        return -1;
        }

        type = packed_object_info(e.p, e.offset, oi->sizep, &rtype,
                                  oi->disk_sizep);
        if (type < 0) {
                mark_bad_packed_object(e.p, sha1);
                type = sha1_object_info_extended(sha1, oi);
        } else if (in_delta_base_cache(e.p, e.offset)) {
                oi->whence = OI_DBCACHED;
        } else {
                oi->whence = OI_PACKED;
                oi->u.packed.offset = e.offset;
                oi->u.packed.pack = e.p;
                oi->u.packed.is_delta = (rtype == OBJ_REF_DELTA ||
                                         rtype == OBJ_OFS_DELTA);
        }

        return type;
}

int sha1_object_info(const unsigned char *sha1, unsigned long *sizep)
{
        struct object_info oi = {0};

        oi.sizep = sizep;
        return sha1_object_info_extended(sha1, &oi);
}

static void *read_packed_sha1(const unsigned char *sha1,
                              enum object_type *type, unsigned long *size)
{
        struct pack_entry e;
        void *data;

        if (!find_pack_entry(sha1, &e))
                return NULL;
        data = cache_or_unpack_entry(e.p, e.offset, size, type, 1);
        if (!data) {
                /*
                 * We're probably in deep shit, but let's try to fetch
                 * the required object anyway from another pack or loose.
                 * This should happen only in the presence of a corrupted
                 * pack, and is better than failing outright.
                 */
                error("failed to read object %s at offset %"PRIuMAX" from %s",
                      sha1_to_hex(sha1), (uintmax_t)e.offset, e.p->pack_name);
                mark_bad_packed_object(e.p, sha1);
                data = read_object(sha1, type, size);
        }
        return data;
}

int pretend_sha1_file(void *buf, unsigned long len, enum object_type type,
                      unsigned char *sha1)
{
        struct cached_object *co;

        hash_sha1_file(buf, len, typename(type), sha1);
        if (has_sha1_file(sha1) || find_cached_object(sha1))
                return 0;
        if (cached_object_alloc <= cached_object_nr) {
                cached_object_alloc = alloc_nr(cached_object_alloc);
                cached_objects = xrealloc(cached_objects,
                                          sizeof(*cached_objects) *
                                          cached_object_alloc);
        }
        co = &cached_objects[cached_object_nr++];
        co->size = len;
        co->type = type;
        co->buf = xmalloc(len);
        memcpy(co->buf, buf, len);
        hashcpy(co->sha1, sha1);
        return 0;
}

static void *read_object(const unsigned char *sha1, enum object_type *type,
                         unsigned long *size)
{
        unsigned long mapsize;
        void *map, *buf;
        struct cached_object *co;

        co = find_cached_object(sha1);
        if (co) {
                *type = co->type;
                *size = co->size;
                return xmemdupz(co->buf, co->size);
        }

        buf = read_packed_sha1(sha1, type, size);
        if (buf)
                return buf;
        map = map_sha1_file(sha1, &mapsize);
        if (map) {
                buf = unpack_sha1_file(map, mapsize, type, size, sha1);
                munmap(map, mapsize);
                return buf;
        }
        reprepare_packed_git();
        return read_packed_sha1(sha1, type, size);
}

/*
 * This function dies on corrupt objects; the callers who want to
 * deal with them should arrange to call read_object() and give error
 * messages themselves.
 */
void *read_sha1_file_extended(const unsigned char *sha1,
                              enum object_type *type,
                              unsigned long *size,
                              unsigned flag)
{
        void *data;
        char *path;
        const struct packed_git *p;
        const unsigned char *repl = (flag & READ_SHA1_FILE_REPLACE)
                ? lookup_replace_object(sha1) : sha1;

        errno = 0;
        data = read_object(repl, type, size);
        if (data)
                return data;

        if (errno && errno != ENOENT)
                die_errno("failed to read object %s", sha1_to_hex(sha1));

        /* die if we replaced an object with one that does not exist */
        if (repl != sha1)
                die("replacement %s not found for %s",
                    sha1_to_hex(repl), sha1_to_hex(sha1));

        if (has_loose_object(repl)) {
                path = sha1_file_name(sha1);
                die("loose object %s (stored in %s) is corrupt",
                    sha1_to_hex(repl), path);
        }

        if ((p = has_packed_and_bad(repl)) != NULL)
                die("packed object %s (stored in %s) is corrupt",
                    sha1_to_hex(repl), p->pack_name);

        return NULL;
}

void *read_object_with_reference(const unsigned char *sha1,
                                 const char *required_type_name,
                                 unsigned long *size,
                                 unsigned char *actual_sha1_return)
{
        enum object_type type, required_type;
        void *buffer;
        unsigned long isize;
        unsigned char actual_sha1[20];

        required_type = type_from_string(required_type_name);
        hashcpy(actual_sha1, sha1);
        while (1) {
                int ref_length = -1;
                const char *ref_type = NULL;

                buffer = read_sha1_file(actual_sha1, &type, &isize);
                if (!buffer)
                        return NULL;
                if (type == required_type) {
                        *size = isize;
                        if (actual_sha1_return)
                                hashcpy(actual_sha1_return, actual_sha1);
                        return buffer;
                }
                /* Handle references */
                else if (type == OBJ_COMMIT)
                        ref_type = "tree ";
                else if (type == OBJ_TAG)
                        ref_type = "object ";
                else {
                        free(buffer);
                        return NULL;
                }
                ref_length = strlen(ref_type);

                if (ref_length + 40 > isize ||
                    memcmp(buffer, ref_type, ref_length) ||
                    get_sha1_hex((char *) buffer + ref_length, actual_sha1)) {
                        free(buffer);
                        return NULL;
                }
                free(buffer);
                /* Now we have the ID of the referred-to object in
                 * actual_sha1.  Check again. */
        }
}

static void write_sha1_file_prepare(const void *buf, unsigned long len,
                                    const char *type, unsigned char *sha1,
                                    char *hdr, int *hdrlen)
{
        git_SHA_CTX c;

        /* Generate the header */
        *hdrlen = sprintf(hdr, "%s %lu", type, len)+1;

        /* Sha1.. */
        git_SHA1_Init(&c);
        git_SHA1_Update(&c, hdr, *hdrlen);
        git_SHA1_Update(&c, buf, len);
        git_SHA1_Final(sha1, &c);
}

/*
 * Move the just written object into its final resting place.
 * NEEDSWORK: this should be renamed to finalize_temp_file() as
 * "moving" is only a part of what it does, when no patch between
 * master to pu changes the call sites of this function.
 */
int move_temp_to_file(const char *tmpfile, const char *filename)
{
        int ret = 0;

        if (object_creation_mode == OBJECT_CREATION_USES_RENAMES)
                goto try_rename;
        else if (link(tmpfile, filename))
                ret = errno;

        /*
         * Coda hack - coda doesn't like cross-directory links,
         * so we fall back to a rename, which will mean that it
         * won't be able to check collisions, but that's not a
         * big deal.
         *
         * The same holds for FAT formatted media.
         *
         * When this succeeds, we just return.  We have nothing
         * left to unlink.
         */
        if (ret && ret != EEXIST) {
        try_rename:
                if (!rename(tmpfile, filename))
                        goto out;
                ret = errno;
        }
        unlink_or_warn(tmpfile);
        if (ret) {
                if (ret != EEXIST) {
                        return error("unable to write sha1 filename %s: %s", filename, strerror(ret));
                }
                /* FIXME!!! Collision check here ? */
        }

out:
        if (adjust_shared_perm(filename))
                return error("unable to set permission to '%s'", filename);
        return 0;
}

static int write_buffer(int fd, const void *buf, size_t len)
{
        if (write_in_full(fd, buf, len) < 0)
                return error("file write error (%s)", strerror(errno));
        return 0;
}

int hash_sha1_file(const void *buf, unsigned long len, const char *type,
                   unsigned char *sha1)
{
        char hdr[32];
        int hdrlen;
        write_sha1_file_prepare(buf, len, type, sha1, hdr, &hdrlen);
        return 0;
}

/* Finalize a file on disk, and close it. */
static void close_sha1_file(int fd)
{
        if (fsync_object_files)
                fsync_or_die(fd, "sha1 file");
        if (close(fd) != 0)
                die_errno("error when closing sha1 file");
}

/* Size of directory component, including the ending '/' */
static inline int directory_size(const char *filename)
{
        const char *s = strrchr(filename, '/');
        if (!s)
                return 0;
        return s - filename + 1;
}

/*
 * This creates a temporary file in the same directory as the final
 * 'filename'
 *
 * We want to avoid cross-directory filename renames, because those
 * can have problems on various filesystems (FAT, NFS, Coda).
 */
static int create_tmpfile(char *buffer, size_t bufsiz, const char *filename)
{
        int fd, dirlen = directory_size(filename);

        if (dirlen + 20 > bufsiz) {
                errno = ENAMETOOLONG;
                return -1;
        }
        memcpy(buffer, filename, dirlen);
        strcpy(buffer + dirlen, "tmp_obj_XXXXXX");
        fd = git_mkstemp_mode(buffer, 0444);
        if (fd < 0 && dirlen && errno == ENOENT) {
                /* Make sure the directory exists */
                memcpy(buffer, filename, dirlen);
                buffer[dirlen-1] = 0;
                if (mkdir(buffer, 0777) || adjust_shared_perm(buffer))
                        return -1;

                /* Try again */
                strcpy(buffer + dirlen - 1, "/tmp_obj_XXXXXX");
                fd = git_mkstemp_mode(buffer, 0444);
        }
        return fd;
}

static int write_loose_object(const unsigned char *sha1, char *hdr, int hdrlen,
                              const void *buf, unsigned long len, time_t mtime)
{
        int fd, ret;
        unsigned char compressed[4096];
        git_zstream stream;
        git_SHA_CTX c;
        unsigned char parano_sha1[20];
        char *filename;
        static char tmp_file[PATH_MAX];

        filename = sha1_file_name(sha1);
        fd = create_tmpfile(tmp_file, sizeof(tmp_file), filename);
        if (fd < 0) {
                if (errno == EACCES)
                        return error("insufficient permission for adding an object to repository database %s", get_object_directory());
                else
                        return error("unable to create temporary file: %s", strerror(errno));
        }

        /* Set it up */
        memset(&stream, 0, sizeof(stream));
        git_deflate_init(&stream, zlib_compression_level);
        stream.next_out = compressed;
        stream.avail_out = sizeof(compressed);
        git_SHA1_Init(&c);

        /* First header.. */
        stream.next_in = (unsigned char *)hdr;
        stream.avail_in = hdrlen;
        while (git_deflate(&stream, 0) == Z_OK)
                ; /* nothing */
        git_SHA1_Update(&c, hdr, hdrlen);

        /* Then the data itself.. */
        stream.next_in = (void *)buf;
        stream.avail_in = len;
        do {
                unsigned char *in0 = stream.next_in;
                ret = git_deflate(&stream, Z_FINISH);
                git_SHA1_Update(&c, in0, stream.next_in - in0);
                if (write_buffer(fd, compressed, stream.next_out - compressed) < 0)
                        die("unable to write sha1 file");
                stream.next_out = compressed;
                stream.avail_out = sizeof(compressed);
        } while (ret == Z_OK);

        if (ret != Z_STREAM_END)
                die("unable to deflate new object %s (%d)", sha1_to_hex(sha1), ret);
        ret = git_deflate_end_gently(&stream);
        if (ret != Z_OK)
                die("deflateEnd on object %s failed (%d)", sha1_to_hex(sha1), ret);
        git_SHA1_Final(parano_sha1, &c);
        if (hashcmp(sha1, parano_sha1) != 0)
                die("confused by unstable object source data for %s", sha1_to_hex(sha1));

        close_sha1_file(fd);

        if (mtime) {
                struct utimbuf utb;
                utb.actime = mtime;
                utb.modtime = mtime;
                if (utime(tmp_file, &utb) < 0)
                        warning("failed utime() on %s: %s",
                                tmp_file, strerror(errno));
        }

        return move_temp_to_file(tmp_file, filename);
}

int write_sha1_file(const void *buf, unsigned long len, const char *type, unsigned char *returnsha1)
{
        unsigned char sha1[20];
        char hdr[32];
        int hdrlen;

        /* Normally if we have it in the pack then we do not bother writing
         * it out into .git/objects/??/?{38} file.
         */
        write_sha1_file_prepare(buf, len, type, sha1, hdr, &hdrlen);
        if (returnsha1)
                hashcpy(returnsha1, sha1);
        if (has_sha1_file(sha1))
                return 0;
        return write_loose_object(sha1, hdr, hdrlen, buf, len, 0);
}

int force_object_loose(const unsigned char *sha1, time_t mtime)
{
        void *buf;
        unsigned long len;
        enum object_type type;
        char hdr[32];
        int hdrlen;
        int ret;

        if (has_loose_object(sha1))
                return 0;
        buf = read_packed_sha1(sha1, &type, &len);
        if (!buf)
                return error("cannot read sha1_file for %s", sha1_to_hex(sha1));
        hdrlen = sprintf(hdr, "%s %lu", typename(type), len) + 1;
        ret = write_loose_object(sha1, hdr, hdrlen, buf, len, mtime);
        free(buf);

        return ret;
}

int has_pack_index(const unsigned char *sha1)
{
        struct stat st;
        if (stat(sha1_pack_index_name(sha1), &st))
                return 0;
        return 1;
}

int has_sha1_pack(const unsigned char *sha1)
{
        struct pack_entry e;
        return find_pack_entry(sha1, &e);
}

int has_sha1_file(const unsigned char *sha1)
{
        struct pack_entry e;

        if (find_pack_entry(sha1, &e))
                return 1;
        return has_loose_object(sha1);
}

static void check_tree(const void *buf, size_t size)
{
        struct tree_desc desc;
        struct name_entry entry;

        init_tree_desc(&desc, buf, size);
        while (tree_entry(&desc, &entry))
                /* do nothing
                 * tree_entry() will die() on malformed entries */
                ;
}

static void check_commit(const void *buf, size_t size)
{
        struct commit c;
        memset(&c, 0, sizeof(c));
        if (parse_commit_buffer(&c, buf, size))
                die("corrupt commit");
}

static void check_tag(const void *buf, size_t size)
{
        struct tag t;
        memset(&t, 0, sizeof(t));
        if (parse_tag_buffer(&t, buf, size))
                die("corrupt tag");
}

static int index_mem(unsigned char *sha1, void *buf, size_t size,
                     enum object_type type,
                     const char *path, unsigned flags)
{
        int ret, re_allocated = 0;
        int write_object = flags & HASH_WRITE_OBJECT;

        if (!type)
                type = OBJ_BLOB;

        /*
         * Convert blobs to git internal format
         */
        if ((type == OBJ_BLOB) && path) {
                struct strbuf nbuf = STRBUF_INIT;
                if (convert_to_git(path, buf, size, &nbuf,
                                   write_object ? safe_crlf : SAFE_CRLF_FALSE)) {
                        buf = strbuf_detach(&nbuf, &size);
                        re_allocated = 1;
                }
        }
        if (flags & HASH_FORMAT_CHECK) {
                if (type == OBJ_TREE)
                        check_tree(buf, size);
                if (type == OBJ_COMMIT)
                        check_commit(buf, size);
                if (type == OBJ_TAG)
                        check_tag(buf, size);
        }

        if (write_object)
                ret = write_sha1_file(buf, size, typename(type), sha1);
        else
                ret = hash_sha1_file(buf, size, typename(type), sha1);
        if (re_allocated)
                free(buf);
        return ret;
}

static int index_pipe(unsigned char *sha1, int fd, enum object_type type,
                      const char *path, unsigned flags)
{
        struct strbuf sbuf = STRBUF_INIT;
        int ret;

        if (strbuf_read(&sbuf, fd, 4096) >= 0)
                ret = index_mem(sha1, sbuf.buf, sbuf.len, type, path, flags);
        else
                ret = -1;
        strbuf_release(&sbuf);
        return ret;
}

#define SMALL_FILE_SIZE (32*1024)

static int index_core(unsigned char *sha1, int fd, size_t size,
                      enum object_type type, const char *path,
                      unsigned flags)
{
        int ret;

        if (!size) {
                ret = index_mem(sha1, NULL, size, type, path, flags);
        } else if (size <= SMALL_FILE_SIZE) {
                char *buf = xmalloc(size);
                if (size == read_in_full(fd, buf, size))
                        ret = index_mem(sha1, buf, size, type, path, flags);
                else
                        ret = error("short read %s", strerror(errno));
                free(buf);
        } else {
                void *buf = xmmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
                ret = index_mem(sha1, buf, size, type, path, flags);
                munmap(buf, size);
        }
        return ret;
}

/*
 * This creates one packfile per large blob unless bulk-checkin
 * machinery is "plugged".
 *
 * This also bypasses the usual "convert-to-git" dance, and that is on
 * purpose. We could write a streaming version of the converting
 * functions and insert that before feeding the data to fast-import
 * (or equivalent in-core API described above). However, that is
 * somewhat complicated, as we do not know the size of the filter
 * result, which we need to know beforehand when writing a git object.
 * Since the primary motivation for trying to stream from the working
 * tree file and to avoid mmaping it in core is to deal with large
 * binary blobs, they generally do not want to get any conversion, and
 * callers should avoid this code path when filters are requested.
 */
static int index_stream(unsigned char *sha1, int fd, size_t size,
                        enum object_type type, const char *path,
                        unsigned flags)
{
        return index_bulk_checkin(sha1, fd, size, type, path, flags);
}

int index_fd(unsigned char *sha1, int fd, struct stat *st,
             enum object_type type, const char *path, unsigned flags)
{
        int ret;
        size_t size = xsize_t(st->st_size);

        if (!S_ISREG(st->st_mode))
                ret = index_pipe(sha1, fd, type, path, flags);
        else if (size <= big_file_threshold || type != OBJ_BLOB ||
                 (path && would_convert_to_git(path, NULL, 0, 0)))
                ret = index_core(sha1, fd, size, type, path, flags);
        else
                ret = index_stream(sha1, fd, size, type, path, flags);
        close(fd);
        return ret;
}

int index_path(unsigned char *sha1, const char *path, struct stat *st, unsigned flags)
{
        int fd;
        struct strbuf sb = STRBUF_INIT;

        switch (st->st_mode & S_IFMT) {
        case S_IFREG:
                fd = open(path, O_RDONLY);
                if (fd < 0)
                        return error("open(\"%s\"): %s", path,
                                     strerror(errno));
                if (index_fd(sha1, fd, st, OBJ_BLOB, path, flags) < 0)
                        return error("%s: failed to insert into database",
                                     path);
                break;
        case S_IFLNK:
                if (strbuf_readlink(&sb, path, st->st_size)) {
                        char *errstr = strerror(errno);
                        return error("readlink(\"%s\"): %s", path,
                                     errstr);
                }
                if (!(flags & HASH_WRITE_OBJECT))
                        hash_sha1_file(sb.buf, sb.len, blob_type, sha1);
                else if (write_sha1_file(sb.buf, sb.len, blob_type, sha1))
                        return error("%s: failed to insert into database",
                                     path);
                strbuf_release(&sb);
                break;
        case S_IFDIR:
                return resolve_gitlink_ref(path, "HEAD", sha1);
        default:
                return error("%s: unsupported file type", path);
        }
        return 0;
}

int read_pack_header(int fd, struct pack_header *header)
{
        if (read_in_full(fd, header, sizeof(*header)) < sizeof(*header))
                /* "eof before pack header was fully read" */
                return PH_ERROR_EOF;

        if (header->hdr_signature != htonl(PACK_SIGNATURE))
                /* "protocol error (pack signature mismatch detected)" */
                return PH_ERROR_PACK_SIGNATURE;
        if (!pack_version_ok(header->hdr_version))
                /* "protocol error (pack version unsupported)" */
                return PH_ERROR_PROTOCOL;
        return 0;
}

void assert_sha1_type(const unsigned char *sha1, enum object_type expect)
{
        enum object_type type = sha1_object_info(sha1, NULL);
        if (type < 0)
                die("%s is not a valid object", sha1_to_hex(sha1));
        if (type != expect)
                die("%s is not a valid '%s' object", sha1_to_hex(sha1),
                    typename(expect));
}