merge-ort: avoid recursing into identical trees

[git] / merge-ort.c

/*
 * "Ostensibly Recursive's Twin" merge strategy, or "ort" for short.  Meant
 * as a drop-in replacement for the "recursive" merge strategy, allowing one
 * to replace
 *
 *   git merge [-s recursive]
 *
 * with
 *
 *   git merge -s ort
 *
 * Note: git's parser allows the space between '-s' and its argument to be
 * missing.  (Should I have backronymed "ham", "alsa", "kip", "nap, "alvo",
 * "cale", "peedy", or "ins" instead of "ort"?)
 */

#include "cache.h"
#include "merge-ort.h"

#include "diff.h"
#include "diffcore.h"
#include "strmap.h"
#include "tree.h"
#include "xdiff-interface.h"

/*
 * We have many arrays of size 3.  Whenever we have such an array, the
 * indices refer to one of the sides of the three-way merge.  This is so
 * pervasive that the constants 0, 1, and 2 are used in many places in the
 * code (especially in arithmetic operations to find the other side's index
 * or to compute a relevant mask), but sometimes these enum names are used
 * to aid code clarity.
 *
 * See also 'filemask' and 'dirmask' in struct conflict_info; the "ith side"
 * referred to there is one of these three sides.
 */
enum merge_side {
        MERGE_BASE = 0,
        MERGE_SIDE1 = 1,
        MERGE_SIDE2 = 2
};

struct merge_options_internal {
        /*
         * paths: primary data structure in all of merge ort.
         *
         * The keys of paths:
         *   * are full relative paths from the toplevel of the repository
         *     (e.g. "drivers/firmware/raspberrypi.c").
         *   * store all relevant paths in the repo, both directories and
         *     files (e.g. drivers, drivers/firmware would also be included)
         *   * these keys serve to intern all the path strings, which allows
         *     us to do pointer comparison on directory names instead of
         *     strcmp; we just have to be careful to use the interned strings.
         *
         * The values of paths:
         *   * either a pointer to a merged_info, or a conflict_info struct
         *   * merged_info contains all relevant information for a
         *     non-conflicted entry.
         *   * conflict_info contains a merged_info, plus any additional
         *     information about a conflict such as the higher orders stages
         *     involved and the names of the paths those came from (handy
         *     once renames get involved).
         *   * a path may start "conflicted" (i.e. point to a conflict_info)
         *     and then a later step (e.g. three-way content merge) determines
         *     it can be cleanly merged, at which point it'll be marked clean
         *     and the algorithm will ignore any data outside the contained
         *     merged_info for that entry
         *   * If an entry remains conflicted, the merged_info portion of a
         *     conflict_info will later be filled with whatever version of
         *     the file should be placed in the working directory (e.g. an
         *     as-merged-as-possible variation that contains conflict markers).
         */
        struct strmap paths;

        /*
         * conflicted: a subset of keys->values from "paths"
         *
         * conflicted is basically an optimization between process_entries()
         * and record_conflicted_index_entries(); the latter could loop over
         * ALL the entries in paths AGAIN and look for the ones that are
         * still conflicted, but since process_entries() has to loop over
         * all of them, it saves the ones it couldn't resolve in this strmap
         * so that record_conflicted_index_entries() can iterate just the
         * relevant entries.
         */
        struct strmap conflicted;

        /*
         * current_dir_name: temporary var used in collect_merge_info_callback()
         *
         * Used to set merged_info.directory_name; see documentation for that
         * variable and the requirements placed on that field.
         */
        const char *current_dir_name;

        /* call_depth: recursion level counter for merging merge bases */
        int call_depth;
};

struct version_info {
        struct object_id oid;
        unsigned short mode;
};

struct merged_info {
        /* if is_null, ignore result.  otherwise result has oid & mode */
        struct version_info result;
        unsigned is_null:1;

        /*
         * clean: whether the path in question is cleanly merged.
         *
         * see conflict_info.merged for more details.
         */
        unsigned clean:1;

        /*
         * basename_offset: offset of basename of path.
         *
         * perf optimization to avoid recomputing offset of final '/'
         * character in pathname (0 if no '/' in pathname).
         */
        size_t basename_offset;

         /*
          * directory_name: containing directory name.
          *
          * Note that we assume directory_name is constructed such that
          *    strcmp(dir1_name, dir2_name) == 0 iff dir1_name == dir2_name,
          * i.e. string equality is equivalent to pointer equality.  For this
          * to hold, we have to be careful setting directory_name.
          */
        const char *directory_name;
};

struct conflict_info {
        /*
         * merged: the version of the path that will be written to working tree
         *
         * WARNING: It is critical to check merged.clean and ensure it is 0
         * before reading any conflict_info fields outside of merged.
         * Allocated merge_info structs will always have clean set to 1.
         * Allocated conflict_info structs will have merged.clean set to 0
         * initially.  The merged.clean field is how we know if it is safe
         * to access other parts of conflict_info besides merged; if a
         * conflict_info's merged.clean is changed to 1, the rest of the
         * algorithm is not allowed to look at anything outside of the
         * merged member anymore.
         */
        struct merged_info merged;

        /* oids & modes from each of the three trees for this path */
        struct version_info stages[3];

        /* pathnames for each stage; may differ due to rename detection */
        const char *pathnames[3];

        /* Whether this path is/was involved in a directory/file conflict */
        unsigned df_conflict:1;

        /*
         * For filemask and dirmask, the ith bit corresponds to whether the
         * ith entry is a file (filemask) or a directory (dirmask).  Thus,
         * filemask & dirmask is always zero, and filemask | dirmask is at
         * most 7 but can be less when a path does not appear as either a
         * file or a directory on at least one side of history.
         *
         * Note that these masks are related to enum merge_side, as the ith
         * entry corresponds to side i.
         *
         * These values come from a traverse_trees() call; more info may be
         * found looking at tree-walk.h's struct traverse_info,
         * particularly the documentation above the "fn" member (note that
         * filemask = mask & ~dirmask from that documentation).
         */
        unsigned filemask:3;
        unsigned dirmask:3;

        /*
         * Optimization to track which stages match, to avoid the need to
         * recompute it in multiple steps. Either 0 or at least 2 bits are
         * set; if at least 2 bits are set, their corresponding stages match.
         */
        unsigned match_mask:3;
};

/*
 * For the next three macros, see warning for conflict_info.merged.
 *
 * In each of the below, mi is a struct merged_info*, and ci was defined
 * as a struct conflict_info* (but we need to verify ci isn't actually
 * pointed at a struct merged_info*).
 *
 * INITIALIZE_CI: Assign ci to mi but only if it's safe; set to NULL otherwise.
 * VERIFY_CI: Ensure that something we assigned to a conflict_info* is one.
 * ASSIGN_AND_VERIFY_CI: Similar to VERIFY_CI but do assignment first.
 */
#define INITIALIZE_CI(ci, mi) do {                                           \
        (ci) = (!(mi) || (mi)->clean) ? NULL : (struct conflict_info *)(mi); \
} while (0)
#define VERIFY_CI(ci) assert(ci && !ci->merged.clean);
#define ASSIGN_AND_VERIFY_CI(ci, mi) do {    \
        (ci) = (struct conflict_info *)(mi);  \
        assert((ci) && !(mi)->clean);        \
} while (0)

static int err(struct merge_options *opt, const char *err, ...)
{
        va_list params;
        struct strbuf sb = STRBUF_INIT;

        strbuf_addstr(&sb, "error: ");
        va_start(params, err);
        strbuf_vaddf(&sb, err, params);
        va_end(params);

        error("%s", sb.buf);
        strbuf_release(&sb);

        return -1;
}

static void setup_path_info(struct merge_options *opt,
                            struct string_list_item *result,
                            const char *current_dir_name,
                            int current_dir_name_len,
                            char *fullpath, /* we'll take over ownership */
                            struct name_entry *names,
                            struct name_entry *merged_version,
                            unsigned is_null,     /* boolean */
                            unsigned df_conflict, /* boolean */
                            unsigned filemask,
                            unsigned dirmask,
                            int resolved          /* boolean */)
{
        /* result->util is void*, so mi is a convenience typed variable */
        struct merged_info *mi;

        assert(!is_null || resolved);
        assert(!df_conflict || !resolved); /* df_conflict implies !resolved */
        assert(resolved == (merged_version != NULL));

        mi = xcalloc(1, resolved ? sizeof(struct merged_info) :
                                   sizeof(struct conflict_info));
        mi->directory_name = current_dir_name;
        mi->basename_offset = current_dir_name_len;
        mi->clean = !!resolved;
        if (resolved) {
                mi->result.mode = merged_version->mode;
                oidcpy(&mi->result.oid, &merged_version->oid);
                mi->is_null = !!is_null;
        } else {
                int i;
                struct conflict_info *ci;

                ASSIGN_AND_VERIFY_CI(ci, mi);
                for (i = MERGE_BASE; i <= MERGE_SIDE2; i++) {
                        ci->pathnames[i] = fullpath;
                        ci->stages[i].mode = names[i].mode;
                        oidcpy(&ci->stages[i].oid, &names[i].oid);
                }
                ci->filemask = filemask;
                ci->dirmask = dirmask;
                ci->df_conflict = !!df_conflict;
                if (dirmask)
                        /*
                         * Assume is_null for now, but if we have entries
                         * under the directory then when it is complete in
                         * write_completed_directory() it'll update this.
                         * Also, for D/F conflicts, we have to handle the
                         * directory first, then clear this bit and process
                         * the file to see how it is handled -- that occurs
                         * near the top of process_entry().
                         */
                        mi->is_null = 1;
        }
        strmap_put(&opt->priv->paths, fullpath, mi);
        result->string = fullpath;
        result->util = mi;
}

static int collect_merge_info_callback(int n,
                                       unsigned long mask,
                                       unsigned long dirmask,
                                       struct name_entry *names,
                                       struct traverse_info *info)
{
        /*
         * n is 3.  Always.
         * common ancestor (mbase) has mask 1, and stored in index 0 of names
         * head of side 1  (side1) has mask 2, and stored in index 1 of names
         * head of side 2  (side2) has mask 4, and stored in index 2 of names
         */
        struct merge_options *opt = info->data;
        struct merge_options_internal *opti = opt->priv;
        struct string_list_item pi;  /* Path Info */
        struct conflict_info *ci; /* typed alias to pi.util (which is void*) */
        struct name_entry *p;
        size_t len;
        char *fullpath;
        const char *dirname = opti->current_dir_name;
        unsigned filemask = mask & ~dirmask;
        unsigned match_mask = 0; /* will be updated below */
        unsigned mbase_null = !(mask & 1);
        unsigned side1_null = !(mask & 2);
        unsigned side2_null = !(mask & 4);
        unsigned side1_matches_mbase = (!side1_null && !mbase_null &&
                                        names[0].mode == names[1].mode &&
                                        oideq(&names[0].oid, &names[1].oid));
        unsigned side2_matches_mbase = (!side2_null && !mbase_null &&
                                        names[0].mode == names[2].mode &&
                                        oideq(&names[0].oid, &names[2].oid));
        unsigned sides_match = (!side1_null && !side2_null &&
                                names[1].mode == names[2].mode &&
                                oideq(&names[1].oid, &names[2].oid));

        /*
         * Note: When a path is a file on one side of history and a directory
         * in another, we have a directory/file conflict.  In such cases, if
         * the conflict doesn't resolve from renames and deletions, then we
         * always leave directories where they are and move files out of the
         * way.  Thus, while struct conflict_info has a df_conflict field to
         * track such conflicts, we ignore that field for any directories at
         * a path and only pay attention to it for files at the given path.
         * The fact that we leave directories were they are also means that
         * we do not need to worry about getting additional df_conflict
         * information propagated from parent directories down to children
         * (unlike, say traverse_trees_recursive() in unpack-trees.c, which
         * sets a newinfo.df_conflicts field specifically to propagate it).
         */
        unsigned df_conflict = (filemask != 0) && (dirmask != 0);

        /* n = 3 is a fundamental assumption. */
        if (n != 3)
                BUG("Called collect_merge_info_callback wrong");

        /*
         * A bunch of sanity checks verifying that traverse_trees() calls
         * us the way I expect.  Could just remove these at some point,
         * though maybe they are helpful to future code readers.
         */
        assert(mbase_null == is_null_oid(&names[0].oid));
        assert(side1_null == is_null_oid(&names[1].oid));
        assert(side2_null == is_null_oid(&names[2].oid));
        assert(!mbase_null || !side1_null || !side2_null);
        assert(mask > 0 && mask < 8);

        /* Determine match_mask */
        if (side1_matches_mbase)
                match_mask = (side2_matches_mbase ? 7 : 3);
        else if (side2_matches_mbase)
                match_mask = 5;
        else if (sides_match)
                match_mask = 6;

        /*
         * Get the name of the relevant filepath, which we'll pass to
         * setup_path_info() for tracking.
         */
        p = names;
        while (!p->mode)
                p++;
        len = traverse_path_len(info, p->pathlen);

        /* +1 in both of the following lines to include the NUL byte */
        fullpath = xmalloc(len + 1);
        make_traverse_path(fullpath, len + 1, info, p->path, p->pathlen);

        /*
         * If mbase, side1, and side2 all match, we can resolve early.  Even
         * if these are trees, there will be no renames or anything
         * underneath.
         */
        if (side1_matches_mbase && side2_matches_mbase) {
                /* mbase, side1, & side2 all match; use mbase as resolution */
                setup_path_info(opt, &pi, dirname, info->pathlen, fullpath,
                                names, names+0, mbase_null, 0,
                                filemask, dirmask, 1);
                return mask;
        }

        /*
         * Record information about the path so we can resolve later in
         * process_entries.
         */
        setup_path_info(opt, &pi, dirname, info->pathlen, fullpath,
                        names, NULL, 0, df_conflict, filemask, dirmask, 0);

        ci = pi.util;
        VERIFY_CI(ci);
        ci->match_mask = match_mask;

        /* If dirmask, recurse into subdirectories */
        if (dirmask) {
                struct traverse_info newinfo;
                struct tree_desc t[3];
                void *buf[3] = {NULL, NULL, NULL};
                const char *original_dir_name;
                int i, ret;

                ci->match_mask &= filemask;
                newinfo = *info;
                newinfo.prev = info;
                newinfo.name = p->path;
                newinfo.namelen = p->pathlen;
                newinfo.pathlen = st_add3(newinfo.pathlen, p->pathlen, 1);
                /*
                 * If this directory we are about to recurse into cared about
                 * its parent directory (the current directory) having a D/F
                 * conflict, then we'd propagate the masks in this way:
                 *    newinfo.df_conflicts |= (mask & ~dirmask);
                 * But we don't worry about propagating D/F conflicts.  (See
                 * comment near setting of local df_conflict variable near
                 * the beginning of this function).
                 */

                for (i = MERGE_BASE; i <= MERGE_SIDE2; i++) {
                        if (i == 1 && side1_matches_mbase)
                                t[1] = t[0];
                        else if (i == 2 && side2_matches_mbase)
                                t[2] = t[0];
                        else if (i == 2 && sides_match)
                                t[2] = t[1];
                        else {
                                const struct object_id *oid = NULL;
                                if (dirmask & 1)
                                        oid = &names[i].oid;
                                buf[i] = fill_tree_descriptor(opt->repo,
                                                              t + i, oid);
                        }
                        dirmask >>= 1;
                }

                original_dir_name = opti->current_dir_name;
                opti->current_dir_name = pi.string;
                ret = traverse_trees(NULL, 3, t, &newinfo);
                opti->current_dir_name = original_dir_name;

                for (i = MERGE_BASE; i <= MERGE_SIDE2; i++)
                        free(buf[i]);

                if (ret < 0)
                        return -1;
        }

        return mask;
}

static int collect_merge_info(struct merge_options *opt,
                              struct tree *merge_base,
                              struct tree *side1,
                              struct tree *side2)
{
        int ret;
        struct tree_desc t[3];
        struct traverse_info info;
        const char *toplevel_dir_placeholder = "";

        opt->priv->current_dir_name = toplevel_dir_placeholder;
        setup_traverse_info(&info, toplevel_dir_placeholder);
        info.fn = collect_merge_info_callback;
        info.data = opt;
        info.show_all_errors = 1;

        parse_tree(merge_base);
        parse_tree(side1);
        parse_tree(side2);
        init_tree_desc(t + 0, merge_base->buffer, merge_base->size);
        init_tree_desc(t + 1, side1->buffer, side1->size);
        init_tree_desc(t + 2, side2->buffer, side2->size);

        ret = traverse_trees(NULL, 3, t, &info);

        return ret;
}

static int detect_and_process_renames(struct merge_options *opt,
                                      struct tree *merge_base,
                                      struct tree *side1,
                                      struct tree *side2)
{
        int clean = 1;

        /*
         * Rename detection works by detecting file similarity.  Here we use
         * a really easy-to-implement scheme: files are similar IFF they have
         * the same filename.  Therefore, by this scheme, there are no renames.
         *
         * TODO: Actually implement a real rename detection scheme.
         */
        return clean;
}

static void process_entries(struct merge_options *opt,
                            struct object_id *result_oid)
{
        die("Not yet implemented.");
}

void merge_switch_to_result(struct merge_options *opt,
                            struct tree *head,
                            struct merge_result *result,
                            int update_worktree_and_index,
                            int display_update_msgs)
{
        die("Not yet implemented");
        merge_finalize(opt, result);
}

void merge_finalize(struct merge_options *opt,
                    struct merge_result *result)
{
        die("Not yet implemented");
}

static void merge_start(struct merge_options *opt, struct merge_result *result)
{
        /* Sanity checks on opt */
        assert(opt->repo);

        assert(opt->branch1 && opt->branch2);

        assert(opt->detect_directory_renames >= MERGE_DIRECTORY_RENAMES_NONE &&
               opt->detect_directory_renames <= MERGE_DIRECTORY_RENAMES_TRUE);
        assert(opt->rename_limit >= -1);
        assert(opt->rename_score >= 0 && opt->rename_score <= MAX_SCORE);
        assert(opt->show_rename_progress >= 0 && opt->show_rename_progress <= 1);

        assert(opt->xdl_opts >= 0);
        assert(opt->recursive_variant >= MERGE_VARIANT_NORMAL &&
               opt->recursive_variant <= MERGE_VARIANT_THEIRS);

        /*
         * detect_renames, verbosity, buffer_output, and obuf are ignored
         * fields that were used by "recursive" rather than "ort" -- but
         * sanity check them anyway.
         */
        assert(opt->detect_renames >= -1 &&
               opt->detect_renames <= DIFF_DETECT_COPY);
        assert(opt->verbosity >= 0 && opt->verbosity <= 5);
        assert(opt->buffer_output <= 2);
        assert(opt->obuf.len == 0);

        assert(opt->priv == NULL);

        /* Default to histogram diff.  Actually, just hardcode it...for now. */
        opt->xdl_opts = DIFF_WITH_ALG(opt, HISTOGRAM_DIFF);

        /* Initialization of opt->priv, our internal merge data */
        opt->priv = xcalloc(1, sizeof(*opt->priv));

        /*
         * Although we initialize opt->priv->paths with strdup_strings=0,
         * that's just to avoid making yet another copy of an allocated
         * string.  Putting the entry into paths means we are taking
         * ownership, so we will later free it.
         *
         * In contrast, conflicted just has a subset of keys from paths, so
         * we don't want to free those (it'd be a duplicate free).
         */
        strmap_init_with_options(&opt->priv->paths, NULL, 0);
        strmap_init_with_options(&opt->priv->conflicted, NULL, 0);
}

/*
 * Originally from merge_trees_internal(); heavily adapted, though.
 */
static void merge_ort_nonrecursive_internal(struct merge_options *opt,
                                            struct tree *merge_base,
                                            struct tree *side1,
                                            struct tree *side2,
                                            struct merge_result *result)
{
        struct object_id working_tree_oid;

        if (collect_merge_info(opt, merge_base, side1, side2) != 0) {
                /*
                 * TRANSLATORS: The %s arguments are: 1) tree hash of a merge
                 * base, and 2-3) the trees for the two trees we're merging.
                 */
                err(opt, _("collecting merge info failed for trees %s, %s, %s"),
                    oid_to_hex(&merge_base->object.oid),
                    oid_to_hex(&side1->object.oid),
                    oid_to_hex(&side2->object.oid));
                result->clean = -1;
                return;
        }

        result->clean = detect_and_process_renames(opt, merge_base,
                                                   side1, side2);
        process_entries(opt, &working_tree_oid);

        /* Set return values */
        result->tree = parse_tree_indirect(&working_tree_oid);
        /* existence of conflicted entries implies unclean */
        result->clean &= strmap_empty(&opt->priv->conflicted);
        if (!opt->priv->call_depth) {
                result->priv = opt->priv;
                opt->priv = NULL;
        }
}

void merge_incore_nonrecursive(struct merge_options *opt,
                               struct tree *merge_base,
                               struct tree *side1,
                               struct tree *side2,
                               struct merge_result *result)
{
        assert(opt->ancestor != NULL);
        merge_start(opt, result);
        merge_ort_nonrecursive_internal(opt, merge_base, side1, side2, result);
}

void merge_incore_recursive(struct merge_options *opt,
                            struct commit_list *merge_bases,
                            struct commit *side1,
                            struct commit *side2,
                            struct merge_result *result)
{
        die("Not yet implemented");
}