fetch-pack: refactor command and capability write

[git] / builtin / checkout.c

#define USE_THE_INDEX_COMPATIBILITY_MACROS
#include "builtin.h"
#include "advice.h"
#include "blob.h"
#include "branch.h"
#include "cache-tree.h"
#include "checkout.h"
#include "commit.h"
#include "config.h"
#include "diff.h"
#include "dir.h"
#include "ll-merge.h"
#include "lockfile.h"
#include "merge-recursive.h"
#include "object-store.h"
#include "parse-options.h"
#include "refs.h"
#include "remote.h"
#include "resolve-undo.h"
#include "revision.h"
#include "run-command.h"
#include "submodule.h"
#include "submodule-config.h"
#include "tree.h"
#include "tree-walk.h"
#include "unpack-trees.h"
#include "wt-status.h"
#include "xdiff-interface.h"
#include "entry.h"

static const char * const checkout_usage[] = {
        N_("git checkout [<options>] <branch>"),
        N_("git checkout [<options>] [<branch>] -- <file>..."),
        NULL,
};

static const char * const switch_branch_usage[] = {
        N_("git switch [<options>] [<branch>]"),
        NULL,
};

static const char * const restore_usage[] = {
        N_("git restore [<options>] [--source=<branch>] <file>..."),
        NULL,
};

struct checkout_opts {
        int patch_mode;
        int quiet;
        int merge;
        int force;
        int force_detach;
        int implicit_detach;
        int writeout_stage;
        int overwrite_ignore;
        int ignore_skipworktree;
        int ignore_other_worktrees;
        int show_progress;
        int count_checkout_paths;
        int overlay_mode;
        int dwim_new_local_branch;
        int discard_changes;
        int accept_ref;
        int accept_pathspec;
        int switch_branch_doing_nothing_is_ok;
        int only_merge_on_switching_branches;
        int can_switch_when_in_progress;
        int orphan_from_empty_tree;
        int empty_pathspec_ok;
        int checkout_index;
        int checkout_worktree;
        const char *ignore_unmerged_opt;
        int ignore_unmerged;
        int pathspec_file_nul;
        const char *pathspec_from_file;

        const char *new_branch;
        const char *new_branch_force;
        const char *new_orphan_branch;
        int new_branch_log;
        enum branch_track track;
        struct diff_options diff_options;
        char *conflict_style;

        int branch_exists;
        const char *prefix;
        struct pathspec pathspec;
        const char *from_treeish;
        struct tree *source_tree;
};

struct branch_info {
        const char *name; /* The short name used */
        const char *path; /* The full name of a real branch */
        struct commit *commit; /* The named commit */
        char *refname; /* The full name of the ref being checked out. */
        struct object_id oid; /* The object ID of the commit being checked out. */
        /*
         * if not null the branch is detached because it's already
         * checked out in this checkout
         */
        char *checkout;
};

static int post_checkout_hook(struct commit *old_commit, struct commit *new_commit,
                              int changed)
{
        return run_hook_le(NULL, "post-checkout",
                           oid_to_hex(old_commit ? &old_commit->object.oid : &null_oid),
                           oid_to_hex(new_commit ? &new_commit->object.oid : &null_oid),
                           changed ? "1" : "0", NULL);
        /* "new_commit" can be NULL when checking out from the index before
           a commit exists. */

}

static int update_some(const struct object_id *oid, struct strbuf *base,
                const char *pathname, unsigned mode, void *context)
{
        int len;
        struct cache_entry *ce;
        int pos;

        if (S_ISDIR(mode))
                return READ_TREE_RECURSIVE;

        len = base->len + strlen(pathname);
        ce = make_empty_cache_entry(&the_index, len);
        oidcpy(&ce->oid, oid);
        memcpy(ce->name, base->buf, base->len);
        memcpy(ce->name + base->len, pathname, len - base->len);
        ce->ce_flags = create_ce_flags(0) | CE_UPDATE;
        ce->ce_namelen = len;
        ce->ce_mode = create_ce_mode(mode);

        /*
         * If the entry is the same as the current index, we can leave the old
         * entry in place. Whether it is UPTODATE or not, checkout_entry will
         * do the right thing.
         */
        pos = cache_name_pos(ce->name, ce->ce_namelen);
        if (pos >= 0) {
                struct cache_entry *old = active_cache[pos];
                if (ce->ce_mode == old->ce_mode &&
                    !ce_intent_to_add(old) &&
                    oideq(&ce->oid, &old->oid)) {
                        old->ce_flags |= CE_UPDATE;
                        discard_cache_entry(ce);
                        return 0;
                }
        }

        add_cache_entry(ce, ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE);
        return 0;
}

static int read_tree_some(struct tree *tree, const struct pathspec *pathspec)
{
        read_tree(the_repository, tree,
                  pathspec, update_some, NULL);

        /* update the index with the given tree's info
         * for all args, expanding wildcards, and exit
         * with any non-zero return code.
         */
        return 0;
}

static int skip_same_name(const struct cache_entry *ce, int pos)
{
        while (++pos < active_nr &&
               !strcmp(active_cache[pos]->name, ce->name))
                ; /* skip */
        return pos;
}

static int check_stage(int stage, const struct cache_entry *ce, int pos,
                       int overlay_mode)
{
        while (pos < active_nr &&
               !strcmp(active_cache[pos]->name, ce->name)) {
                if (ce_stage(active_cache[pos]) == stage)
                        return 0;
                pos++;
        }
        if (!overlay_mode)
                return 0;
        if (stage == 2)
                return error(_("path '%s' does not have our version"), ce->name);
        else
                return error(_("path '%s' does not have their version"), ce->name);
}

static int check_stages(unsigned stages, const struct cache_entry *ce, int pos)
{
        unsigned seen = 0;
        const char *name = ce->name;

        while (pos < active_nr) {
                ce = active_cache[pos];
                if (strcmp(name, ce->name))
                        break;
                seen |= (1 << ce_stage(ce));
                pos++;
        }
        if ((stages & seen) != stages)
                return error(_("path '%s' does not have all necessary versions"),
                             name);
        return 0;
}

static int checkout_stage(int stage, const struct cache_entry *ce, int pos,
                          const struct checkout *state, int *nr_checkouts,
                          int overlay_mode)
{
        while (pos < active_nr &&
               !strcmp(active_cache[pos]->name, ce->name)) {
                if (ce_stage(active_cache[pos]) == stage)
                        return checkout_entry(active_cache[pos], state,
                                              NULL, nr_checkouts);
                pos++;
        }
        if (!overlay_mode) {
                unlink_entry(ce);
                return 0;
        }
        if (stage == 2)
                return error(_("path '%s' does not have our version"), ce->name);
        else
                return error(_("path '%s' does not have their version"), ce->name);
}

static int checkout_merged(int pos, const struct checkout *state, int *nr_checkouts)
{
        struct cache_entry *ce = active_cache[pos];
        const char *path = ce->name;
        mmfile_t ancestor, ours, theirs;
        int status;
        struct object_id oid;
        mmbuffer_t result_buf;
        struct object_id threeway[3];
        unsigned mode = 0;
        struct ll_merge_options ll_opts;
        int renormalize = 0;

        memset(threeway, 0, sizeof(threeway));
        while (pos < active_nr) {
                int stage;
                stage = ce_stage(ce);
                if (!stage || strcmp(path, ce->name))
                        break;
                oidcpy(&threeway[stage - 1], &ce->oid);
                if (stage == 2)
                        mode = create_ce_mode(ce->ce_mode);
                pos++;
                ce = active_cache[pos];
        }
        if (is_null_oid(&threeway[1]) || is_null_oid(&threeway[2]))
                return error(_("path '%s' does not have necessary versions"), path);

        read_mmblob(&ancestor, &threeway[0]);
        read_mmblob(&ours, &threeway[1]);
        read_mmblob(&theirs, &threeway[2]);

        memset(&ll_opts, 0, sizeof(ll_opts));
        git_config_get_bool("merge.renormalize", &renormalize);
        ll_opts.renormalize = renormalize;
        status = ll_merge(&result_buf, path, &ancestor, "base",
                          &ours, "ours", &theirs, "theirs",
                          state->istate, &ll_opts);
        free(ancestor.ptr);
        free(ours.ptr);
        free(theirs.ptr);
        if (status < 0 || !result_buf.ptr) {
                free(result_buf.ptr);
                return error(_("path '%s': cannot merge"), path);
        }

        /*
         * NEEDSWORK:
         * There is absolutely no reason to write this as a blob object
         * and create a phony cache entry.  This hack is primarily to get
         * to the write_entry() machinery that massages the contents to
         * work-tree format and writes out which only allows it for a
         * cache entry.  The code in write_entry() needs to be refactored
         * to allow us to feed a <buffer, size, mode> instead of a cache
         * entry.  Such a refactoring would help merge_recursive as well
         * (it also writes the merge result to the object database even
         * when it may contain conflicts).
         */
        if (write_object_file(result_buf.ptr, result_buf.size, blob_type, &oid))
                die(_("Unable to add merge result for '%s'"), path);
        free(result_buf.ptr);
        ce = make_transient_cache_entry(mode, &oid, path, 2);
        if (!ce)
                die(_("make_cache_entry failed for path '%s'"), path);
        status = checkout_entry(ce, state, NULL, nr_checkouts);
        discard_cache_entry(ce);
        return status;
}

static void mark_ce_for_checkout_overlay(struct cache_entry *ce,
                                         char *ps_matched,
                                         const struct checkout_opts *opts)
{
        ce->ce_flags &= ~CE_MATCHED;
        if (!opts->ignore_skipworktree && ce_skip_worktree(ce))
                return;
        if (opts->source_tree && !(ce->ce_flags & CE_UPDATE))
                /*
                 * "git checkout tree-ish -- path", but this entry
                 * is in the original index but is not in tree-ish
                 * or does not match the pathspec; it will not be
                 * checked out to the working tree.  We will not do
                 * anything to this entry at all.
                 */
                return;
        /*
         * Either this entry came from the tree-ish we are
         * checking the paths out of, or we are checking out
         * of the index.
         *
         * If it comes from the tree-ish, we already know it
         * matches the pathspec and could just stamp
         * CE_MATCHED to it from update_some(). But we still
         * need ps_matched and read_tree (and
         * eventually tree_entry_interesting) cannot fill
         * ps_matched yet. Once it can, we can avoid calling
         * match_pathspec() for _all_ entries when
         * opts->source_tree != NULL.
         */
        if (ce_path_match(&the_index, ce, &opts->pathspec, ps_matched))
                ce->ce_flags |= CE_MATCHED;
}

static void mark_ce_for_checkout_no_overlay(struct cache_entry *ce,
                                            char *ps_matched,
                                            const struct checkout_opts *opts)
{
        ce->ce_flags &= ~CE_MATCHED;
        if (!opts->ignore_skipworktree && ce_skip_worktree(ce))
                return;
        if (ce_path_match(&the_index, ce, &opts->pathspec, ps_matched)) {
                ce->ce_flags |= CE_MATCHED;
                if (opts->source_tree && !(ce->ce_flags & CE_UPDATE))
                        /*
                         * In overlay mode, but the path is not in
                         * tree-ish, which means we should remove it
                         * from the index and the working tree.
                         */
                        ce->ce_flags |= CE_REMOVE | CE_WT_REMOVE;
        }
}

static int checkout_worktree(const struct checkout_opts *opts,
                             const struct branch_info *info)
{
        struct checkout state = CHECKOUT_INIT;
        int nr_checkouts = 0, nr_unmerged = 0;
        int errs = 0;
        int pos;

        state.force = 1;
        state.refresh_cache = 1;
        state.istate = &the_index;

        init_checkout_metadata(&state.meta, info->refname,
                               info->commit ? &info->commit->object.oid : &info->oid,
                               NULL);

        enable_delayed_checkout(&state);
        for (pos = 0; pos < active_nr; pos++) {
                struct cache_entry *ce = active_cache[pos];
                if (ce->ce_flags & CE_MATCHED) {
                        if (!ce_stage(ce)) {
                                errs |= checkout_entry(ce, &state,
                                                       NULL, &nr_checkouts);
                                continue;
                        }
                        if (opts->writeout_stage)
                                errs |= checkout_stage(opts->writeout_stage,
                                                       ce, pos,
                                                       &state,
                                                       &nr_checkouts, opts->overlay_mode);
                        else if (opts->merge)
                                errs |= checkout_merged(pos, &state,
                                                        &nr_unmerged);
                        pos = skip_same_name(ce, pos) - 1;
                }
        }
        remove_marked_cache_entries(&the_index, 1);
        remove_scheduled_dirs();
        errs |= finish_delayed_checkout(&state, &nr_checkouts);

        if (opts->count_checkout_paths) {
                if (nr_unmerged)
                        fprintf_ln(stderr, Q_("Recreated %d merge conflict",
                                              "Recreated %d merge conflicts",
                                              nr_unmerged),
                                   nr_unmerged);
                if (opts->source_tree)
                        fprintf_ln(stderr, Q_("Updated %d path from %s",
                                              "Updated %d paths from %s",
                                              nr_checkouts),
                                   nr_checkouts,
                                   find_unique_abbrev(&opts->source_tree->object.oid,
                                                      DEFAULT_ABBREV));
                else if (!nr_unmerged || nr_checkouts)
                        fprintf_ln(stderr, Q_("Updated %d path from the index",
                                              "Updated %d paths from the index",
                                              nr_checkouts),
                                   nr_checkouts);
        }

        return errs;
}

static int checkout_paths(const struct checkout_opts *opts,
                          const struct branch_info *new_branch_info)
{
        int pos;
        static char *ps_matched;
        struct object_id rev;
        struct commit *head;
        int errs = 0;
        struct lock_file lock_file = LOCK_INIT;
        int checkout_index;

        trace2_cmd_mode(opts->patch_mode ? "patch" : "path");

        if (opts->track != BRANCH_TRACK_UNSPECIFIED)
                die(_("'%s' cannot be used with updating paths"), "--track");

        if (opts->new_branch_log)
                die(_("'%s' cannot be used with updating paths"), "-l");

        if (opts->ignore_unmerged && opts->patch_mode)
                die(_("'%s' cannot be used with updating paths"),
                    opts->ignore_unmerged_opt);

        if (opts->force_detach)
                die(_("'%s' cannot be used with updating paths"), "--detach");

        if (opts->merge && opts->patch_mode)
                die(_("'%s' cannot be used with %s"), "--merge", "--patch");

        if (opts->ignore_unmerged && opts->merge)
                die(_("'%s' cannot be used with %s"),
                    opts->ignore_unmerged_opt, "-m");

        if (opts->new_branch)
                die(_("Cannot update paths and switch to branch '%s' at the same time."),
                    opts->new_branch);

        if (!opts->checkout_worktree && !opts->checkout_index)
                die(_("neither '%s' or '%s' is specified"),
                    "--staged", "--worktree");

        if (!opts->checkout_worktree && !opts->from_treeish)
                die(_("'%s' must be used when '%s' is not specified"),
                    "--worktree", "--source");

        if (opts->checkout_index && !opts->checkout_worktree &&
            opts->writeout_stage)
                die(_("'%s' or '%s' cannot be used with %s"),
                    "--ours", "--theirs", "--staged");

        if (opts->checkout_index && !opts->checkout_worktree &&
            opts->merge)
                die(_("'%s' or '%s' cannot be used with %s"),
                    "--merge", "--conflict", "--staged");

        if (opts->patch_mode) {
                const char *patch_mode;
                const char *rev = new_branch_info->name;
                char rev_oid[GIT_MAX_HEXSZ + 1];

                /*
                 * Since rev can be in the form of `<a>...<b>` (which is not
                 * recognized by diff-index), we will always replace the name
                 * with the hex of the commit (whether it's in `...` form or
                 * not) for the run_add_interactive() machinery to work
                 * properly. However, there is special logic for the HEAD case
                 * so we mustn't replace that.  Also, when we were given a
                 * tree-object, new_branch_info->commit would be NULL, but we
                 * do not have to do any replacement, either.
                 */
                if (rev && new_branch_info->commit && strcmp(rev, "HEAD"))
                        rev = oid_to_hex_r(rev_oid, &new_branch_info->commit->object.oid);

                if (opts->checkout_index && opts->checkout_worktree)
                        patch_mode = "--patch=checkout";
                else if (opts->checkout_index && !opts->checkout_worktree)
                        patch_mode = "--patch=reset";
                else if (!opts->checkout_index && opts->checkout_worktree)
                        patch_mode = "--patch=worktree";
                else
                        BUG("either flag must have been set, worktree=%d, index=%d",
                            opts->checkout_worktree, opts->checkout_index);
                return run_add_interactive(rev, patch_mode, &opts->pathspec);
        }

        repo_hold_locked_index(the_repository, &lock_file, LOCK_DIE_ON_ERROR);
        if (read_cache_preload(&opts->pathspec) < 0)
                return error(_("index file corrupt"));

        if (opts->source_tree)
                read_tree_some(opts->source_tree, &opts->pathspec);

        ps_matched = xcalloc(opts->pathspec.nr, 1);

        /*
         * Make sure all pathspecs participated in locating the paths
         * to be checked out.
         */
        for (pos = 0; pos < active_nr; pos++)
                if (opts->overlay_mode)
                        mark_ce_for_checkout_overlay(active_cache[pos],
                                                     ps_matched,
                                                     opts);
                else
                        mark_ce_for_checkout_no_overlay(active_cache[pos],
                                                        ps_matched,
                                                        opts);

        if (report_path_error(ps_matched, &opts->pathspec)) {
                free(ps_matched);
                return 1;
        }
        free(ps_matched);

        /* "checkout -m path" to recreate conflicted state */
        if (opts->merge)
                unmerge_marked_index(&the_index);

        /* Any unmerged paths? */
        for (pos = 0; pos < active_nr; pos++) {
                const struct cache_entry *ce = active_cache[pos];
                if (ce->ce_flags & CE_MATCHED) {
                        if (!ce_stage(ce))
                                continue;
                        if (opts->ignore_unmerged) {
                                if (!opts->quiet)
                                        warning(_("path '%s' is unmerged"), ce->name);
                        } else if (opts->writeout_stage) {
                                errs |= check_stage(opts->writeout_stage, ce, pos, opts->overlay_mode);
                        } else if (opts->merge) {
                                errs |= check_stages((1<<2) | (1<<3), ce, pos);
                        } else {
                                errs = 1;
                                error(_("path '%s' is unmerged"), ce->name);
                        }
                        pos = skip_same_name(ce, pos) - 1;
                }
        }
        if (errs)
                return 1;

        /* Now we are committed to check them out */
        if (opts->checkout_worktree)
                errs |= checkout_worktree(opts, new_branch_info);
        else
                remove_marked_cache_entries(&the_index, 1);

        /*
         * Allow updating the index when checking out from the index.
         * This is to save new stat info.
         */
        if (opts->checkout_worktree && !opts->checkout_index && !opts->source_tree)
                checkout_index = 1;
        else
                checkout_index = opts->checkout_index;

        if (checkout_index) {
                if (write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
                        die(_("unable to write new index file"));
        } else {
                /*
                 * NEEDSWORK: if --worktree is not specified, we
                 * should save stat info of checked out files in the
                 * index to avoid the next (potentially costly)
                 * refresh. But it's a bit tricker to do...
                 */
                rollback_lock_file(&lock_file);
        }

        read_ref_full("HEAD", 0, &rev, NULL);
        head = lookup_commit_reference_gently(the_repository, &rev, 1);

        errs |= post_checkout_hook(head, head, 0);
        return errs;
}

static void show_local_changes(struct object *head,
                               const struct diff_options *opts)
{
        struct rev_info rev;
        /* I think we want full paths, even if we're in a subdirectory. */
        repo_init_revisions(the_repository, &rev, NULL);
        rev.diffopt.flags = opts->flags;
        rev.diffopt.output_format |= DIFF_FORMAT_NAME_STATUS;
        diff_setup_done(&rev.diffopt);
        add_pending_object(&rev, head, NULL);
        run_diff_index(&rev, 0);
}

static void describe_detached_head(const char *msg, struct commit *commit)
{
        struct strbuf sb = STRBUF_INIT;

        if (!parse_commit(commit))
                pp_commit_easy(CMIT_FMT_ONELINE, commit, &sb);
        if (print_sha1_ellipsis()) {
                fprintf(stderr, "%s %s... %s\n", msg,
                        find_unique_abbrev(&commit->object.oid, DEFAULT_ABBREV), sb.buf);
        } else {
                fprintf(stderr, "%s %s %s\n", msg,
                        find_unique_abbrev(&commit->object.oid, DEFAULT_ABBREV), sb.buf);
        }
        strbuf_release(&sb);
}

static int reset_tree(struct tree *tree, const struct checkout_opts *o,
                      int worktree, int *writeout_error,
                      struct branch_info *info)
{
        struct unpack_trees_options opts;
        struct tree_desc tree_desc;

        memset(&opts, 0, sizeof(opts));
        opts.head_idx = -1;
        opts.update = worktree;
        opts.skip_unmerged = !worktree;
        opts.reset = 1;
        opts.merge = 1;
        opts.fn = oneway_merge;
        opts.verbose_update = o->show_progress;
        opts.src_index = &the_index;
        opts.dst_index = &the_index;
        init_checkout_metadata(&opts.meta, info->refname,
                               info->commit ? &info->commit->object.oid : &null_oid,
                               NULL);
        parse_tree(tree);
        init_tree_desc(&tree_desc, tree->buffer, tree->size);
        switch (unpack_trees(1, &tree_desc, &opts)) {
        case -2:
                *writeout_error = 1;
                /*
                 * We return 0 nevertheless, as the index is all right
                 * and more importantly we have made best efforts to
                 * update paths in the work tree, and we cannot revert
                 * them.
                 */
                /* fallthrough */
        case 0:
                return 0;
        default:
                return 128;
        }
}

static void setup_branch_path(struct branch_info *branch)
{
        struct strbuf buf = STRBUF_INIT;

        /*
         * If this is a ref, resolve it; otherwise, look up the OID for our
         * expression.  Failure here is okay.
         */
        if (!dwim_ref(branch->name, strlen(branch->name), &branch->oid, &branch->refname, 0))
                repo_get_oid_committish(the_repository, branch->name, &branch->oid);

        strbuf_branchname(&buf, branch->name, INTERPRET_BRANCH_LOCAL);
        if (strcmp(buf.buf, branch->name))
                branch->name = xstrdup(buf.buf);
        strbuf_splice(&buf, 0, 0, "refs/heads/", 11);
        branch->path = strbuf_detach(&buf, NULL);
}

static int merge_working_tree(const struct checkout_opts *opts,
                              struct branch_info *old_branch_info,
                              struct branch_info *new_branch_info,
                              int *writeout_error)
{
        int ret;
        struct lock_file lock_file = LOCK_INIT;
        struct tree *new_tree;

        hold_locked_index(&lock_file, LOCK_DIE_ON_ERROR);
        if (read_cache_preload(NULL) < 0)
                return error(_("index file corrupt"));

        resolve_undo_clear();
        if (opts->new_orphan_branch && opts->orphan_from_empty_tree) {
                if (new_branch_info->commit)
                        BUG("'switch --orphan' should never accept a commit as starting point");
                new_tree = parse_tree_indirect(the_hash_algo->empty_tree);
        } else
                new_tree = get_commit_tree(new_branch_info->commit);
        if (opts->discard_changes) {
                ret = reset_tree(new_tree, opts, 1, writeout_error, new_branch_info);
                if (ret)
                        return ret;
        } else {
                struct tree_desc trees[2];
                struct tree *tree;
                struct unpack_trees_options topts;

                memset(&topts, 0, sizeof(topts));
                topts.head_idx = -1;
                topts.src_index = &the_index;
                topts.dst_index = &the_index;

                setup_unpack_trees_porcelain(&topts, "checkout");

                refresh_cache(REFRESH_QUIET);

                if (unmerged_cache()) {
                        error(_("you need to resolve your current index first"));
                        return 1;
                }

                /* 2-way merge to the new branch */
                topts.initial_checkout = is_cache_unborn();
                topts.update = 1;
                topts.merge = 1;
                topts.quiet = opts->merge && old_branch_info->commit;
                topts.verbose_update = opts->show_progress;
                topts.fn = twoway_merge;
                init_checkout_metadata(&topts.meta, new_branch_info->refname,
                                       new_branch_info->commit ?
                                       &new_branch_info->commit->object.oid :
                                       &new_branch_info->oid, NULL);
                if (opts->overwrite_ignore) {
                        topts.dir = xcalloc(1, sizeof(*topts.dir));
                        topts.dir->flags |= DIR_SHOW_IGNORED;
                        setup_standard_excludes(topts.dir);
                }
                tree = parse_tree_indirect(old_branch_info->commit ?
                                           &old_branch_info->commit->object.oid :
                                           the_hash_algo->empty_tree);
                init_tree_desc(&trees[0], tree->buffer, tree->size);
                parse_tree(new_tree);
                tree = new_tree;
                init_tree_desc(&trees[1], tree->buffer, tree->size);

                ret = unpack_trees(2, trees, &topts);
                clear_unpack_trees_porcelain(&topts);
                if (ret == -1) {
                        /*
                         * Unpack couldn't do a trivial merge; either
                         * give up or do a real merge, depending on
                         * whether the merge flag was used.
                         */
                        struct tree *work;
                        struct tree *old_tree;
                        struct merge_options o;
                        struct strbuf sb = STRBUF_INIT;
                        struct strbuf old_commit_shortname = STRBUF_INIT;

                        if (!opts->merge)
                                return 1;

                        /*
                         * Without old_branch_info->commit, the below is the same as
                         * the two-tree unpack we already tried and failed.
                         */
                        if (!old_branch_info->commit)
                                return 1;
                        old_tree = get_commit_tree(old_branch_info->commit);

                        if (repo_index_has_changes(the_repository, old_tree, &sb))
                                die(_("cannot continue with staged changes in "
                                      "the following files:\n%s"), sb.buf);
                        strbuf_release(&sb);

                        /* Do more real merge */

                        /*
                         * We update the index fully, then write the
                         * tree from the index, then merge the new
                         * branch with the current tree, with the old
                         * branch as the base. Then we reset the index
                         * (but not the working tree) to the new
                         * branch, leaving the working tree as the
                         * merged version, but skipping unmerged
                         * entries in the index.
                         */

                        add_files_to_cache(NULL, NULL, 0);
                        init_merge_options(&o, the_repository);
                        o.verbosity = 0;
                        work = write_in_core_index_as_tree(the_repository);

                        ret = reset_tree(new_tree,
                                         opts, 1,
                                         writeout_error, new_branch_info);
                        if (ret)
                                return ret;
                        o.ancestor = old_branch_info->name;
                        if (old_branch_info->name == NULL) {
                                strbuf_add_unique_abbrev(&old_commit_shortname,
                                                         &old_branch_info->commit->object.oid,
                                                         DEFAULT_ABBREV);
                                o.ancestor = old_commit_shortname.buf;
                        }
                        o.branch1 = new_branch_info->name;
                        o.branch2 = "local";
                        ret = merge_trees(&o,
                                          new_tree,
                                          work,
                                          old_tree);
                        if (ret < 0)
                                exit(128);
                        ret = reset_tree(new_tree,
                                         opts, 0,
                                         writeout_error, new_branch_info);
                        strbuf_release(&o.obuf);
                        strbuf_release(&old_commit_shortname);
                        if (ret)
                                return ret;
                }
        }

        if (!cache_tree_fully_valid(active_cache_tree))
                cache_tree_update(&the_index, WRITE_TREE_SILENT | WRITE_TREE_REPAIR);

        if (write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
                die(_("unable to write new index file"));

        if (!opts->discard_changes && !opts->quiet && new_branch_info->commit)
                show_local_changes(&new_branch_info->commit->object, &opts->diff_options);

        return 0;
}

static void report_tracking(struct branch_info *new_branch_info)
{
        struct strbuf sb = STRBUF_INIT;
        struct branch *branch = branch_get(new_branch_info->name);

        if (!format_tracking_info(branch, &sb, AHEAD_BEHIND_FULL))
                return;
        fputs(sb.buf, stdout);
        strbuf_release(&sb);
}

static void update_refs_for_switch(const struct checkout_opts *opts,
                                   struct branch_info *old_branch_info,
                                   struct branch_info *new_branch_info)
{
        struct strbuf msg = STRBUF_INIT;
        const char *old_desc, *reflog_msg;
        if (opts->new_branch) {
                if (opts->new_orphan_branch) {
                        char *refname;

                        refname = mkpathdup("refs/heads/%s", opts->new_orphan_branch);
                        if (opts->new_branch_log &&
                            !should_autocreate_reflog(refname)) {
                                int ret;
                                struct strbuf err = STRBUF_INIT;

                                ret = safe_create_reflog(refname, 1, &err);
                                if (ret) {
                                        fprintf(stderr, _("Can not do reflog for '%s': %s\n"),
                                                opts->new_orphan_branch, err.buf);
                                        strbuf_release(&err);
                                        free(refname);
                                        return;
                                }
                                strbuf_release(&err);
                        }
                        free(refname);
                }
                else
                        create_branch(the_repository,
                                      opts->new_branch, new_branch_info->name,
                                      opts->new_branch_force ? 1 : 0,
                                      opts->new_branch_force ? 1 : 0,
                                      opts->new_branch_log,
                                      opts->quiet,
                                      opts->track);
                new_branch_info->name = opts->new_branch;
                setup_branch_path(new_branch_info);
        }

        old_desc = old_branch_info->name;
        if (!old_desc && old_branch_info->commit)
                old_desc = oid_to_hex(&old_branch_info->commit->object.oid);

        reflog_msg = getenv("GIT_REFLOG_ACTION");
        if (!reflog_msg)
                strbuf_addf(&msg, "checkout: moving from %s to %s",
                        old_desc ? old_desc : "(invalid)", new_branch_info->name);
        else
                strbuf_insertstr(&msg, 0, reflog_msg);

        if (!strcmp(new_branch_info->name, "HEAD") && !new_branch_info->path && !opts->force_detach) {
                /* Nothing to do. */
        } else if (opts->force_detach || !new_branch_info->path) {      /* No longer on any branch. */
                update_ref(msg.buf, "HEAD", &new_branch_info->commit->object.oid, NULL,
                           REF_NO_DEREF, UPDATE_REFS_DIE_ON_ERR);
                if (!opts->quiet) {
                        if (old_branch_info->path &&
                            advice_detached_head && !opts->force_detach)
                                detach_advice(new_branch_info->name);
                        describe_detached_head(_("HEAD is now at"), new_branch_info->commit);
                }
        } else if (new_branch_info->path) {     /* Switch branches. */
                if (create_symref("HEAD", new_branch_info->path, msg.buf) < 0)
                        die(_("unable to update HEAD"));
                if (!opts->quiet) {
                        if (old_branch_info->path && !strcmp(new_branch_info->path, old_branch_info->path)) {
                                if (opts->new_branch_force)
                                        fprintf(stderr, _("Reset branch '%s'\n"),
                                                new_branch_info->name);
                                else
                                        fprintf(stderr, _("Already on '%s'\n"),
                                                new_branch_info->name);
                        } else if (opts->new_branch) {
                                if (opts->branch_exists)
                                        fprintf(stderr, _("Switched to and reset branch '%s'\n"), new_branch_info->name);
                                else
                                        fprintf(stderr, _("Switched to a new branch '%s'\n"), new_branch_info->name);
                        } else {
                                fprintf(stderr, _("Switched to branch '%s'\n"),
                                        new_branch_info->name);
                        }
                }
                if (old_branch_info->path && old_branch_info->name) {
                        if (!ref_exists(old_branch_info->path) && reflog_exists(old_branch_info->path))
                                delete_reflog(old_branch_info->path);
                }
        }
        remove_branch_state(the_repository, !opts->quiet);
        strbuf_release(&msg);
        if (!opts->quiet &&
            (new_branch_info->path || (!opts->force_detach && !strcmp(new_branch_info->name, "HEAD"))))
                report_tracking(new_branch_info);
}

static int add_pending_uninteresting_ref(const char *refname,
                                         const struct object_id *oid,
                                         int flags, void *cb_data)
{
        add_pending_oid(cb_data, refname, oid, UNINTERESTING);
        return 0;
}

static void describe_one_orphan(struct strbuf *sb, struct commit *commit)
{
        strbuf_addstr(sb, "  ");
        strbuf_add_unique_abbrev(sb, &commit->object.oid, DEFAULT_ABBREV);
        strbuf_addch(sb, ' ');
        if (!parse_commit(commit))
                pp_commit_easy(CMIT_FMT_ONELINE, commit, sb);
        strbuf_addch(sb, '\n');
}

#define ORPHAN_CUTOFF 4
static void suggest_reattach(struct commit *commit, struct rev_info *revs)
{
        struct commit *c, *last = NULL;
        struct strbuf sb = STRBUF_INIT;
        int lost = 0;
        while ((c = get_revision(revs)) != NULL) {
                if (lost < ORPHAN_CUTOFF)
                        describe_one_orphan(&sb, c);
                last = c;
                lost++;
        }
        if (ORPHAN_CUTOFF < lost) {
                int more = lost - ORPHAN_CUTOFF;
                if (more == 1)
                        describe_one_orphan(&sb, last);
                else
                        strbuf_addf(&sb, _(" ... and %d more.\n"), more);
        }

        fprintf(stderr,
                Q_(
                /* The singular version */
                "Warning: you are leaving %d commit behind, "
                "not connected to\n"
                "any of your branches:\n\n"
                "%s\n",
                /* The plural version */
                "Warning: you are leaving %d commits behind, "
                "not connected to\n"
                "any of your branches:\n\n"
                "%s\n",
                /* Give ngettext() the count */
                lost),
                lost,
                sb.buf);
        strbuf_release(&sb);

        if (advice_detached_head)
                fprintf(stderr,
                        Q_(
                        /* The singular version */
                        "If you want to keep it by creating a new branch, "
                        "this may be a good time\nto do so with:\n\n"
                        " git branch <new-branch-name> %s\n\n",
                        /* The plural version */
                        "If you want to keep them by creating a new branch, "
                        "this may be a good time\nto do so with:\n\n"
                        " git branch <new-branch-name> %s\n\n",
                        /* Give ngettext() the count */
                        lost),
                        find_unique_abbrev(&commit->object.oid, DEFAULT_ABBREV));
}

/*
 * We are about to leave commit that was at the tip of a detached
 * HEAD.  If it is not reachable from any ref, this is the last chance
 * for the user to do so without resorting to reflog.
 */
static void orphaned_commit_warning(struct commit *old_commit, struct commit *new_commit)
{
        struct rev_info revs;
        struct object *object = &old_commit->object;

        repo_init_revisions(the_repository, &revs, NULL);
        setup_revisions(0, NULL, &revs, NULL);

        object->flags &= ~UNINTERESTING;
        add_pending_object(&revs, object, oid_to_hex(&object->oid));

        for_each_ref(add_pending_uninteresting_ref, &revs);
        if (new_commit)
                add_pending_oid(&revs, "HEAD",
                                &new_commit->object.oid,
                                UNINTERESTING);

        if (prepare_revision_walk(&revs))
                die(_("internal error in revision walk"));
        if (!(old_commit->object.flags & UNINTERESTING))
                suggest_reattach(old_commit, &revs);
        else
                describe_detached_head(_("Previous HEAD position was"), old_commit);

        /* Clean up objects used, as they will be reused. */
        repo_clear_commit_marks(the_repository, ALL_REV_FLAGS);
}

static int switch_branches(const struct checkout_opts *opts,
                           struct branch_info *new_branch_info)
{
        int ret = 0;
        struct branch_info old_branch_info;
        void *path_to_free;
        struct object_id rev;
        int flag, writeout_error = 0;
        int do_merge = 1;

        trace2_cmd_mode("branch");

        memset(&old_branch_info, 0, sizeof(old_branch_info));
        old_branch_info.path = path_to_free = resolve_refdup("HEAD", 0, &rev, &flag);
        if (old_branch_info.path)
                old_branch_info.commit = lookup_commit_reference_gently(the_repository, &rev, 1);
        if (!(flag & REF_ISSYMREF))
                old_branch_info.path = NULL;

        if (old_branch_info.path)
                skip_prefix(old_branch_info.path, "refs/heads/", &old_branch_info.name);

        if (opts->new_orphan_branch && opts->orphan_from_empty_tree) {
                if (new_branch_info->name)
                        BUG("'switch --orphan' should never accept a commit as starting point");
                new_branch_info->commit = NULL;
                new_branch_info->name = "(empty)";
                do_merge = 1;
        }

        if (!new_branch_info->name) {
                new_branch_info->name = "HEAD";
                new_branch_info->commit = old_branch_info.commit;
                if (!new_branch_info->commit)
                        die(_("You are on a branch yet to be born"));
                parse_commit_or_die(new_branch_info->commit);

                if (opts->only_merge_on_switching_branches)
                        do_merge = 0;
        }

        if (do_merge) {
                ret = merge_working_tree(opts, &old_branch_info, new_branch_info, &writeout_error);
                if (ret) {
                        free(path_to_free);
                        return ret;
                }
        }

        if (!opts->quiet && !old_branch_info.path && old_branch_info.commit && new_branch_info->commit != old_branch_info.commit)
                orphaned_commit_warning(old_branch_info.commit, new_branch_info->commit);

        update_refs_for_switch(opts, &old_branch_info, new_branch_info);

        ret = post_checkout_hook(old_branch_info.commit, new_branch_info->commit, 1);
        free(path_to_free);
        return ret || writeout_error;
}

static int git_checkout_config(const char *var, const char *value, void *cb)
{
        struct checkout_opts *opts = cb;

        if (!strcmp(var, "diff.ignoresubmodules")) {
                handle_ignore_submodules_arg(&opts->diff_options, value);
                return 0;
        }
        if (!strcmp(var, "checkout.guess")) {
                opts->dwim_new_local_branch = git_config_bool(var, value);
                return 0;
        }

        if (starts_with(var, "submodule."))
                return git_default_submodule_config(var, value, NULL);

        return git_xmerge_config(var, value, NULL);
}

static void setup_new_branch_info_and_source_tree(
        struct branch_info *new_branch_info,
        struct checkout_opts *opts,
        struct object_id *rev,
        const char *arg)
{
        struct tree **source_tree = &opts->source_tree;
        struct object_id branch_rev;

        new_branch_info->name = arg;
        setup_branch_path(new_branch_info);

        if (!check_refname_format(new_branch_info->path, 0) &&
            !read_ref(new_branch_info->path, &branch_rev))
                oidcpy(rev, &branch_rev);
        else {
                free((char *)new_branch_info->path);
                new_branch_info->path = NULL; /* not an existing branch */
        }

        new_branch_info->commit = lookup_commit_reference_gently(the_repository, rev, 1);
        if (!new_branch_info->commit) {
                /* not a commit */
                *source_tree = parse_tree_indirect(rev);
        } else {
                parse_commit_or_die(new_branch_info->commit);
                *source_tree = get_commit_tree(new_branch_info->commit);
        }
}

static const char *parse_remote_branch(const char *arg,
                                       struct object_id *rev,
                                       int could_be_checkout_paths)
{
        int num_matches = 0;
        const char *remote = unique_tracking_name(arg, rev, &num_matches);

        if (remote && could_be_checkout_paths) {
                die(_("'%s' could be both a local file and a tracking branch.\n"
                        "Please use -- (and optionally --no-guess) to disambiguate"),
                    arg);
        }

        if (!remote && num_matches > 1) {
            if (advice_checkout_ambiguous_remote_branch_name) {
                    advise(_("If you meant to check out a remote tracking branch on, e.g. 'origin',\n"
                             "you can do so by fully qualifying the name with the --track option:\n"
                             "\n"
                             "    git checkout --track origin/<name>\n"
                             "\n"
                             "If you'd like to always have checkouts of an ambiguous <name> prefer\n"
                             "one remote, e.g. the 'origin' remote, consider setting\n"
                             "checkout.defaultRemote=origin in your config."));
            }

            die(_("'%s' matched multiple (%d) remote tracking branches"),
                arg, num_matches);
        }

        return remote;
}

static int parse_branchname_arg(int argc, const char **argv,
                                int dwim_new_local_branch_ok,
                                struct branch_info *new_branch_info,
                                struct checkout_opts *opts,
                                struct object_id *rev)
{
        const char **new_branch = &opts->new_branch;
        int argcount = 0;
        const char *arg;
        int dash_dash_pos;
        int has_dash_dash = 0;
        int i;

        /*
         * case 1: git checkout <ref> -- [<paths>]
         *
         *   <ref> must be a valid tree, everything after the '--' must be
         *   a path.
         *
         * case 2: git checkout -- [<paths>]
         *
         *   everything after the '--' must be paths.
         *
         * case 3: git checkout <something> [--]
         *
         *   (a) If <something> is a commit, that is to
         *       switch to the branch or detach HEAD at it.  As a special case,
         *       if <something> is A...B (missing A or B means HEAD but you can
         *       omit at most one side), and if there is a unique merge base
         *       between A and B, A...B names that merge base.
         *
         *   (b) If <something> is _not_ a commit, either "--" is present
         *       or <something> is not a path, no -t or -b was given, and
         *       and there is a tracking branch whose name is <something>
         *       in one and only one remote (or if the branch exists on the
         *       remote named in checkout.defaultRemote), then this is a
         *       short-hand to fork local <something> from that
         *       remote-tracking branch.
         *
         *   (c) Otherwise, if "--" is present, treat it like case (1).
         *
         *   (d) Otherwise :
         *       - if it's a reference, treat it like case (1)
         *       - else if it's a path, treat it like case (2)
         *       - else: fail.
         *
         * case 4: git checkout <something> <paths>
         *
         *   The first argument must not be ambiguous.
         *   - If it's *only* a reference, treat it like case (1).
         *   - If it's only a path, treat it like case (2).
         *   - else: fail.
         *
         */
        if (!argc)
                return 0;

        if (!opts->accept_pathspec) {
                if (argc > 1)
                        die(_("only one reference expected"));
                has_dash_dash = 1; /* helps disambiguate */
        }

        arg = argv[0];
        dash_dash_pos = -1;
        for (i = 0; i < argc; i++) {
                if (opts->accept_pathspec && !strcmp(argv[i], "--")) {
                        dash_dash_pos = i;
                        break;
                }
        }
        if (dash_dash_pos == 0)
                return 1; /* case (2) */
        else if (dash_dash_pos == 1)
                has_dash_dash = 1; /* case (3) or (1) */
        else if (dash_dash_pos >= 2)
                die(_("only one reference expected, %d given."), dash_dash_pos);
        opts->count_checkout_paths = !opts->quiet && !has_dash_dash;

        if (!strcmp(arg, "-"))
                arg = "@{-1}";

        if (get_oid_mb(arg, rev)) {
                /*
                 * Either case (3) or (4), with <something> not being
                 * a commit, or an attempt to use case (1) with an
                 * invalid ref.
                 *
                 * It's likely an error, but we need to find out if
                 * we should auto-create the branch, case (3).(b).
                 */
                int recover_with_dwim = dwim_new_local_branch_ok;

                int could_be_checkout_paths = !has_dash_dash &&
                        check_filename(opts->prefix, arg);

                if (!has_dash_dash && !no_wildcard(arg))
                        recover_with_dwim = 0;

                /*
                 * Accept "git checkout foo", "git checkout foo --"
                 * and "git switch foo" as candidates for dwim.
                 */
                if (!(argc == 1 && !has_dash_dash) &&
                    !(argc == 2 && has_dash_dash) &&
                    opts->accept_pathspec)
                        recover_with_dwim = 0;

                if (recover_with_dwim) {
                        const char *remote = parse_remote_branch(arg, rev,
                                                                 could_be_checkout_paths);
                        if (remote) {
                                *new_branch = arg;
                                arg = remote;
                                /* DWIMmed to create local branch, case (3).(b) */
                        } else {
                                recover_with_dwim = 0;
                        }
                }

                if (!recover_with_dwim) {
                        if (has_dash_dash)
                                die(_("invalid reference: %s"), arg);
                        return argcount;
                }
        }

        /* we can't end up being in (2) anymore, eat the argument */
        argcount++;
        argv++;
        argc--;

        setup_new_branch_info_and_source_tree(new_branch_info, opts, rev, arg);

        if (!opts->source_tree)                   /* case (1): want a tree */
                die(_("reference is not a tree: %s"), arg);

        if (!has_dash_dash) {   /* case (3).(d) -> (1) */
                /*
                 * Do not complain the most common case
                 *      git checkout branch
                 * even if there happen to be a file called 'branch';
                 * it would be extremely annoying.
                 */
                if (argc)
                        verify_non_filename(opts->prefix, arg);
        } else if (opts->accept_pathspec) {
                argcount++;
                argv++;
                argc--;
        }

        return argcount;
}

static int switch_unborn_to_new_branch(const struct checkout_opts *opts)
{
        int status;
        struct strbuf branch_ref = STRBUF_INIT;

        trace2_cmd_mode("unborn");

        if (!opts->new_branch)
                die(_("You are on a branch yet to be born"));
        strbuf_addf(&branch_ref, "refs/heads/%s", opts->new_branch);
        status = create_symref("HEAD", branch_ref.buf, "checkout -b");
        strbuf_release(&branch_ref);
        if (!opts->quiet)
                fprintf(stderr, _("Switched to a new branch '%s'\n"),
                        opts->new_branch);
        return status;
}

static void die_expecting_a_branch(const struct branch_info *branch_info)
{
        struct object_id oid;
        char *to_free;

        if (dwim_ref(branch_info->name, strlen(branch_info->name), &oid, &to_free, 0) == 1) {
                const char *ref = to_free;

                if (skip_prefix(ref, "refs/tags/", &ref))
                        die(_("a branch is expected, got tag '%s'"), ref);
                if (skip_prefix(ref, "refs/remotes/", &ref))
                        die(_("a branch is expected, got remote branch '%s'"), ref);
                die(_("a branch is expected, got '%s'"), ref);
        }
        if (branch_info->commit)
                die(_("a branch is expected, got commit '%s'"), branch_info->name);
        /*
         * This case should never happen because we already die() on
         * non-commit, but just in case.
         */
        die(_("a branch is expected, got '%s'"), branch_info->name);
}

static void die_if_some_operation_in_progress(void)
{
        struct wt_status_state state;

        memset(&state, 0, sizeof(state));
        wt_status_get_state(the_repository, &state, 0);

        if (state.merge_in_progress)
                die(_("cannot switch branch while merging\n"
                      "Consider \"git merge --quit\" "
                      "or \"git worktree add\"."));
        if (state.am_in_progress)
                die(_("cannot switch branch in the middle of an am session\n"
                      "Consider \"git am --quit\" "
                      "or \"git worktree add\"."));
        if (state.rebase_interactive_in_progress || state.rebase_in_progress)
                die(_("cannot switch branch while rebasing\n"
                      "Consider \"git rebase --quit\" "
                      "or \"git worktree add\"."));
        if (state.cherry_pick_in_progress)
                die(_("cannot switch branch while cherry-picking\n"
                      "Consider \"git cherry-pick --quit\" "
                      "or \"git worktree add\"."));
        if (state.revert_in_progress)
                die(_("cannot switch branch while reverting\n"
                      "Consider \"git revert --quit\" "
                      "or \"git worktree add\"."));
        if (state.bisect_in_progress)
                warning(_("you are switching branch while bisecting"));
}

static int checkout_branch(struct checkout_opts *opts,
                           struct branch_info *new_branch_info)
{
        if (opts->pathspec.nr)
                die(_("paths cannot be used with switching branches"));

        if (opts->patch_mode)
                die(_("'%s' cannot be used with switching branches"),
                    "--patch");

        if (opts->overlay_mode != -1)
                die(_("'%s' cannot be used with switching branches"),
                    "--[no]-overlay");

        if (opts->writeout_stage)
                die(_("'%s' cannot be used with switching branches"),
                    "--ours/--theirs");

        if (opts->force && opts->merge)
                die(_("'%s' cannot be used with '%s'"), "-f", "-m");

        if (opts->discard_changes && opts->merge)
                die(_("'%s' cannot be used with '%s'"), "--discard-changes", "--merge");

        if (opts->force_detach && opts->new_branch)
                die(_("'%s' cannot be used with '%s'"),
                    "--detach", "-b/-B/--orphan");

        if (opts->new_orphan_branch) {
                if (opts->track != BRANCH_TRACK_UNSPECIFIED)
                        die(_("'%s' cannot be used with '%s'"), "--orphan", "-t");
                if (opts->orphan_from_empty_tree && new_branch_info->name)
                        die(_("'%s' cannot take <start-point>"), "--orphan");
        } else if (opts->force_detach) {
                if (opts->track != BRANCH_TRACK_UNSPECIFIED)
                        die(_("'%s' cannot be used with '%s'"), "--detach", "-t");
        } else if (opts->track == BRANCH_TRACK_UNSPECIFIED)
                opts->track = git_branch_track;

        if (new_branch_info->name && !new_branch_info->commit)
                die(_("Cannot switch branch to a non-commit '%s'"),
                    new_branch_info->name);

        if (!opts->switch_branch_doing_nothing_is_ok &&
            !new_branch_info->name &&
            !opts->new_branch &&
            !opts->force_detach)
                die(_("missing branch or commit argument"));

        if (!opts->implicit_detach &&
            !opts->force_detach &&
            !opts->new_branch &&
            !opts->new_branch_force &&
            new_branch_info->name &&
            !new_branch_info->path)
                die_expecting_a_branch(new_branch_info);

        if (!opts->can_switch_when_in_progress)
                die_if_some_operation_in_progress();

        if (new_branch_info->path && !opts->force_detach && !opts->new_branch &&
            !opts->ignore_other_worktrees) {
                int flag;
                char *head_ref = resolve_refdup("HEAD", 0, NULL, &flag);
                if (head_ref &&
                    (!(flag & REF_ISSYMREF) || strcmp(head_ref, new_branch_info->path)))
                        die_if_checked_out(new_branch_info->path, 1);
                free(head_ref);
        }

        if (!new_branch_info->commit && opts->new_branch) {
                struct object_id rev;
                int flag;

                if (!read_ref_full("HEAD", 0, &rev, &flag) &&
                    (flag & REF_ISSYMREF) && is_null_oid(&rev))
                        return switch_unborn_to_new_branch(opts);
        }
        return switch_branches(opts, new_branch_info);
}

static struct option *add_common_options(struct checkout_opts *opts,
                                         struct option *prevopts)
{
        struct option options[] = {
                OPT__QUIET(&opts->quiet, N_("suppress progress reporting")),
                OPT_CALLBACK_F(0, "recurse-submodules", NULL,
                            "checkout", "control recursive updating of submodules",
                            PARSE_OPT_OPTARG, option_parse_recurse_submodules_worktree_updater),
                OPT_BOOL(0, "progress", &opts->show_progress, N_("force progress reporting")),
                OPT_BOOL('m', "merge", &opts->merge, N_("perform a 3-way merge with the new branch")),
                OPT_STRING(0, "conflict", &opts->conflict_style, N_("style"),
                           N_("conflict style (merge or diff3)")),
                OPT_END()
        };
        struct option *newopts = parse_options_concat(prevopts, options);
        free(prevopts);
        return newopts;
}

static struct option *add_common_switch_branch_options(
        struct checkout_opts *opts, struct option *prevopts)
{
        struct option options[] = {
                OPT_BOOL('d', "detach", &opts->force_detach, N_("detach HEAD at named commit")),
                OPT_SET_INT('t', "track",  &opts->track, N_("set upstream info for new branch"),
                        BRANCH_TRACK_EXPLICIT),
                OPT__FORCE(&opts->force, N_("force checkout (throw away local modifications)"),
                           PARSE_OPT_NOCOMPLETE),
                OPT_STRING(0, "orphan", &opts->new_orphan_branch, N_("new-branch"), N_("new unparented branch")),
                OPT_BOOL_F(0, "overwrite-ignore", &opts->overwrite_ignore,
                           N_("update ignored files (default)"),
                           PARSE_OPT_NOCOMPLETE),
                OPT_BOOL(0, "ignore-other-worktrees", &opts->ignore_other_worktrees,
                         N_("do not check if another worktree is holding the given ref")),
                OPT_END()
        };
        struct option *newopts = parse_options_concat(prevopts, options);
        free(prevopts);
        return newopts;
}

static struct option *add_checkout_path_options(struct checkout_opts *opts,
                                                struct option *prevopts)
{
        struct option options[] = {
                OPT_SET_INT_F('2', "ours", &opts->writeout_stage,
                              N_("checkout our version for unmerged files"),
                              2, PARSE_OPT_NONEG),
                OPT_SET_INT_F('3', "theirs", &opts->writeout_stage,
                              N_("checkout their version for unmerged files"),
                              3, PARSE_OPT_NONEG),
                OPT_BOOL('p', "patch", &opts->patch_mode, N_("select hunks interactively")),
                OPT_BOOL(0, "ignore-skip-worktree-bits", &opts->ignore_skipworktree,
                         N_("do not limit pathspecs to sparse entries only")),
                OPT_PATHSPEC_FROM_FILE(&opts->pathspec_from_file),
                OPT_PATHSPEC_FILE_NUL(&opts->pathspec_file_nul),
                OPT_END()
        };
        struct option *newopts = parse_options_concat(prevopts, options);
        free(prevopts);
        return newopts;
}

/* create-branch option (either b or c) */
static char cb_option = 'b';

static int checkout_main(int argc, const char **argv, const char *prefix,
                         struct checkout_opts *opts, struct option *options,
                         const char * const usagestr[])
{
        struct branch_info new_branch_info;
        int parseopt_flags = 0;

        memset(&new_branch_info, 0, sizeof(new_branch_info));
        opts->overwrite_ignore = 1;
        opts->prefix = prefix;
        opts->show_progress = -1;

        git_config(git_checkout_config, opts);

        opts->track = BRANCH_TRACK_UNSPECIFIED;

        if (!opts->accept_pathspec && !opts->accept_ref)
                BUG("make up your mind, you need to take _something_");
        if (opts->accept_pathspec && opts->accept_ref)
                parseopt_flags = PARSE_OPT_KEEP_DASHDASH;

        argc = parse_options(argc, argv, prefix, options,
                             usagestr, parseopt_flags);

        if (opts->show_progress < 0) {
                if (opts->quiet)
                        opts->show_progress = 0;
                else
                        opts->show_progress = isatty(2);
        }

        if (opts->conflict_style) {
                opts->merge = 1; /* implied */
                git_xmerge_config("merge.conflictstyle", opts->conflict_style, NULL);
        }
        if (opts->force) {
                opts->discard_changes = 1;
                opts->ignore_unmerged_opt = "--force";
                opts->ignore_unmerged = 1;
        }

        if ((!!opts->new_branch + !!opts->new_branch_force + !!opts->new_orphan_branch) > 1)
                die(_("-%c, -%c and --orphan are mutually exclusive"),
                                cb_option, toupper(cb_option));

        if (opts->overlay_mode == 1 && opts->patch_mode)
                die(_("-p and --overlay are mutually exclusive"));

        if (opts->checkout_index >= 0 || opts->checkout_worktree >= 0) {
                if (opts->checkout_index < 0)
                        opts->checkout_index = 0;
                if (opts->checkout_worktree < 0)
                        opts->checkout_worktree = 0;
        } else {
                if (opts->checkout_index < 0)
                        opts->checkout_index = -opts->checkout_index - 1;
                if (opts->checkout_worktree < 0)
                        opts->checkout_worktree = -opts->checkout_worktree - 1;
        }
        if (opts->checkout_index < 0 || opts->checkout_worktree < 0)
                BUG("these flags should be non-negative by now");
        /*
         * convenient shortcut: "git restore --staged [--worktree]" equals
         * "git restore --staged [--worktree] --source HEAD"
         */
        if (!opts->from_treeish && opts->checkout_index)
                opts->from_treeish = "HEAD";

        /*
         * From here on, new_branch will contain the branch to be checked out,
         * and new_branch_force and new_orphan_branch will tell us which one of
         * -b/-B/-c/-C/--orphan is being used.
         */
        if (opts->new_branch_force)
                opts->new_branch = opts->new_branch_force;

        if (opts->new_orphan_branch)
                opts->new_branch = opts->new_orphan_branch;

        /* --track without -c/-C/-b/-B/--orphan should DWIM */
        if (opts->track != BRANCH_TRACK_UNSPECIFIED && !opts->new_branch) {
                const char *argv0 = argv[0];
                if (!argc || !strcmp(argv0, "--"))
                        die(_("--track needs a branch name"));
                skip_prefix(argv0, "refs/", &argv0);
                skip_prefix(argv0, "remotes/", &argv0);
                argv0 = strchr(argv0, '/');
                if (!argv0 || !argv0[1])
                        die(_("missing branch name; try -%c"), cb_option);
                opts->new_branch = argv0 + 1;
        }

        /*
         * Extract branch name from command line arguments, so
         * all that is left is pathspecs.
         *
         * Handle
         *
         *  1) git checkout <tree> -- [<paths>]
         *  2) git checkout -- [<paths>]
         *  3) git checkout <something> [<paths>]
         *
         * including "last branch" syntax and DWIM-ery for names of
         * remote branches, erroring out for invalid or ambiguous cases.
         */
        if (argc && opts->accept_ref) {
                struct object_id rev;
                int dwim_ok =
                        !opts->patch_mode &&
                        opts->dwim_new_local_branch &&
                        opts->track == BRANCH_TRACK_UNSPECIFIED &&
                        !opts->new_branch;
                int n = parse_branchname_arg(argc, argv, dwim_ok,
                                             &new_branch_info, opts, &rev);
                argv += n;
                argc -= n;
        } else if (!opts->accept_ref && opts->from_treeish) {
                struct object_id rev;

                if (get_oid_mb(opts->from_treeish, &rev))
                        die(_("could not resolve %s"), opts->from_treeish);

                setup_new_branch_info_and_source_tree(&new_branch_info,
                                                      opts, &rev,
                                                      opts->from_treeish);

                if (!opts->source_tree)
                        die(_("reference is not a tree: %s"), opts->from_treeish);
        }

        if (argc) {
                parse_pathspec(&opts->pathspec, 0,
                               opts->patch_mode ? PATHSPEC_PREFIX_ORIGIN : 0,
                               prefix, argv);

                if (!opts->pathspec.nr)
                        die(_("invalid path specification"));

                /*
                 * Try to give more helpful suggestion.
                 * new_branch && argc > 1 will be caught later.
                 */
                if (opts->new_branch && argc == 1 && !new_branch_info.commit)
                        die(_("'%s' is not a commit and a branch '%s' cannot be created from it"),
                                argv[0], opts->new_branch);

                if (opts->force_detach)
                        die(_("git checkout: --detach does not take a path argument '%s'"),
                            argv[0]);
        }

        if (opts->pathspec_from_file) {
                if (opts->pathspec.nr)
                        die(_("--pathspec-from-file is incompatible with pathspec arguments"));

                if (opts->force_detach)
                        die(_("--pathspec-from-file is incompatible with --detach"));

                if (opts->patch_mode)
                        die(_("--pathspec-from-file is incompatible with --patch"));

                parse_pathspec_file(&opts->pathspec, 0,
                                    0,
                                    prefix, opts->pathspec_from_file, opts->pathspec_file_nul);
        } else if (opts->pathspec_file_nul) {
                die(_("--pathspec-file-nul requires --pathspec-from-file"));
        }

        opts->pathspec.recursive = 1;

        if (opts->pathspec.nr) {
                if (1 < !!opts->writeout_stage + !!opts->force + !!opts->merge)
                        die(_("git checkout: --ours/--theirs, --force and --merge are incompatible when\n"
                              "checking out of the index."));
        } else {
                if (opts->accept_pathspec && !opts->empty_pathspec_ok &&
                    !opts->patch_mode)  /* patch mode is special */
                        die(_("you must specify path(s) to restore"));
        }

        if (opts->new_branch) {
                struct strbuf buf = STRBUF_INIT;

                if (opts->new_branch_force)
                        opts->branch_exists = validate_branchname(opts->new_branch, &buf);
                else
                        opts->branch_exists =
                                validate_new_branchname(opts->new_branch, &buf, 0);
                strbuf_release(&buf);
        }

        UNLEAK(opts);
        if (opts->patch_mode || opts->pathspec.nr)
                return checkout_paths(opts, &new_branch_info);
        else
                return checkout_branch(opts, &new_branch_info);
}

int cmd_checkout(int argc, const char **argv, const char *prefix)
{
        struct checkout_opts opts;
        struct option *options;
        struct option checkout_options[] = {
                OPT_STRING('b', NULL, &opts.new_branch, N_("branch"),
                           N_("create and checkout a new branch")),
                OPT_STRING('B', NULL, &opts.new_branch_force, N_("branch"),
                           N_("create/reset and checkout a branch")),
                OPT_BOOL('l', NULL, &opts.new_branch_log, N_("create reflog for new branch")),
                OPT_BOOL(0, "guess", &opts.dwim_new_local_branch,
                         N_("second guess 'git checkout <no-such-branch>' (default)")),
                OPT_BOOL(0, "overlay", &opts.overlay_mode, N_("use overlay mode (default)")),
                OPT_END()
        };
        int ret;

        memset(&opts, 0, sizeof(opts));
        opts.dwim_new_local_branch = 1;
        opts.switch_branch_doing_nothing_is_ok = 1;
        opts.only_merge_on_switching_branches = 0;
        opts.accept_ref = 1;
        opts.accept_pathspec = 1;
        opts.implicit_detach = 1;
        opts.can_switch_when_in_progress = 1;
        opts.orphan_from_empty_tree = 0;
        opts.empty_pathspec_ok = 1;
        opts.overlay_mode = -1;
        opts.checkout_index = -2;    /* default on */
        opts.checkout_worktree = -2; /* default on */

        if (argc == 3 && !strcmp(argv[1], "-b")) {
                /*
                 * User ran 'git checkout -b <branch>' and expects
                 * the same behavior as 'git switch -c <branch>'.
                 */
                opts.switch_branch_doing_nothing_is_ok = 0;
                opts.only_merge_on_switching_branches = 1;
        }

        options = parse_options_dup(checkout_options);
        options = add_common_options(&opts, options);
        options = add_common_switch_branch_options(&opts, options);
        options = add_checkout_path_options(&opts, options);

        ret = checkout_main(argc, argv, prefix, &opts,
                            options, checkout_usage);
        FREE_AND_NULL(options);
        return ret;
}

int cmd_switch(int argc, const char **argv, const char *prefix)
{
        struct checkout_opts opts;
        struct option *options = NULL;
        struct option switch_options[] = {
                OPT_STRING('c', "create", &opts.new_branch, N_("branch"),
                           N_("create and switch to a new branch")),
                OPT_STRING('C', "force-create", &opts.new_branch_force, N_("branch"),
                           N_("create/reset and switch to a branch")),
                OPT_BOOL(0, "guess", &opts.dwim_new_local_branch,
                         N_("second guess 'git switch <no-such-branch>'")),
                OPT_BOOL(0, "discard-changes", &opts.discard_changes,
                         N_("throw away local modifications")),
                OPT_END()
        };
        int ret;

        memset(&opts, 0, sizeof(opts));
        opts.dwim_new_local_branch = 1;
        opts.accept_ref = 1;
        opts.accept_pathspec = 0;
        opts.switch_branch_doing_nothing_is_ok = 0;
        opts.only_merge_on_switching_branches = 1;
        opts.implicit_detach = 0;
        opts.can_switch_when_in_progress = 0;
        opts.orphan_from_empty_tree = 1;
        opts.overlay_mode = -1;

        options = parse_options_dup(switch_options);
        options = add_common_options(&opts, options);
        options = add_common_switch_branch_options(&opts, options);

        cb_option = 'c';

        ret = checkout_main(argc, argv, prefix, &opts,
                            options, switch_branch_usage);
        FREE_AND_NULL(options);
        return ret;
}

int cmd_restore(int argc, const char **argv, const char *prefix)
{
        struct checkout_opts opts;
        struct option *options;
        struct option restore_options[] = {
                OPT_STRING('s', "source", &opts.from_treeish, "<tree-ish>",
                           N_("which tree-ish to checkout from")),
                OPT_BOOL('S', "staged", &opts.checkout_index,
                           N_("restore the index")),
                OPT_BOOL('W', "worktree", &opts.checkout_worktree,
                           N_("restore the working tree (default)")),
                OPT_BOOL(0, "ignore-unmerged", &opts.ignore_unmerged,
                         N_("ignore unmerged entries")),
                OPT_BOOL(0, "overlay", &opts.overlay_mode, N_("use overlay mode")),
                OPT_END()
        };
        int ret;

        memset(&opts, 0, sizeof(opts));
        opts.accept_ref = 0;
        opts.accept_pathspec = 1;
        opts.empty_pathspec_ok = 0;
        opts.overlay_mode = 0;
        opts.checkout_index = -1;    /* default off */
        opts.checkout_worktree = -2; /* default on */
        opts.ignore_unmerged_opt = "--ignore-unmerged";

        options = parse_options_dup(restore_options);
        options = add_common_options(&opts, options);
        options = add_checkout_path_options(&opts, options);

        ret = checkout_main(argc, argv, prefix, &opts,
                            options, restore_usage);
        FREE_AND_NULL(options);
        return ret;
}