pack-bitmap: introduce bitmap_walk_contains()

[git.git] / unpack-trees.c

#include "cache.h"
#include "argv-array.h"
#include "repository.h"
#include "config.h"
#include "dir.h"
#include "tree.h"
#include "tree-walk.h"
#include "cache-tree.h"
#include "unpack-trees.h"
#include "progress.h"
#include "refs.h"
#include "attr.h"
#include "split-index.h"
#include "dir.h"
#include "submodule.h"
#include "submodule-config.h"
#include "fsmonitor.h"
#include "object-store.h"
#include "fetch-object.h"

/*
 * Error messages expected by scripts out of plumbing commands such as
 * read-tree.  Non-scripted Porcelain is not required to use these messages
 * and in fact are encouraged to reword them to better suit their particular
 * situation better.  See how "git checkout" and "git merge" replaces
 * them using setup_unpack_trees_porcelain(), for example.
 */
static const char *unpack_plumbing_errors[NB_UNPACK_TREES_ERROR_TYPES] = {
        /* ERROR_WOULD_OVERWRITE */
        "Entry '%s' would be overwritten by merge. Cannot merge.",

        /* ERROR_NOT_UPTODATE_FILE */
        "Entry '%s' not uptodate. Cannot merge.",

        /* ERROR_NOT_UPTODATE_DIR */
        "Updating '%s' would lose untracked files in it",

        /* ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN */
        "Untracked working tree file '%s' would be overwritten by merge.",

        /* ERROR_WOULD_LOSE_UNTRACKED_REMOVED */
        "Untracked working tree file '%s' would be removed by merge.",

        /* ERROR_BIND_OVERLAP */
        "Entry '%s' overlaps with '%s'.  Cannot bind.",

        /* ERROR_SPARSE_NOT_UPTODATE_FILE */
        "Entry '%s' not uptodate. Cannot update sparse checkout.",

        /* ERROR_WOULD_LOSE_ORPHANED_OVERWRITTEN */
        "Working tree file '%s' would be overwritten by sparse checkout update.",

        /* ERROR_WOULD_LOSE_ORPHANED_REMOVED */
        "Working tree file '%s' would be removed by sparse checkout update.",

        /* ERROR_WOULD_LOSE_SUBMODULE */
        "Submodule '%s' cannot checkout new HEAD.",
};

#define ERRORMSG(o,type) \
        ( ((o) && (o)->msgs[(type)]) \
          ? ((o)->msgs[(type)])      \
          : (unpack_plumbing_errors[(type)]) )

static const char *super_prefixed(const char *path)
{
        /*
         * It is necessary and sufficient to have two static buffers
         * here, as the return value of this function is fed to
         * error() using the unpack_*_errors[] templates we see above.
         */
        static struct strbuf buf[2] = {STRBUF_INIT, STRBUF_INIT};
        static int super_prefix_len = -1;
        static unsigned idx = ARRAY_SIZE(buf) - 1;

        if (super_prefix_len < 0) {
                const char *super_prefix = get_super_prefix();
                if (!super_prefix) {
                        super_prefix_len = 0;
                } else {
                        int i;
                        for (i = 0; i < ARRAY_SIZE(buf); i++)
                                strbuf_addstr(&buf[i], super_prefix);
                        super_prefix_len = buf[0].len;
                }
        }

        if (!super_prefix_len)
                return path;

        if (++idx >= ARRAY_SIZE(buf))
                idx = 0;

        strbuf_setlen(&buf[idx], super_prefix_len);
        strbuf_addstr(&buf[idx], path);

        return buf[idx].buf;
}

void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
                                  const char *cmd)
{
        int i;
        const char **msgs = opts->msgs;
        const char *msg;

        argv_array_init(&opts->msgs_to_free);

        if (!strcmp(cmd, "checkout"))
                msg = advice_commit_before_merge
                      ? _("Your local changes to the following files would be overwritten by checkout:\n%%s"
                          "Please commit your changes or stash them before you switch branches.")
                      : _("Your local changes to the following files would be overwritten by checkout:\n%%s");
        else if (!strcmp(cmd, "merge"))
                msg = advice_commit_before_merge
                      ? _("Your local changes to the following files would be overwritten by merge:\n%%s"
                          "Please commit your changes or stash them before you merge.")
                      : _("Your local changes to the following files would be overwritten by merge:\n%%s");
        else
                msg = advice_commit_before_merge
                      ? _("Your local changes to the following files would be overwritten by %s:\n%%s"
                          "Please commit your changes or stash them before you %s.")
                      : _("Your local changes to the following files would be overwritten by %s:\n%%s");
        msgs[ERROR_WOULD_OVERWRITE] = msgs[ERROR_NOT_UPTODATE_FILE] =
                argv_array_pushf(&opts->msgs_to_free, msg, cmd, cmd);

        msgs[ERROR_NOT_UPTODATE_DIR] =
                _("Updating the following directories would lose untracked files in them:\n%s");

        if (!strcmp(cmd, "checkout"))
                msg = advice_commit_before_merge
                      ? _("The following untracked working tree files would be removed by checkout:\n%%s"
                          "Please move or remove them before you switch branches.")
                      : _("The following untracked working tree files would be removed by checkout:\n%%s");
        else if (!strcmp(cmd, "merge"))
                msg = advice_commit_before_merge
                      ? _("The following untracked working tree files would be removed by merge:\n%%s"
                          "Please move or remove them before you merge.")
                      : _("The following untracked working tree files would be removed by merge:\n%%s");
        else
                msg = advice_commit_before_merge
                      ? _("The following untracked working tree files would be removed by %s:\n%%s"
                          "Please move or remove them before you %s.")
                      : _("The following untracked working tree files would be removed by %s:\n%%s");
        msgs[ERROR_WOULD_LOSE_UNTRACKED_REMOVED] =
                argv_array_pushf(&opts->msgs_to_free, msg, cmd, cmd);

        if (!strcmp(cmd, "checkout"))
                msg = advice_commit_before_merge
                      ? _("The following untracked working tree files would be overwritten by checkout:\n%%s"
                          "Please move or remove them before you switch branches.")
                      : _("The following untracked working tree files would be overwritten by checkout:\n%%s");
        else if (!strcmp(cmd, "merge"))
                msg = advice_commit_before_merge
                      ? _("The following untracked working tree files would be overwritten by merge:\n%%s"
                          "Please move or remove them before you merge.")
                      : _("The following untracked working tree files would be overwritten by merge:\n%%s");
        else
                msg = advice_commit_before_merge
                      ? _("The following untracked working tree files would be overwritten by %s:\n%%s"
                          "Please move or remove them before you %s.")
                      : _("The following untracked working tree files would be overwritten by %s:\n%%s");
        msgs[ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN] =
                argv_array_pushf(&opts->msgs_to_free, msg, cmd, cmd);

        /*
         * Special case: ERROR_BIND_OVERLAP refers to a pair of paths, we
         * cannot easily display it as a list.
         */
        msgs[ERROR_BIND_OVERLAP] = _("Entry '%s' overlaps with '%s'.  Cannot bind.");

        msgs[ERROR_SPARSE_NOT_UPTODATE_FILE] =
                _("Cannot update sparse checkout: the following entries are not up to date:\n%s");
        msgs[ERROR_WOULD_LOSE_ORPHANED_OVERWRITTEN] =
                _("The following working tree files would be overwritten by sparse checkout update:\n%s");
        msgs[ERROR_WOULD_LOSE_ORPHANED_REMOVED] =
                _("The following working tree files would be removed by sparse checkout update:\n%s");
        msgs[ERROR_WOULD_LOSE_SUBMODULE] =
                _("Cannot update submodule:\n%s");

        opts->show_all_errors = 1;
        /* rejected paths may not have a static buffer */
        for (i = 0; i < ARRAY_SIZE(opts->unpack_rejects); i++)
                opts->unpack_rejects[i].strdup_strings = 1;
}

void clear_unpack_trees_porcelain(struct unpack_trees_options *opts)
{
        argv_array_clear(&opts->msgs_to_free);
        memset(opts->msgs, 0, sizeof(opts->msgs));
}

static int do_add_entry(struct unpack_trees_options *o, struct cache_entry *ce,
                         unsigned int set, unsigned int clear)
{
        clear |= CE_HASHED;

        if (set & CE_REMOVE)
                set |= CE_WT_REMOVE;

        ce->ce_flags = (ce->ce_flags & ~clear) | set;
        return add_index_entry(&o->result, ce,
                               ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE);
}

static void add_entry(struct unpack_trees_options *o,
                      const struct cache_entry *ce,
                      unsigned int set, unsigned int clear)
{
        do_add_entry(o, dup_cache_entry(ce, &o->result), set, clear);
}

/*
 * add error messages on path <path>
 * corresponding to the type <e> with the message <msg>
 * indicating if it should be display in porcelain or not
 */
static int add_rejected_path(struct unpack_trees_options *o,
                             enum unpack_trees_error_types e,
                             const char *path)
{
        if (o->quiet)
                return -1;

        if (!o->show_all_errors)
                return error(ERRORMSG(o, e), super_prefixed(path));

        /*
         * Otherwise, insert in a list for future display by
         * display_error_msgs()
         */
        string_list_append(&o->unpack_rejects[e], path);
        return -1;
}

/*
 * display all the error messages stored in a nice way
 */
static void display_error_msgs(struct unpack_trees_options *o)
{
        int e, i;
        int something_displayed = 0;
        for (e = 0; e < NB_UNPACK_TREES_ERROR_TYPES; e++) {
                struct string_list *rejects = &o->unpack_rejects[e];
                if (rejects->nr > 0) {
                        struct strbuf path = STRBUF_INIT;
                        something_displayed = 1;
                        for (i = 0; i < rejects->nr; i++)
                                strbuf_addf(&path, "\t%s\n", rejects->items[i].string);
                        error(ERRORMSG(o, e), super_prefixed(path.buf));
                        strbuf_release(&path);
                }
                string_list_clear(rejects, 0);
        }
        if (something_displayed)
                fprintf(stderr, _("Aborting\n"));
}

static int check_submodule_move_head(const struct cache_entry *ce,
                                     const char *old_id,
                                     const char *new_id,
                                     struct unpack_trees_options *o)
{
        unsigned flags = SUBMODULE_MOVE_HEAD_DRY_RUN;
        const struct submodule *sub = submodule_from_ce(ce);

        if (!sub)
                return 0;

        if (o->reset)
                flags |= SUBMODULE_MOVE_HEAD_FORCE;

        if (submodule_move_head(ce->name, old_id, new_id, flags))
                return add_rejected_path(o, ERROR_WOULD_LOSE_SUBMODULE, ce->name);
        return 0;
}

/*
 * Preform the loading of the repository's gitmodules file.  This function is
 * used by 'check_update()' to perform loading of the gitmodules file in two
 * differnt situations:
 * (1) before removing entries from the working tree if the gitmodules file has
 *     been marked for removal.  This situation is specified by 'state' == NULL.
 * (2) before checking out entries to the working tree if the gitmodules file
 *     has been marked for update.  This situation is specified by 'state' != NULL.
 */
static void load_gitmodules_file(struct index_state *index,
                                 struct checkout *state)
{
        int pos = index_name_pos(index, GITMODULES_FILE, strlen(GITMODULES_FILE));

        if (pos >= 0) {
                struct cache_entry *ce = index->cache[pos];
                if (!state && ce->ce_flags & CE_WT_REMOVE) {
                        repo_read_gitmodules(the_repository);
                } else if (state && (ce->ce_flags & CE_UPDATE)) {
                        submodule_free(the_repository);
                        checkout_entry(ce, state, NULL, NULL);
                        repo_read_gitmodules(the_repository);
                }
        }
}

static struct progress *get_progress(struct unpack_trees_options *o)
{
        unsigned cnt = 0, total = 0;
        struct index_state *index = &o->result;

        if (!o->update || !o->verbose_update)
                return NULL;

        for (; cnt < index->cache_nr; cnt++) {
                const struct cache_entry *ce = index->cache[cnt];
                if (ce->ce_flags & (CE_UPDATE | CE_WT_REMOVE))
                        total++;
        }

        return start_delayed_progress(_("Updating files"), total);
}

static void setup_collided_checkout_detection(struct checkout *state,
                                              struct index_state *index)
{
        int i;

        state->clone = 1;
        for (i = 0; i < index->cache_nr; i++)
                index->cache[i]->ce_flags &= ~CE_MATCHED;
}

static void report_collided_checkout(struct index_state *index)
{
        struct string_list list = STRING_LIST_INIT_NODUP;
        int i;

        for (i = 0; i < index->cache_nr; i++) {
                struct cache_entry *ce = index->cache[i];

                if (!(ce->ce_flags & CE_MATCHED))
                        continue;

                string_list_append(&list, ce->name);
                ce->ce_flags &= ~CE_MATCHED;
        }

        list.cmp = fspathcmp;
        string_list_sort(&list);

        if (list.nr) {
                warning(_("the following paths have collided (e.g. case-sensitive paths\n"
                          "on a case-insensitive filesystem) and only one from the same\n"
                          "colliding group is in the working tree:\n"));

                for (i = 0; i < list.nr; i++)
                        fprintf(stderr, "  '%s'\n", list.items[i].string);
        }

        string_list_clear(&list, 0);
}

static int check_updates(struct unpack_trees_options *o)
{
        unsigned cnt = 0;
        int errs = 0;
        struct progress *progress;
        struct index_state *index = &o->result;
        struct checkout state = CHECKOUT_INIT;
        int i;

        trace_performance_enter();
        state.force = 1;
        state.quiet = 1;
        state.refresh_cache = 1;
        state.istate = index;

        if (o->clone)
                setup_collided_checkout_detection(&state, index);

        progress = get_progress(o);

        if (o->update)
                git_attr_set_direction(GIT_ATTR_CHECKOUT);

        if (should_update_submodules() && o->update && !o->dry_run)
                load_gitmodules_file(index, NULL);

        for (i = 0; i < index->cache_nr; i++) {
                const struct cache_entry *ce = index->cache[i];

                if (ce->ce_flags & CE_WT_REMOVE) {
                        display_progress(progress, ++cnt);
                        if (o->update && !o->dry_run)
                                unlink_entry(ce);
                }
        }
        remove_marked_cache_entries(index, 0);
        remove_scheduled_dirs();

        if (should_update_submodules() && o->update && !o->dry_run)
                load_gitmodules_file(index, &state);

        enable_delayed_checkout(&state);
        if (repository_format_partial_clone && o->update && !o->dry_run) {
                /*
                 * Prefetch the objects that are to be checked out in the loop
                 * below.
                 */
                struct oid_array to_fetch = OID_ARRAY_INIT;
                for (i = 0; i < index->cache_nr; i++) {
                        struct cache_entry *ce = index->cache[i];

                        if (!(ce->ce_flags & CE_UPDATE) ||
                            S_ISGITLINK(ce->ce_mode))
                                continue;
                        if (!oid_object_info_extended(the_repository, &ce->oid,
                                                      NULL,
                                                      OBJECT_INFO_FOR_PREFETCH))
                                continue;
                        oid_array_append(&to_fetch, &ce->oid);
                }
                if (to_fetch.nr)
                        fetch_objects(repository_format_partial_clone,
                                      to_fetch.oid, to_fetch.nr);
                oid_array_clear(&to_fetch);
        }
        for (i = 0; i < index->cache_nr; i++) {
                struct cache_entry *ce = index->cache[i];

                if (ce->ce_flags & CE_UPDATE) {
                        if (ce->ce_flags & CE_WT_REMOVE)
                                BUG("both update and delete flags are set on %s",
                                    ce->name);
                        display_progress(progress, ++cnt);
                        ce->ce_flags &= ~CE_UPDATE;
                        if (o->update && !o->dry_run) {
                                errs |= checkout_entry(ce, &state, NULL, NULL);
                        }
                }
        }
        stop_progress(&progress);
        errs |= finish_delayed_checkout(&state, NULL);
        if (o->update)
                git_attr_set_direction(GIT_ATTR_CHECKIN);

        if (o->clone)
                report_collided_checkout(index);

        trace_performance_leave("check_updates");
        return errs != 0;
}

static int verify_uptodate_sparse(const struct cache_entry *ce,
                                  struct unpack_trees_options *o);
static int verify_absent_sparse(const struct cache_entry *ce,
                                enum unpack_trees_error_types,
                                struct unpack_trees_options *o);

static int apply_sparse_checkout(struct index_state *istate,
                                 struct cache_entry *ce,
                                 struct unpack_trees_options *o)
{
        int was_skip_worktree = ce_skip_worktree(ce);

        if (ce->ce_flags & CE_NEW_SKIP_WORKTREE)
                ce->ce_flags |= CE_SKIP_WORKTREE;
        else
                ce->ce_flags &= ~CE_SKIP_WORKTREE;
        if (was_skip_worktree != ce_skip_worktree(ce)) {
                ce->ce_flags |= CE_UPDATE_IN_BASE;
                mark_fsmonitor_invalid(istate, ce);
                istate->cache_changed |= CE_ENTRY_CHANGED;
        }

        /*
         * if (!was_skip_worktree && !ce_skip_worktree()) {
         *      This is perfectly normal. Move on;
         * }
         */

        /*
         * Merge strategies may set CE_UPDATE|CE_REMOVE outside checkout
         * area as a result of ce_skip_worktree() shortcuts in
         * verify_absent() and verify_uptodate().
         * Make sure they don't modify worktree if they are already
         * outside checkout area
         */
        if (was_skip_worktree && ce_skip_worktree(ce)) {
                ce->ce_flags &= ~CE_UPDATE;

                /*
                 * By default, when CE_REMOVE is on, CE_WT_REMOVE is also
                 * on to get that file removed from both index and worktree.
                 * If that file is already outside worktree area, don't
                 * bother remove it.
                 */
                if (ce->ce_flags & CE_REMOVE)
                        ce->ce_flags &= ~CE_WT_REMOVE;
        }

        if (!was_skip_worktree && ce_skip_worktree(ce)) {
                /*
                 * If CE_UPDATE is set, verify_uptodate() must be called already
                 * also stat info may have lost after merged_entry() so calling
                 * verify_uptodate() again may fail
                 */
                if (!(ce->ce_flags & CE_UPDATE) && verify_uptodate_sparse(ce, o))
                        return -1;
                ce->ce_flags |= CE_WT_REMOVE;
                ce->ce_flags &= ~CE_UPDATE;
        }
        if (was_skip_worktree && !ce_skip_worktree(ce)) {
                if (verify_absent_sparse(ce, ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o))
                        return -1;
                ce->ce_flags |= CE_UPDATE;
        }
        return 0;
}

static inline int call_unpack_fn(const struct cache_entry * const *src,
                                 struct unpack_trees_options *o)
{
        int ret = o->fn(src, o);
        if (ret > 0)
                ret = 0;
        return ret;
}

static void mark_ce_used(struct cache_entry *ce, struct unpack_trees_options *o)
{
        ce->ce_flags |= CE_UNPACKED;

        if (o->cache_bottom < o->src_index->cache_nr &&
            o->src_index->cache[o->cache_bottom] == ce) {
                int bottom = o->cache_bottom;
                while (bottom < o->src_index->cache_nr &&
                       o->src_index->cache[bottom]->ce_flags & CE_UNPACKED)
                        bottom++;
                o->cache_bottom = bottom;
        }
}

static void mark_all_ce_unused(struct index_state *index)
{
        int i;
        for (i = 0; i < index->cache_nr; i++)
                index->cache[i]->ce_flags &= ~(CE_UNPACKED | CE_ADDED | CE_NEW_SKIP_WORKTREE);
}

static int locate_in_src_index(const struct cache_entry *ce,
                               struct unpack_trees_options *o)
{
        struct index_state *index = o->src_index;
        int len = ce_namelen(ce);
        int pos = index_name_pos(index, ce->name, len);
        if (pos < 0)
                pos = -1 - pos;
        return pos;
}

/*
 * We call unpack_index_entry() with an unmerged cache entry
 * only in diff-index, and it wants a single callback.  Skip
 * the other unmerged entry with the same name.
 */
static void mark_ce_used_same_name(struct cache_entry *ce,
                                   struct unpack_trees_options *o)
{
        struct index_state *index = o->src_index;
        int len = ce_namelen(ce);
        int pos;

        for (pos = locate_in_src_index(ce, o); pos < index->cache_nr; pos++) {
                struct cache_entry *next = index->cache[pos];
                if (len != ce_namelen(next) ||
                    memcmp(ce->name, next->name, len))
                        break;
                mark_ce_used(next, o);
        }
}

static struct cache_entry *next_cache_entry(struct unpack_trees_options *o)
{
        const struct index_state *index = o->src_index;
        int pos = o->cache_bottom;

        while (pos < index->cache_nr) {
                struct cache_entry *ce = index->cache[pos];
                if (!(ce->ce_flags & CE_UNPACKED))
                        return ce;
                pos++;
        }
        return NULL;
}

static void add_same_unmerged(const struct cache_entry *ce,
                              struct unpack_trees_options *o)
{
        struct index_state *index = o->src_index;
        int len = ce_namelen(ce);
        int pos = index_name_pos(index, ce->name, len);

        if (0 <= pos)
                die("programming error in a caller of mark_ce_used_same_name");
        for (pos = -pos - 1; pos < index->cache_nr; pos++) {
                struct cache_entry *next = index->cache[pos];
                if (len != ce_namelen(next) ||
                    memcmp(ce->name, next->name, len))
                        break;
                add_entry(o, next, 0, 0);
                mark_ce_used(next, o);
        }
}

static int unpack_index_entry(struct cache_entry *ce,
                              struct unpack_trees_options *o)
{
        const struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
        int ret;

        src[0] = ce;

        mark_ce_used(ce, o);
        if (ce_stage(ce)) {
                if (o->skip_unmerged) {
                        add_entry(o, ce, 0, 0);
                        return 0;
                }
        }
        ret = call_unpack_fn(src, o);
        if (ce_stage(ce))
                mark_ce_used_same_name(ce, o);
        return ret;
}

static int find_cache_pos(struct traverse_info *, const struct name_entry *);

static void restore_cache_bottom(struct traverse_info *info, int bottom)
{
        struct unpack_trees_options *o = info->data;

        if (o->diff_index_cached)
                return;
        o->cache_bottom = bottom;
}

static int switch_cache_bottom(struct traverse_info *info)
{
        struct unpack_trees_options *o = info->data;
        int ret, pos;

        if (o->diff_index_cached)
                return 0;
        ret = o->cache_bottom;
        pos = find_cache_pos(info->prev, &info->name);

        if (pos < -1)
                o->cache_bottom = -2 - pos;
        else if (pos < 0)
                o->cache_bottom = o->src_index->cache_nr;
        return ret;
}

static inline int are_same_oid(struct name_entry *name_j, struct name_entry *name_k)
{
        return !is_null_oid(&name_j->oid) && !is_null_oid(&name_k->oid) && oideq(&name_j->oid, &name_k->oid);
}

static int all_trees_same_as_cache_tree(int n, unsigned long dirmask,
                                        struct name_entry *names,
                                        struct traverse_info *info)
{
        struct unpack_trees_options *o = info->data;
        int i;

        if (!o->merge || dirmask != ((1 << n) - 1))
                return 0;

        for (i = 1; i < n; i++)
                if (!are_same_oid(names, names + i))
                        return 0;

        return cache_tree_matches_traversal(o->src_index->cache_tree, names, info);
}

static int index_pos_by_traverse_info(struct name_entry *names,
                                      struct traverse_info *info)
{
        struct unpack_trees_options *o = info->data;
        int len = traverse_path_len(info, names);
        char *name = xmalloc(len + 1 /* slash */ + 1 /* NUL */);
        int pos;

        make_traverse_path(name, info, names);
        name[len++] = '/';
        name[len] = '\0';
        pos = index_name_pos(o->src_index, name, len);
        if (pos >= 0)
                BUG("This is a directory and should not exist in index");
        pos = -pos - 1;
        if (!starts_with(o->src_index->cache[pos]->name, name) ||
            (pos > 0 && starts_with(o->src_index->cache[pos-1]->name, name)))
                BUG("pos must point at the first entry in this directory");
        free(name);
        return pos;
}

/*
 * Fast path if we detect that all trees are the same as cache-tree at this
 * path. We'll walk these trees in an iterative loop using cache-tree/index
 * instead of ODB since we already know what these trees contain.
 */
static int traverse_by_cache_tree(int pos, int nr_entries, int nr_names,
                                  struct traverse_info *info)
{
        struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
        struct unpack_trees_options *o = info->data;
        struct cache_entry *tree_ce = NULL;
        int ce_len = 0;
        int i, d;

        if (!o->merge)
                BUG("We need cache-tree to do this optimization");

        /*
         * Do what unpack_callback() and unpack_nondirectories() normally
         * do. But we walk all paths in an iterative loop instead.
         *
         * D/F conflicts and higher stage entries are not a concern
         * because cache-tree would be invalidated and we would never
         * get here in the first place.
         */
        for (i = 0; i < nr_entries; i++) {
                int new_ce_len, len, rc;

                src[0] = o->src_index->cache[pos + i];

                len = ce_namelen(src[0]);
                new_ce_len = cache_entry_size(len);

                if (new_ce_len > ce_len) {
                        new_ce_len <<= 1;
                        tree_ce = xrealloc(tree_ce, new_ce_len);
                        memset(tree_ce, 0, new_ce_len);
                        ce_len = new_ce_len;

                        tree_ce->ce_flags = create_ce_flags(0);

                        for (d = 1; d <= nr_names; d++)
                                src[d] = tree_ce;
                }

                tree_ce->ce_mode = src[0]->ce_mode;
                tree_ce->ce_namelen = len;
                oidcpy(&tree_ce->oid, &src[0]->oid);
                memcpy(tree_ce->name, src[0]->name, len + 1);

                rc = call_unpack_fn((const struct cache_entry * const *)src, o);
                if (rc < 0) {
                        free(tree_ce);
                        return rc;
                }

                mark_ce_used(src[0], o);
        }
        free(tree_ce);
        if (o->debug_unpack)
                printf("Unpacked %d entries from %s to %s using cache-tree\n",
                       nr_entries,
                       o->src_index->cache[pos]->name,
                       o->src_index->cache[pos + nr_entries - 1]->name);
        return 0;
}

static int traverse_trees_recursive(int n, unsigned long dirmask,
                                    unsigned long df_conflicts,
                                    struct name_entry *names,
                                    struct traverse_info *info)
{
        struct unpack_trees_options *o = info->data;
        int i, ret, bottom;
        int nr_buf = 0;
        struct tree_desc t[MAX_UNPACK_TREES];
        void *buf[MAX_UNPACK_TREES];
        struct traverse_info newinfo;
        struct name_entry *p;
        int nr_entries;

        nr_entries = all_trees_same_as_cache_tree(n, dirmask, names, info);
        if (nr_entries > 0) {
                int pos = index_pos_by_traverse_info(names, info);

                if (!o->merge || df_conflicts)
                        BUG("Wrong condition to get here buddy");

                /*
                 * All entries up to 'pos' must have been processed
                 * (i.e. marked CE_UNPACKED) at this point. But to be safe,
                 * save and restore cache_bottom anyway to not miss
                 * unprocessed entries before 'pos'.
                 */
                bottom = o->cache_bottom;
                ret = traverse_by_cache_tree(pos, nr_entries, n, info);
                o->cache_bottom = bottom;
                return ret;
        }

        p = names;
        while (!p->mode)
                p++;

        newinfo = *info;
        newinfo.prev = info;
        newinfo.pathspec = info->pathspec;
        newinfo.name = *p;
        newinfo.pathlen += tree_entry_len(p) + 1;
        newinfo.df_conflicts |= df_conflicts;

        /*
         * Fetch the tree from the ODB for each peer directory in the
         * n commits.
         *
         * For 2- and 3-way traversals, we try to avoid hitting the
         * ODB twice for the same OID.  This should yield a nice speed
         * up in checkouts and merges when the commits are similar.
         *
         * We don't bother doing the full O(n^2) search for larger n,
         * because wider traversals don't happen that often and we
         * avoid the search setup.
         *
         * When 2 peer OIDs are the same, we just copy the tree
         * descriptor data.  This implicitly borrows the buffer
         * data from the earlier cell.
         */
        for (i = 0; i < n; i++, dirmask >>= 1) {
                if (i > 0 && are_same_oid(&names[i], &names[i - 1]))
                        t[i] = t[i - 1];
                else if (i > 1 && are_same_oid(&names[i], &names[i - 2]))
                        t[i] = t[i - 2];
                else {
                        const struct object_id *oid = NULL;
                        if (dirmask & 1)
                                oid = &names[i].oid;
                        buf[nr_buf++] = fill_tree_descriptor(the_repository, t + i, oid);
                }
        }

        bottom = switch_cache_bottom(&newinfo);
        ret = traverse_trees(o->src_index, n, t, &newinfo);
        restore_cache_bottom(&newinfo, bottom);

        for (i = 0; i < nr_buf; i++)
                free(buf[i]);

        return ret;
}

/*
 * Compare the traverse-path to the cache entry without actually
 * having to generate the textual representation of the traverse
 * path.
 *
 * NOTE! This *only* compares up to the size of the traverse path
 * itself - the caller needs to do the final check for the cache
 * entry having more data at the end!
 */
static int do_compare_entry_piecewise(const struct cache_entry *ce, const struct traverse_info *info, const struct name_entry *n)
{
        int len, pathlen, ce_len;
        const char *ce_name;

        if (info->prev) {
                int cmp = do_compare_entry_piecewise(ce, info->prev,
                                                     &info->name);
                if (cmp)
                        return cmp;
        }
        pathlen = info->pathlen;
        ce_len = ce_namelen(ce);

        /* If ce_len < pathlen then we must have previously hit "name == directory" entry */
        if (ce_len < pathlen)
                return -1;

        ce_len -= pathlen;
        ce_name = ce->name + pathlen;

        len = tree_entry_len(n);
        return df_name_compare(ce_name, ce_len, S_IFREG, n->path, len, n->mode);
}

static int do_compare_entry(const struct cache_entry *ce,
                            const struct traverse_info *info,
                            const struct name_entry *n)
{
        int len, pathlen, ce_len;
        const char *ce_name;
        int cmp;

        /*
         * If we have not precomputed the traverse path, it is quicker
         * to avoid doing so.  But if we have precomputed it,
         * it is quicker to use the precomputed version.
         */
        if (!info->traverse_path)
                return do_compare_entry_piecewise(ce, info, n);

        cmp = strncmp(ce->name, info->traverse_path, info->pathlen);
        if (cmp)
                return cmp;

        pathlen = info->pathlen;
        ce_len = ce_namelen(ce);

        if (ce_len < pathlen)
                return -1;

        ce_len -= pathlen;
        ce_name = ce->name + pathlen;

        len = tree_entry_len(n);
        return df_name_compare(ce_name, ce_len, S_IFREG, n->path, len, n->mode);
}

static int compare_entry(const struct cache_entry *ce, const struct traverse_info *info, const struct name_entry *n)
{
        int cmp = do_compare_entry(ce, info, n);
        if (cmp)
                return cmp;

        /*
         * Even if the beginning compared identically, the ce should
         * compare as bigger than a directory leading up to it!
         */
        return ce_namelen(ce) > traverse_path_len(info, n);
}

static int ce_in_traverse_path(const struct cache_entry *ce,
                               const struct traverse_info *info)
{
        if (!info->prev)
                return 1;
        if (do_compare_entry(ce, info->prev, &info->name))
                return 0;
        /*
         * If ce (blob) is the same name as the path (which is a tree
         * we will be descending into), it won't be inside it.
         */
        return (info->pathlen < ce_namelen(ce));
}

static struct cache_entry *create_ce_entry(const struct traverse_info *info,
        const struct name_entry *n,
        int stage,
        struct index_state *istate,
        int is_transient)
{
        int len = traverse_path_len(info, n);
        struct cache_entry *ce =
                is_transient ?
                make_empty_transient_cache_entry(len) :
                make_empty_cache_entry(istate, len);

        ce->ce_mode = create_ce_mode(n->mode);
        ce->ce_flags = create_ce_flags(stage);
        ce->ce_namelen = len;
        oidcpy(&ce->oid, &n->oid);
        make_traverse_path(ce->name, info, n);

        return ce;
}

/*
 * Note that traverse_by_cache_tree() duplicates some logic in this function
 * without actually calling it. If you change the logic here you may need to
 * check and change there as well.
 */
static int unpack_nondirectories(int n, unsigned long mask,
                                 unsigned long dirmask,
                                 struct cache_entry **src,
                                 const struct name_entry *names,
                                 const struct traverse_info *info)
{
        int i;
        struct unpack_trees_options *o = info->data;
        unsigned long conflicts = info->df_conflicts | dirmask;

        /* Do we have *only* directories? Nothing to do */
        if (mask == dirmask && !src[0])
                return 0;

        /*
         * Ok, we've filled in up to any potential index entry in src[0],
         * now do the rest.
         */
        for (i = 0; i < n; i++) {
                int stage;
                unsigned int bit = 1ul << i;
                if (conflicts & bit) {
                        src[i + o->merge] = o->df_conflict_entry;
                        continue;
                }
                if (!(mask & bit))
                        continue;
                if (!o->merge)
                        stage = 0;
                else if (i + 1 < o->head_idx)
                        stage = 1;
                else if (i + 1 > o->head_idx)
                        stage = 3;
                else
                        stage = 2;

                /*
                 * If the merge bit is set, then the cache entries are
                 * discarded in the following block.  In this case,
                 * construct "transient" cache_entries, as they are
                 * not stored in the index.  otherwise construct the
                 * cache entry from the index aware logic.
                 */
                src[i + o->merge] = create_ce_entry(info, names + i, stage, &o->result, o->merge);
        }

        if (o->merge) {
                int rc = call_unpack_fn((const struct cache_entry * const *)src,
                                        o);
                for (i = 0; i < n; i++) {
                        struct cache_entry *ce = src[i + o->merge];
                        if (ce != o->df_conflict_entry)
                                discard_cache_entry(ce);
                }
                return rc;
        }

        for (i = 0; i < n; i++)
                if (src[i] && src[i] != o->df_conflict_entry)
                        if (do_add_entry(o, src[i], 0, 0))
                                return -1;

        return 0;
}

static int unpack_failed(struct unpack_trees_options *o, const char *message)
{
        discard_index(&o->result);
        if (!o->quiet && !o->exiting_early) {
                if (message)
                        return error("%s", message);
                return -1;
        }
        return -1;
}

/*
 * The tree traversal is looking at name p.  If we have a matching entry,
 * return it.  If name p is a directory in the index, do not return
 * anything, as we will want to match it when the traversal descends into
 * the directory.
 */
static int find_cache_pos(struct traverse_info *info,
                          const struct name_entry *p)
{
        int pos;
        struct unpack_trees_options *o = info->data;
        struct index_state *index = o->src_index;
        int pfxlen = info->pathlen;
        int p_len = tree_entry_len(p);

        for (pos = o->cache_bottom; pos < index->cache_nr; pos++) {
                const struct cache_entry *ce = index->cache[pos];
                const char *ce_name, *ce_slash;
                int cmp, ce_len;

                if (ce->ce_flags & CE_UNPACKED) {
                        /*
                         * cache_bottom entry is already unpacked, so
                         * we can never match it; don't check it
                         * again.
                         */
                        if (pos == o->cache_bottom)
                                ++o->cache_bottom;
                        continue;
                }
                if (!ce_in_traverse_path(ce, info)) {
                        /*
                         * Check if we can skip future cache checks
                         * (because we're already past all possible
                         * entries in the traverse path).
                         */
                        if (info->traverse_path) {
                                if (strncmp(ce->name, info->traverse_path,
                                            info->pathlen) > 0)
                                        break;
                        }
                        continue;
                }
                ce_name = ce->name + pfxlen;
                ce_slash = strchr(ce_name, '/');
                if (ce_slash)
                        ce_len = ce_slash - ce_name;
                else
                        ce_len = ce_namelen(ce) - pfxlen;
                cmp = name_compare(p->path, p_len, ce_name, ce_len);
                /*
                 * Exact match; if we have a directory we need to
                 * delay returning it.
                 */
                if (!cmp)
                        return ce_slash ? -2 - pos : pos;
                if (0 < cmp)
                        continue; /* keep looking */
                /*
                 * ce_name sorts after p->path; could it be that we
                 * have files under p->path directory in the index?
                 * E.g.  ce_name == "t-i", and p->path == "t"; we may
                 * have "t/a" in the index.
                 */
                if (p_len < ce_len && !memcmp(ce_name, p->path, p_len) &&
                    ce_name[p_len] < '/')
                        continue; /* keep looking */
                break;
        }
        return -1;
}

static struct cache_entry *find_cache_entry(struct traverse_info *info,
                                            const struct name_entry *p)
{
        int pos = find_cache_pos(info, p);
        struct unpack_trees_options *o = info->data;

        if (0 <= pos)
                return o->src_index->cache[pos];
        else
                return NULL;
}

static void debug_path(struct traverse_info *info)
{
        if (info->prev) {
                debug_path(info->prev);
                if (*info->prev->name.path)
                        putchar('/');
        }
        printf("%s", info->name.path);
}

static void debug_name_entry(int i, struct name_entry *n)
{
        printf("ent#%d %06o %s\n", i,
               n->path ? n->mode : 0,
               n->path ? n->path : "(missing)");
}

static void debug_unpack_callback(int n,
                                  unsigned long mask,
                                  unsigned long dirmask,
                                  struct name_entry *names,
                                  struct traverse_info *info)
{
        int i;
        printf("* unpack mask %lu, dirmask %lu, cnt %d ",
               mask, dirmask, n);
        debug_path(info);
        putchar('\n');
        for (i = 0; i < n; i++)
                debug_name_entry(i, names + i);
}

/*
 * Note that traverse_by_cache_tree() duplicates some logic in this function
 * without actually calling it. If you change the logic here you may need to
 * check and change there as well.
 */
static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, struct name_entry *names, struct traverse_info *info)
{
        struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
        struct unpack_trees_options *o = info->data;
        const struct name_entry *p = names;

        /* Find first entry with a real name (we could use "mask" too) */
        while (!p->mode)
                p++;

        if (o->debug_unpack)
                debug_unpack_callback(n, mask, dirmask, names, info);

        /* Are we supposed to look at the index too? */
        if (o->merge) {
                while (1) {
                        int cmp;
                        struct cache_entry *ce;

                        if (o->diff_index_cached)
                                ce = next_cache_entry(o);
                        else
                                ce = find_cache_entry(info, p);

                        if (!ce)
                                break;
                        cmp = compare_entry(ce, info, p);
                        if (cmp < 0) {
                                if (unpack_index_entry(ce, o) < 0)
                                        return unpack_failed(o, NULL);
                                continue;
                        }
                        if (!cmp) {
                                if (ce_stage(ce)) {
                                        /*
                                         * If we skip unmerged index
                                         * entries, we'll skip this
                                         * entry *and* the tree
                                         * entries associated with it!
                                         */
                                        if (o->skip_unmerged) {
                                                add_same_unmerged(ce, o);
                                                return mask;
                                        }
                                }
                                src[0] = ce;
                        }
                        break;
                }
        }

        if (unpack_nondirectories(n, mask, dirmask, src, names, info) < 0)
                return -1;

        if (o->merge && src[0]) {
                if (ce_stage(src[0]))
                        mark_ce_used_same_name(src[0], o);
                else
                        mark_ce_used(src[0], o);
        }

        /* Now handle any directories.. */
        if (dirmask) {
                /* special case: "diff-index --cached" looking at a tree */
                if (o->diff_index_cached &&
                    n == 1 && dirmask == 1 && S_ISDIR(names->mode)) {
                        int matches;
                        matches = cache_tree_matches_traversal(o->src_index->cache_tree,
                                                               names, info);
                        /*
                         * Everything under the name matches; skip the
                         * entire hierarchy.  diff_index_cached codepath
                         * special cases D/F conflicts in such a way that
                         * it does not do any look-ahead, so this is safe.
                         */
                        if (matches) {
                                o->cache_bottom += matches;
                                return mask;
                        }
                }

                if (traverse_trees_recursive(n, dirmask, mask & ~dirmask,
                                             names, info) < 0)
                        return -1;
                return mask;
        }

        return mask;
}

static int clear_ce_flags_1(struct index_state *istate,
                            struct cache_entry **cache, int nr,
                            struct strbuf *prefix,
                            int select_mask, int clear_mask,
                            struct exclude_list *el, int defval);

/* Whole directory matching */
static int clear_ce_flags_dir(struct index_state *istate,
                              struct cache_entry **cache, int nr,
                              struct strbuf *prefix,
                              char *basename,
                              int select_mask, int clear_mask,
                              struct exclude_list *el, int defval)
{
        struct cache_entry **cache_end;
        int dtype = DT_DIR;
        int ret = is_excluded_from_list(prefix->buf, prefix->len,
                                        basename, &dtype, el, istate);
        int rc;

        strbuf_addch(prefix, '/');

        /* If undecided, use matching result of parent dir in defval */
        if (ret < 0)
                ret = defval;

        for (cache_end = cache; cache_end != cache + nr; cache_end++) {
                struct cache_entry *ce = *cache_end;
                if (strncmp(ce->name, prefix->buf, prefix->len))
                        break;
        }

        /*
         * TODO: check el, if there are no patterns that may conflict
         * with ret (iow, we know in advance the incl/excl
         * decision for the entire directory), clear flag here without
         * calling clear_ce_flags_1(). That function will call
         * the expensive is_excluded_from_list() on every entry.
         */
        rc = clear_ce_flags_1(istate, cache, cache_end - cache,
                              prefix,
                              select_mask, clear_mask,
                              el, ret);
        strbuf_setlen(prefix, prefix->len - 1);
        return rc;
}

/*
 * Traverse the index, find every entry that matches according to
 * o->el. Do "ce_flags &= ~clear_mask" on those entries. Return the
 * number of traversed entries.
 *
 * If select_mask is non-zero, only entries whose ce_flags has on of
 * those bits enabled are traversed.
 *
 * cache        : pointer to an index entry
 * prefix_len   : an offset to its path
 *
 * The current path ("prefix") including the trailing '/' is
 *   cache[0]->name[0..(prefix_len-1)]
 * Top level path has prefix_len zero.
 */
static int clear_ce_flags_1(struct index_state *istate,
                            struct cache_entry **cache, int nr,
                            struct strbuf *prefix,
                            int select_mask, int clear_mask,
                            struct exclude_list *el, int defval)
{
        struct cache_entry **cache_end = cache + nr;

        /*
         * Process all entries that have the given prefix and meet
         * select_mask condition
         */
        while(cache != cache_end) {
                struct cache_entry *ce = *cache;
                const char *name, *slash;
                int len, dtype, ret;

                if (select_mask && !(ce->ce_flags & select_mask)) {
                        cache++;
                        continue;
                }

                if (prefix->len && strncmp(ce->name, prefix->buf, prefix->len))
                        break;

                name = ce->name + prefix->len;
                slash = strchr(name, '/');

                /* If it's a directory, try whole directory match first */
                if (slash) {
                        int processed;

                        len = slash - name;
                        strbuf_add(prefix, name, len);

                        processed = clear_ce_flags_dir(istate, cache, cache_end - cache,
                                                       prefix,
                                                       prefix->buf + prefix->len - len,
                                                       select_mask, clear_mask,
                                                       el, defval);

                        /* clear_c_f_dir eats a whole dir already? */
                        if (processed) {
                                cache += processed;
                                strbuf_setlen(prefix, prefix->len - len);
                                continue;
                        }

                        strbuf_addch(prefix, '/');
                        cache += clear_ce_flags_1(istate, cache, cache_end - cache,
                                                  prefix,
                                                  select_mask, clear_mask, el, defval);
                        strbuf_setlen(prefix, prefix->len - len - 1);
                        continue;
                }

                /* Non-directory */
                dtype = ce_to_dtype(ce);
                ret = is_excluded_from_list(ce->name, ce_namelen(ce),
                                            name, &dtype, el, istate);
                if (ret < 0)
                        ret = defval;
                if (ret > 0)
                        ce->ce_flags &= ~clear_mask;
                cache++;
        }
        return nr - (cache_end - cache);
}

static int clear_ce_flags(struct index_state *istate,
                          int select_mask, int clear_mask,
                          struct exclude_list *el)
{
        static struct strbuf prefix = STRBUF_INIT;

        strbuf_reset(&prefix);

        return clear_ce_flags_1(istate,
                                istate->cache,
                                istate->cache_nr,
                                &prefix,
                                select_mask, clear_mask,
                                el, 0);
}

/*
 * Set/Clear CE_NEW_SKIP_WORKTREE according to $GIT_DIR/info/sparse-checkout
 */
static void mark_new_skip_worktree(struct exclude_list *el,
                                   struct index_state *istate,
                                   int select_flag, int skip_wt_flag)
{
        int i;

        /*
         * 1. Pretend the narrowest worktree: only unmerged entries
         * are checked out
         */
        for (i = 0; i < istate->cache_nr; i++) {
                struct cache_entry *ce = istate->cache[i];

                if (select_flag && !(ce->ce_flags & select_flag))
                        continue;

                if (!ce_stage(ce) && !(ce->ce_flags & CE_CONFLICTED))
                        ce->ce_flags |= skip_wt_flag;
                else
                        ce->ce_flags &= ~skip_wt_flag;
        }

        /*
         * 2. Widen worktree according to sparse-checkout file.
         * Matched entries will have skip_wt_flag cleared (i.e. "in")
         */
        clear_ce_flags(istate, select_flag, skip_wt_flag, el);
}

static int verify_absent(const struct cache_entry *,
                         enum unpack_trees_error_types,
                         struct unpack_trees_options *);
/*
 * N-way merge "len" trees.  Returns 0 on success, -1 on failure to manipulate the
 * resulting index, -2 on failure to reflect the changes to the work tree.
 *
 * CE_ADDED, CE_UNPACKED and CE_NEW_SKIP_WORKTREE are used internally
 */
int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options *o)
{
        int i, ret;
        static struct cache_entry *dfc;
        struct exclude_list el;

        if (len > MAX_UNPACK_TREES)
                die("unpack_trees takes at most %d trees", MAX_UNPACK_TREES);

        trace_performance_enter();
        memset(&el, 0, sizeof(el));
        if (!core_apply_sparse_checkout || !o->update)
                o->skip_sparse_checkout = 1;
        if (!o->skip_sparse_checkout) {
                char *sparse = git_pathdup("info/sparse-checkout");
                if (add_excludes_from_file_to_list(sparse, "", 0, &el, NULL) < 0)
                        o->skip_sparse_checkout = 1;
                else
                        o->el = &el;
                free(sparse);
        }

        memset(&o->result, 0, sizeof(o->result));
        o->result.initialized = 1;
        o->result.timestamp.sec = o->src_index->timestamp.sec;
        o->result.timestamp.nsec = o->src_index->timestamp.nsec;
        o->result.version = o->src_index->version;
        if (!o->src_index->split_index) {
                o->result.split_index = NULL;
        } else if (o->src_index == o->dst_index) {
                /*
                 * o->dst_index (and thus o->src_index) will be discarded
                 * and overwritten with o->result at the end of this function,
                 * so just use src_index's split_index to avoid having to
                 * create a new one.
                 */
                o->result.split_index = o->src_index->split_index;
                o->result.split_index->refcount++;
        } else {
                o->result.split_index = init_split_index(&o->result);
        }
        oidcpy(&o->result.oid, &o->src_index->oid);
        o->merge_size = len;
        mark_all_ce_unused(o->src_index);

        /*
         * Sparse checkout loop #1: set NEW_SKIP_WORKTREE on existing entries
         */
        if (!o->skip_sparse_checkout)
                mark_new_skip_worktree(o->el, o->src_index, 0, CE_NEW_SKIP_WORKTREE);

        if (!dfc)
                dfc = xcalloc(1, cache_entry_size(0));
        o->df_conflict_entry = dfc;

        if (len) {
                const char *prefix = o->prefix ? o->prefix : "";
                struct traverse_info info;

                setup_traverse_info(&info, prefix);
                info.fn = unpack_callback;
                info.data = o;
                info.show_all_errors = o->show_all_errors;
                info.pathspec = o->pathspec;

                if (o->prefix) {
                        /*
                         * Unpack existing index entries that sort before the
                         * prefix the tree is spliced into.  Note that o->merge
                         * is always true in this case.
                         */
                        while (1) {
                                struct cache_entry *ce = next_cache_entry(o);
                                if (!ce)
                                        break;
                                if (ce_in_traverse_path(ce, &info))
                                        break;
                                if (unpack_index_entry(ce, o) < 0)
                                        goto return_failed;
                        }
                }

                trace_performance_enter();
                ret = traverse_trees(o->src_index, len, t, &info);
                trace_performance_leave("traverse_trees");
                if (ret < 0)
                        goto return_failed;
        }

        /* Any left-over entries in the index? */
        if (o->merge) {
                while (1) {
                        struct cache_entry *ce = next_cache_entry(o);
                        if (!ce)
                                break;
                        if (unpack_index_entry(ce, o) < 0)
                                goto return_failed;
                }
        }
        mark_all_ce_unused(o->src_index);

        if (o->trivial_merges_only && o->nontrivial_merge) {
                ret = unpack_failed(o, "Merge requires file-level merging");
                goto done;
        }

        if (!o->skip_sparse_checkout) {
                int empty_worktree = 1;

                /*
                 * Sparse checkout loop #2: set NEW_SKIP_WORKTREE on entries not in loop #1
                 * If the will have NEW_SKIP_WORKTREE, also set CE_SKIP_WORKTREE
                 * so apply_sparse_checkout() won't attempt to remove it from worktree
                 */
                mark_new_skip_worktree(o->el, &o->result, CE_ADDED, CE_SKIP_WORKTREE | CE_NEW_SKIP_WORKTREE);

                ret = 0;
                for (i = 0; i < o->result.cache_nr; i++) {
                        struct cache_entry *ce = o->result.cache[i];

                        /*
                         * Entries marked with CE_ADDED in merged_entry() do not have
                         * verify_absent() check (the check is effectively disabled
                         * because CE_NEW_SKIP_WORKTREE is set unconditionally).
                         *
                         * Do the real check now because we have had
                         * correct CE_NEW_SKIP_WORKTREE
                         */
                        if (ce->ce_flags & CE_ADDED &&
                            verify_absent(ce, ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o)) {
                                if (!o->show_all_errors)
                                        goto return_failed;
                                ret = -1;
                        }

                        if (apply_sparse_checkout(&o->result, ce, o)) {
                                if (!o->show_all_errors)
                                        goto return_failed;
                                ret = -1;
                        }
                        if (!ce_skip_worktree(ce))
                                empty_worktree = 0;

                }
                if (ret < 0)
                        goto return_failed;
                /*
                 * Sparse checkout is meant to narrow down checkout area
                 * but it does not make sense to narrow down to empty working
                 * tree. This is usually a mistake in sparse checkout rules.
                 * Do not allow users to do that.
                 */
                if (o->result.cache_nr && empty_worktree) {
                        ret = unpack_failed(o, "Sparse checkout leaves no entry on working directory");
                        goto done;
                }
        }

        ret = check_updates(o) ? (-2) : 0;
        if (o->dst_index) {
                move_index_extensions(&o->result, o->src_index);
                if (!ret) {
                        if (git_env_bool("GIT_TEST_CHECK_CACHE_TREE", 0))
                                cache_tree_verify(the_repository, &o->result);
                        if (!o->result.cache_tree)
                                o->result.cache_tree = cache_tree();
                        if (!cache_tree_fully_valid(o->result.cache_tree))
                                cache_tree_update(&o->result,
                                                  WRITE_TREE_SILENT |
                                                  WRITE_TREE_REPAIR);
                }

                o->result.updated_workdir = 1;
                discard_index(o->dst_index);
                *o->dst_index = o->result;
        } else {
                discard_index(&o->result);
        }
        o->src_index = NULL;

done:
        trace_performance_leave("unpack_trees");
        clear_exclude_list(&el);
        return ret;

return_failed:
        if (o->show_all_errors)
                display_error_msgs(o);
        mark_all_ce_unused(o->src_index);
        ret = unpack_failed(o, NULL);
        if (o->exiting_early)
                ret = 0;
        goto done;
}

/* Here come the merge functions */

static int reject_merge(const struct cache_entry *ce,
                        struct unpack_trees_options *o)
{
        return add_rejected_path(o, ERROR_WOULD_OVERWRITE, ce->name);
}

static int same(const struct cache_entry *a, const struct cache_entry *b)
{
        if (!!a != !!b)
                return 0;
        if (!a && !b)
                return 1;
        if ((a->ce_flags | b->ce_flags) & CE_CONFLICTED)
                return 0;
        return a->ce_mode == b->ce_mode &&
               oideq(&a->oid, &b->oid);
}


/*
 * When a CE gets turned into an unmerged entry, we
 * want it to be up-to-date
 */
static int verify_uptodate_1(const struct cache_entry *ce,
                             struct unpack_trees_options *o,
                             enum unpack_trees_error_types error_type)
{
        struct stat st;

        if (o->index_only)
                return 0;

        /*
         * CE_VALID and CE_SKIP_WORKTREE cheat, we better check again
         * if this entry is truly up-to-date because this file may be
         * overwritten.
         */
        if ((ce->ce_flags & CE_VALID) || ce_skip_worktree(ce))
                ; /* keep checking */
        else if (o->reset || ce_uptodate(ce))
                return 0;

        if (!lstat(ce->name, &st)) {
                int flags = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE;
                unsigned changed = ie_match_stat(o->src_index, ce, &st, flags);

                if (submodule_from_ce(ce)) {
                        int r = check_submodule_move_head(ce,
                                "HEAD", oid_to_hex(&ce->oid), o);
                        if (r)
                                return add_rejected_path(o, error_type, ce->name);
                        return 0;
                }

                if (!changed)
                        return 0;
                /*
                 * Historic default policy was to allow submodule to be out
                 * of sync wrt the superproject index. If the submodule was
                 * not considered interesting above, we don't care here.
                 */
                if (S_ISGITLINK(ce->ce_mode))
                        return 0;

                errno = 0;
        }
        if (errno == ENOENT)
                return 0;
        return add_rejected_path(o, error_type, ce->name);
}

int verify_uptodate(const struct cache_entry *ce,
                    struct unpack_trees_options *o)
{
        if (!o->skip_sparse_checkout && (ce->ce_flags & CE_NEW_SKIP_WORKTREE))
                return 0;
        return verify_uptodate_1(ce, o, ERROR_NOT_UPTODATE_FILE);
}

static int verify_uptodate_sparse(const struct cache_entry *ce,
                                  struct unpack_trees_options *o)
{
        return verify_uptodate_1(ce, o, ERROR_SPARSE_NOT_UPTODATE_FILE);
}

/*
 * TODO: We should actually invalidate o->result, not src_index [1].
 * But since cache tree and untracked cache both are not copied to
 * o->result until unpacking is complete, we invalidate them on
 * src_index instead with the assumption that they will be copied to
 * dst_index at the end.
 *
 * [1] src_index->cache_tree is also used in unpack_callback() so if
 * we invalidate o->result, we need to update it to use
 * o->result.cache_tree as well.
 */
static void invalidate_ce_path(const struct cache_entry *ce,
                               struct unpack_trees_options *o)
{
        if (!ce)
                return;
        cache_tree_invalidate_path(o->src_index, ce->name);
        untracked_cache_invalidate_path(o->src_index, ce->name, 1);
}

/*
 * Check that checking out ce->sha1 in subdir ce->name is not
 * going to overwrite any working files.
 *
 * Currently, git does not checkout subprojects during a superproject
 * checkout, so it is not going to overwrite anything.
 */
static int verify_clean_submodule(const char *old_sha1,
                                  const struct cache_entry *ce,
                                  struct unpack_trees_options *o)
{
        if (!submodule_from_ce(ce))
                return 0;

        return check_submodule_move_head(ce, old_sha1,
                                         oid_to_hex(&ce->oid), o);
}

static int verify_clean_subdirectory(const struct cache_entry *ce,
                                     struct unpack_trees_options *o)
{
        /*
         * we are about to extract "ce->name"; we would not want to lose
         * anything in the existing directory there.
         */
        int namelen;
        int i;
        struct dir_struct d;
        char *pathbuf;
        int cnt = 0;

        if (S_ISGITLINK(ce->ce_mode)) {
                struct object_id oid;
                int sub_head = resolve_gitlink_ref(ce->name, "HEAD", &oid);
                /*
                 * If we are not going to update the submodule, then
                 * we don't care.
                 */
                if (!sub_head && oideq(&oid, &ce->oid))
                        return 0;
                return verify_clean_submodule(sub_head ? NULL : oid_to_hex(&oid),
                                              ce, o);
        }

        /*
         * First let's make sure we do not have a local modification
         * in that directory.
         */
        namelen = ce_namelen(ce);
        for (i = locate_in_src_index(ce, o);
             i < o->src_index->cache_nr;
             i++) {
                struct cache_entry *ce2 = o->src_index->cache[i];
                int len = ce_namelen(ce2);
                if (len < namelen ||
                    strncmp(ce->name, ce2->name, namelen) ||
                    ce2->name[namelen] != '/')
                        break;
                /*
                 * ce2->name is an entry in the subdirectory to be
                 * removed.
                 */
                if (!ce_stage(ce2)) {
                        if (verify_uptodate(ce2, o))
                                return -1;
                        add_entry(o, ce2, CE_REMOVE, 0);
                        invalidate_ce_path(ce, o);
                        mark_ce_used(ce2, o);
                }
                cnt++;
        }

        /*
         * Then we need to make sure that we do not lose a locally
         * present file that is not ignored.
         */
        pathbuf = xstrfmt("%.*s/", namelen, ce->name);

        memset(&d, 0, sizeof(d));
        if (o->dir)
                d.exclude_per_dir = o->dir->exclude_per_dir;
        i = read_directory(&d, o->src_index, pathbuf, namelen+1, NULL);
        if (i)
                return add_rejected_path(o, ERROR_NOT_UPTODATE_DIR, ce->name);
        free(pathbuf);
        return cnt;
}

/*
 * This gets called when there was no index entry for the tree entry 'dst',
 * but we found a file in the working tree that 'lstat()' said was fine,
 * and we're on a case-insensitive filesystem.
 *
 * See if we can find a case-insensitive match in the index that also
 * matches the stat information, and assume it's that other file!
 */
static int icase_exists(struct unpack_trees_options *o, const char *name, int len, struct stat *st)
{
        const struct cache_entry *src;

        src = index_file_exists(o->src_index, name, len, 1);
        return src && !ie_match_stat(o->src_index, src, st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE);
}

static int check_ok_to_remove(const char *name, int len, int dtype,
                              const struct cache_entry *ce, struct stat *st,
                              enum unpack_trees_error_types error_type,
                              struct unpack_trees_options *o)
{
        const struct cache_entry *result;

        /*
         * It may be that the 'lstat()' succeeded even though
         * target 'ce' was absent, because there is an old
         * entry that is different only in case..
         *
         * Ignore that lstat() if it matches.
         */
        if (ignore_case && icase_exists(o, name, len, st))
                return 0;

        if (o->dir &&
            is_excluded(o->dir, o->src_index, name, &dtype))
                /*
                 * ce->name is explicitly excluded, so it is Ok to
                 * overwrite it.
                 */
                return 0;
        if (S_ISDIR(st->st_mode)) {
                /*
                 * We are checking out path "foo" and
                 * found "foo/." in the working tree.
                 * This is tricky -- if we have modified
                 * files that are in "foo/" we would lose
                 * them.
                 */
                if (verify_clean_subdirectory(ce, o) < 0)
                        return -1;
                return 0;
        }

        /*
         * The previous round may already have decided to
         * delete this path, which is in a subdirectory that
         * is being replaced with a blob.
         */
        result = index_file_exists(&o->result, name, len, 0);
        if (result) {
                if (result->ce_flags & CE_REMOVE)
                        return 0;
        }

        return add_rejected_path(o, error_type, name);
}

/*
 * We do not want to remove or overwrite a working tree file that
 * is not tracked, unless it is ignored.
 */
static int verify_absent_1(const struct cache_entry *ce,
                           enum unpack_trees_error_types error_type,
                           struct unpack_trees_options *o)
{
        int len;
        struct stat st;

        if (o->index_only || o->reset || !o->update)
                return 0;

        len = check_leading_path(ce->name, ce_namelen(ce));
        if (!len)
                return 0;
        else if (len > 0) {
                char *path;
                int ret;

                path = xmemdupz(ce->name, len);
                if (lstat(path, &st))
                        ret = error_errno("cannot stat '%s'", path);
                else {
                        if (submodule_from_ce(ce))
                                ret = check_submodule_move_head(ce,
                                                                oid_to_hex(&ce->oid),
                                                                NULL, o);
                        else
                                ret = check_ok_to_remove(path, len, DT_UNKNOWN, NULL,
                                                         &st, error_type, o);
                }
                free(path);
                return ret;
        } else if (lstat(ce->name, &st)) {
                if (errno != ENOENT)
                        return error_errno("cannot stat '%s'", ce->name);
                return 0;
        } else {
                if (submodule_from_ce(ce))
                        return check_submodule_move_head(ce, oid_to_hex(&ce->oid),
                                                         NULL, o);

                return check_ok_to_remove(ce->name, ce_namelen(ce),
                                          ce_to_dtype(ce), ce, &st,
                                          error_type, o);
        }
}

static int verify_absent(const struct cache_entry *ce,
                         enum unpack_trees_error_types error_type,
                         struct unpack_trees_options *o)
{
        if (!o->skip_sparse_checkout && (ce->ce_flags & CE_NEW_SKIP_WORKTREE))
                return 0;
        return verify_absent_1(ce, error_type, o);
}

static int verify_absent_sparse(const struct cache_entry *ce,
                                enum unpack_trees_error_types error_type,
                                struct unpack_trees_options *o)
{
        enum unpack_trees_error_types orphaned_error = error_type;
        if (orphaned_error == ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN)
                orphaned_error = ERROR_WOULD_LOSE_ORPHANED_OVERWRITTEN;

        return verify_absent_1(ce, orphaned_error, o);
}

static int merged_entry(const struct cache_entry *ce,
                        const struct cache_entry *old,
                        struct unpack_trees_options *o)
{
        int update = CE_UPDATE;
        struct cache_entry *merge = dup_cache_entry(ce, &o->result);

        if (!old) {
                /*
                 * New index entries. In sparse checkout, the following
                 * verify_absent() will be delayed until after
                 * traverse_trees() finishes in unpack_trees(), then:
                 *
                 *  - CE_NEW_SKIP_WORKTREE will be computed correctly
                 *  - verify_absent() be called again, this time with
                 *    correct CE_NEW_SKIP_WORKTREE
                 *
                 * verify_absent() call here does nothing in sparse
                 * checkout (i.e. o->skip_sparse_checkout == 0)
                 */
                update |= CE_ADDED;
                merge->ce_flags |= CE_NEW_SKIP_WORKTREE;

                if (verify_absent(merge,
                                  ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN, o)) {
                        discard_cache_entry(merge);
                        return -1;
                }
                invalidate_ce_path(merge, o);

                if (submodule_from_ce(ce)) {
                        int ret = check_submodule_move_head(ce, NULL,
                                                            oid_to_hex(&ce->oid),
                                                            o);
                        if (ret)
                                return ret;
                }

        } else if (!(old->ce_flags & CE_CONFLICTED)) {
                /*
                 * See if we can re-use the old CE directly?
                 * That way we get the uptodate stat info.
                 *
                 * This also removes the UPDATE flag on a match; otherwise
                 * we will end up overwriting local changes in the work tree.
                 */
                if (same(old, merge)) {
                        copy_cache_entry(merge, old);
                        update = 0;
                } else {
                        if (verify_uptodate(old, o)) {
                                discard_cache_entry(merge);
                                return -1;
                        }
                        /* Migrate old flags over */
                        update |= old->ce_flags & (CE_SKIP_WORKTREE | CE_NEW_SKIP_WORKTREE);
                        invalidate_ce_path(old, o);
                }

                if (submodule_from_ce(ce)) {
                        int ret = check_submodule_move_head(ce, oid_to_hex(&old->oid),
                                                            oid_to_hex(&ce->oid),
                                                            o);
                        if (ret)
                                return ret;
                }
        } else {
                /*
                 * Previously unmerged entry left as an existence
                 * marker by read_index_unmerged();
                 */
                invalidate_ce_path(old, o);
        }

        do_add_entry(o, merge, update, CE_STAGEMASK);
        return 1;
}

static int deleted_entry(const struct cache_entry *ce,
                         const struct cache_entry *old,
                         struct unpack_trees_options *o)
{
        /* Did it exist in the index? */
        if (!old) {
                if (verify_absent(ce, ERROR_WOULD_LOSE_UNTRACKED_REMOVED, o))
                        return -1;
                return 0;
        }
        if (!(old->ce_flags & CE_CONFLICTED) && verify_uptodate(old, o))
                return -1;
        add_entry(o, ce, CE_REMOVE, 0);
        invalidate_ce_path(ce, o);
        return 1;
}

static int keep_entry(const struct cache_entry *ce,
                      struct unpack_trees_options *o)
{
        add_entry(o, ce, 0, 0);
        if (ce_stage(ce))
                invalidate_ce_path(ce, o);
        return 1;
}

#if DBRT_DEBUG
static void show_stage_entry(FILE *o,
                             const char *label, const struct cache_entry *ce)
{
        if (!ce)
                fprintf(o, "%s (missing)\n", label);
        else
                fprintf(o, "%s%06o %s %d\t%s\n",
                        label,
                        ce->ce_mode,
                        oid_to_hex(&ce->oid),
                        ce_stage(ce),
                        ce->name);
}
#endif

int threeway_merge(const struct cache_entry * const *stages,
                   struct unpack_trees_options *o)
{
        const struct cache_entry *index;
        const struct cache_entry *head;
        const struct cache_entry *remote = stages[o->head_idx + 1];
        int count;
        int head_match = 0;
        int remote_match = 0;

        int df_conflict_head = 0;
        int df_conflict_remote = 0;

        int any_anc_missing = 0;
        int no_anc_exists = 1;
        int i;

        for (i = 1; i < o->head_idx; i++) {
                if (!stages[i] || stages[i] == o->df_conflict_entry)
                        any_anc_missing = 1;
                else
                        no_anc_exists = 0;
        }

        index = stages[0];
        head = stages[o->head_idx];

        if (head == o->df_conflict_entry) {
                df_conflict_head = 1;
                head = NULL;
        }

        if (remote == o->df_conflict_entry) {
                df_conflict_remote = 1;
                remote = NULL;
        }

        /*
         * First, if there's a #16 situation, note that to prevent #13
         * and #14.
         */
        if (!same(remote, head)) {
                for (i = 1; i < o->head_idx; i++) {
                        if (same(stages[i], head)) {
                                head_match = i;
                        }
                        if (same(stages[i], remote)) {
                                remote_match = i;
                        }
                }
        }

        /*
         * We start with cases where the index is allowed to match
         * something other than the head: #14(ALT) and #2ALT, where it
         * is permitted to match the result instead.
         */
        /* #14, #14ALT, #2ALT */
        if (remote && !df_conflict_head && head_match && !remote_match) {
                if (index && !same(index, remote) && !same(index, head))
                        return reject_merge(index, o);
                return merged_entry(remote, index, o);
        }
        /*
         * If we have an entry in the index cache, then we want to
         * make sure that it matches head.
         */
        if (index && !same(index, head))
                return reject_merge(index, o);

        if (head) {
                /* #5ALT, #15 */
                if (same(head, remote))
                        return merged_entry(head, index, o);
                /* #13, #3ALT */
                if (!df_conflict_remote && remote_match && !head_match)
                        return merged_entry(head, index, o);
        }

        /* #1 */
        if (!head && !remote && any_anc_missing)
                return 0;

        /*
         * Under the "aggressive" rule, we resolve mostly trivial
         * cases that we historically had git-merge-one-file resolve.
         */
        if (o->aggressive) {
                int head_deleted = !head;
                int remote_deleted = !remote;
                const struct cache_entry *ce = NULL;

                if (index)
                        ce = index;
                else if (head)
                        ce = head;
                else if (remote)
                        ce = remote;
                else {
                        for (i = 1; i < o->head_idx; i++) {
                                if (stages[i] && stages[i] != o->df_conflict_entry) {
                                        ce = stages[i];
                                        break;
                                }
                        }
                }

                /*
                 * Deleted in both.
                 * Deleted in one and unchanged in the other.
                 */
                if ((head_deleted && remote_deleted) ||
                    (head_deleted && remote && remote_match) ||
                    (remote_deleted && head && head_match)) {
                        if (index)
                                return deleted_entry(index, index, o);
                        if (ce && !head_deleted) {
                                if (verify_absent(ce, ERROR_WOULD_LOSE_UNTRACKED_REMOVED, o))
                                        return -1;
                        }
                        return 0;
                }
                /*
                 * Added in both, identically.
                 */
                if (no_anc_exists && head && remote && same(head, remote))
                        return merged_entry(head, index, o);

        }

        /* Below are "no merge" cases, which require that the index be
         * up-to-date to avoid the files getting overwritten with
         * conflict resolution files.
         */
        if (index) {
                if (verify_uptodate(index, o))
                        return -1;
        }

        o->nontrivial_merge = 1;

        /* #2, #3, #4, #6, #7, #9, #10, #11. */
        count = 0;
        if (!head_match || !remote_match) {
                for (i = 1; i < o->head_idx; i++) {
                        if (stages[i] && stages[i] != o->df_conflict_entry) {
                                keep_entry(stages[i], o);
                                count++;
                                break;
                        }
                }
        }
#if DBRT_DEBUG
        else {
                fprintf(stderr, "read-tree: warning #16 detected\n");
                show_stage_entry(stderr, "head   ", stages[head_match]);
                show_stage_entry(stderr, "remote ", stages[remote_match]);
        }
#endif
        if (head) { count += keep_entry(head, o); }
        if (remote) { count += keep_entry(remote, o); }
        return count;
}

/*
 * Two-way merge.
 *
 * The rule is to "carry forward" what is in the index without losing
 * information across a "fast-forward", favoring a successful merge
 * over a merge failure when it makes sense.  For details of the
 * "carry forward" rule, please see <Documentation/git-read-tree.txt>.
 *
 */
int twoway_merge(const struct cache_entry * const *src,
                 struct unpack_trees_options *o)
{
        const struct cache_entry *current = src[0];
        const struct cache_entry *oldtree = src[1];
        const struct cache_entry *newtree = src[2];

        if (o->merge_size != 2)
                return error("Cannot do a twoway merge of %d trees",
                             o->merge_size);

        if (oldtree == o->df_conflict_entry)
                oldtree = NULL;
        if (newtree == o->df_conflict_entry)
                newtree = NULL;

        if (current) {
                if (current->ce_flags & CE_CONFLICTED) {
                        if (same(oldtree, newtree) || o->reset) {
                                if (!newtree)
                                        return deleted_entry(current, current, o);
                                else
                                        return merged_entry(newtree, current, o);
                        }
                        return reject_merge(current, o);
                } else if ((!oldtree && !newtree) || /* 4 and 5 */
                         (!oldtree && newtree &&
                          same(current, newtree)) || /* 6 and 7 */
                         (oldtree && newtree &&
                          same(oldtree, newtree)) || /* 14 and 15 */
                         (oldtree && newtree &&
                          !same(oldtree, newtree) && /* 18 and 19 */
                          same(current, newtree))) {
                        return keep_entry(current, o);
                } else if (oldtree && !newtree && same(current, oldtree)) {
                        /* 10 or 11 */
                        return deleted_entry(oldtree, current, o);
                } else if (oldtree && newtree &&
                         same(current, oldtree) && !same(current, newtree)) {
                        /* 20 or 21 */
                        return merged_entry(newtree, current, o);
                } else
                        return reject_merge(current, o);
        }
        else if (newtree) {
                if (oldtree && !o->initial_checkout) {
                        /*
                         * deletion of the path was staged;
                         */
                        if (same(oldtree, newtree))
                                return 1;
                        return reject_merge(oldtree, o);
                }
                return merged_entry(newtree, current, o);
        }
        return deleted_entry(oldtree, current, o);
}

/*
 * Bind merge.
 *
 * Keep the index entries at stage0, collapse stage1 but make sure
 * stage0 does not have anything there.
 */
int bind_merge(const struct cache_entry * const *src,
               struct unpack_trees_options *o)
{
        const struct cache_entry *old = src[0];
        const struct cache_entry *a = src[1];

        if (o->merge_size != 1)
                return error("Cannot do a bind merge of %d trees",
                             o->merge_size);
        if (a && old)
                return o->quiet ? -1 :
                        error(ERRORMSG(o, ERROR_BIND_OVERLAP),
                              super_prefixed(a->name),
                              super_prefixed(old->name));
        if (!a)
                return keep_entry(old, o);
        else
                return merged_entry(a, NULL, o);
}

/*
 * One-way merge.
 *
 * The rule is:
 * - take the stat information from stage0, take the data from stage1
 */
int oneway_merge(const struct cache_entry * const *src,
                 struct unpack_trees_options *o)
{
        const struct cache_entry *old = src[0];
        const struct cache_entry *a = src[1];

        if (o->merge_size != 1)
                return error("Cannot do a oneway merge of %d trees",
                             o->merge_size);

        if (!a || a == o->df_conflict_entry)
                return deleted_entry(old, old, o);

        if (old && same(old, a)) {
                int update = 0;
                if (o->reset && o->update && !ce_uptodate(old) && !ce_skip_worktree(old)) {
                        struct stat st;
                        if (lstat(old->name, &st) ||
                            ie_match_stat(o->src_index, old, &st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE))
                                update |= CE_UPDATE;
                }
                if (o->update && S_ISGITLINK(old->ce_mode) &&
                    should_update_submodules() && !verify_uptodate(old, o))
                        update |= CE_UPDATE;
                add_entry(o, old, update, CE_STAGEMASK);
                return 0;
        }
        return merged_entry(a, old, o);
}