Merge branch 'dw/subtree-split-do-not-drop-merge' into maint

[alt-git.git] / rerere.c

#include "cache.h"
#include "lockfile.h"
#include "string-list.h"
#include "rerere.h"
#include "xdiff-interface.h"
#include "dir.h"
#include "resolve-undo.h"
#include "ll-merge.h"
#include "attr.h"
#include "pathspec.h"

#define RESOLVED 0
#define PUNTED 1
#define THREE_STAGED 2
void *RERERE_RESOLVED = &RERERE_RESOLVED;

/* if rerere_enabled == -1, fall back to detection of .git/rr-cache */
static int rerere_enabled = -1;

/* automatically update cleanly resolved paths to the index */
static int rerere_autoupdate;

static void free_rerere_id(struct string_list_item *item)
{
        free(item->util);
}

static const char *rerere_id_hex(const struct rerere_id *id)
{
        return id->hex;
}

const char *rerere_path(const struct rerere_id *id, const char *file)
{
        if (!file)
                return git_path("rr-cache/%s", rerere_id_hex(id));

        return git_path("rr-cache/%s/%s", rerere_id_hex(id), file);
}

static int has_rerere_resolution(const struct rerere_id *id)
{
        struct stat st;

        return !stat(rerere_path(id, "postimage"), &st);
}

static struct rerere_id *new_rerere_id_hex(char *hex)
{
        struct rerere_id *id = xmalloc(sizeof(*id));
        strcpy(id->hex, hex);
        return id;
}

static struct rerere_id *new_rerere_id(unsigned char *sha1)
{
        return new_rerere_id_hex(sha1_to_hex(sha1));
}

/*
 * $GIT_DIR/MERGE_RR file is a collection of records, each of which is
 * "conflict ID", a HT and pathname, terminated with a NUL, and is
 * used to keep track of the set of paths that "rerere" may need to
 * work on (i.e. what is left by the previous invocation of "git
 * rerere" during the current conflict resolution session).
 */
static void read_rr(struct string_list *rr)
{
        struct strbuf buf = STRBUF_INIT;
        FILE *in = fopen(git_path_merge_rr(), "r");

        if (!in)
                return;
        while (!strbuf_getwholeline(&buf, in, '\0')) {
                char *path;
                unsigned char sha1[20];
                struct rerere_id *id;

                /* There has to be the hash, tab, path and then NUL */
                if (buf.len < 42 || get_sha1_hex(buf.buf, sha1))
                        die("corrupt MERGE_RR");

                if (buf.buf[40] != '\t')
                        die("corrupt MERGE_RR");
                buf.buf[40] = '\0';
                path = buf.buf + 41;
                id = new_rerere_id_hex(buf.buf);
                string_list_insert(rr, path)->util = id;
        }
        strbuf_release(&buf);
        fclose(in);
}

static struct lock_file write_lock;

static int write_rr(struct string_list *rr, int out_fd)
{
        int i;
        for (i = 0; i < rr->nr; i++) {
                struct strbuf buf = STRBUF_INIT;
                struct rerere_id *id;

                assert(rr->items[i].util != RERERE_RESOLVED);

                id = rr->items[i].util;
                if (!id)
                        continue;
                strbuf_addf(&buf, "%s\t%s%c",
                            rerere_id_hex(id),
                            rr->items[i].string, 0);
                if (write_in_full(out_fd, buf.buf, buf.len) != buf.len)
                        die("unable to write rerere record");

                strbuf_release(&buf);
        }
        if (commit_lock_file(&write_lock) != 0)
                die("unable to write rerere record");
        return 0;
}

/*
 * "rerere" interacts with conflicted file contents using this I/O
 * abstraction.  It reads a conflicted contents from one place via
 * "getline()" method, and optionally can write it out after
 * normalizing the conflicted hunks to the "output".  Subclasses of
 * rerere_io embed this structure at the beginning of their own
 * rerere_io object.
 */
struct rerere_io {
        int (*getline)(struct strbuf *, struct rerere_io *);
        FILE *output;
        int wrerror;
        /* some more stuff */
};

static void ferr_write(const void *p, size_t count, FILE *fp, int *err)
{
        if (!count || *err)
                return;
        if (fwrite(p, count, 1, fp) != 1)
                *err = errno;
}

static inline void ferr_puts(const char *s, FILE *fp, int *err)
{
        ferr_write(s, strlen(s), fp, err);
}

static void rerere_io_putstr(const char *str, struct rerere_io *io)
{
        if (io->output)
                ferr_puts(str, io->output, &io->wrerror);
}

/*
 * Write a conflict marker to io->output (if defined).
 */
static void rerere_io_putconflict(int ch, int size, struct rerere_io *io)
{
        char buf[64];

        while (size) {
                if (size <= sizeof(buf) - 2) {
                        memset(buf, ch, size);
                        buf[size] = '\n';
                        buf[size + 1] = '\0';
                        size = 0;
                } else {
                        int sz = sizeof(buf) - 1;

                        /*
                         * Make sure we will not write everything out
                         * in this round by leaving at least 1 byte
                         * for the next round, giving the next round
                         * a chance to add the terminating LF.  Yuck.
                         */
                        if (size <= sz)
                                sz -= (sz - size) + 1;
                        memset(buf, ch, sz);
                        buf[sz] = '\0';
                        size -= sz;
                }
                rerere_io_putstr(buf, io);
        }
}

static void rerere_io_putmem(const char *mem, size_t sz, struct rerere_io *io)
{
        if (io->output)
                ferr_write(mem, sz, io->output, &io->wrerror);
}

/*
 * Subclass of rerere_io that reads from an on-disk file
 */
struct rerere_io_file {
        struct rerere_io io;
        FILE *input;
};

/*
 * ... and its getline() method implementation
 */
static int rerere_file_getline(struct strbuf *sb, struct rerere_io *io_)
{
        struct rerere_io_file *io = (struct rerere_io_file *)io_;
        return strbuf_getwholeline(sb, io->input, '\n');
}

/*
 * Require the exact number of conflict marker letters, no more, no
 * less, followed by SP or any whitespace
 * (including LF).
 */
static int is_cmarker(char *buf, int marker_char, int marker_size)
{
        int want_sp;

        /*
         * The beginning of our version and the end of their version
         * always are labeled like "<<<<< ours" or ">>>>> theirs",
         * hence we set want_sp for them.  Note that the version from
         * the common ancestor in diff3-style output is not always
         * labelled (e.g. "||||| common" is often seen but "|||||"
         * alone is also valid), so we do not set want_sp.
         */
        want_sp = (marker_char == '<') || (marker_char == '>');

        while (marker_size--)
                if (*buf++ != marker_char)
                        return 0;
        if (want_sp && *buf != ' ')
                return 0;
        return isspace(*buf);
}

/*
 * Read contents a file with conflicts, normalize the conflicts
 * by (1) discarding the common ancestor version in diff3-style,
 * (2) reordering our side and their side so that whichever sorts
 * alphabetically earlier comes before the other one, while
 * computing the "conflict ID", which is just an SHA-1 hash of
 * one side of the conflict, NUL, the other side of the conflict,
 * and NUL concatenated together.
 *
 * Return the number of conflict hunks found.
 *
 * NEEDSWORK: the logic and theory of operation behind this conflict
 * normalization may deserve to be documented somewhere, perhaps in
 * Documentation/technical/rerere.txt.
 */
static int handle_path(unsigned char *sha1, struct rerere_io *io, int marker_size)
{
        git_SHA_CTX ctx;
        int hunk_no = 0;
        enum {
                RR_CONTEXT = 0, RR_SIDE_1, RR_SIDE_2, RR_ORIGINAL
        } hunk = RR_CONTEXT;
        struct strbuf one = STRBUF_INIT, two = STRBUF_INIT;
        struct strbuf buf = STRBUF_INIT;

        if (sha1)
                git_SHA1_Init(&ctx);

        while (!io->getline(&buf, io)) {
                if (is_cmarker(buf.buf, '<', marker_size)) {
                        if (hunk != RR_CONTEXT)
                                goto bad;
                        hunk = RR_SIDE_1;
                } else if (is_cmarker(buf.buf, '|', marker_size)) {
                        if (hunk != RR_SIDE_1)
                                goto bad;
                        hunk = RR_ORIGINAL;
                } else if (is_cmarker(buf.buf, '=', marker_size)) {
                        if (hunk != RR_SIDE_1 && hunk != RR_ORIGINAL)
                                goto bad;
                        hunk = RR_SIDE_2;
                } else if (is_cmarker(buf.buf, '>', marker_size)) {
                        if (hunk != RR_SIDE_2)
                                goto bad;
                        if (strbuf_cmp(&one, &two) > 0)
                                strbuf_swap(&one, &two);
                        hunk_no++;
                        hunk = RR_CONTEXT;
                        rerere_io_putconflict('<', marker_size, io);
                        rerere_io_putmem(one.buf, one.len, io);
                        rerere_io_putconflict('=', marker_size, io);
                        rerere_io_putmem(two.buf, two.len, io);
                        rerere_io_putconflict('>', marker_size, io);
                        if (sha1) {
                                git_SHA1_Update(&ctx, one.buf ? one.buf : "",
                                            one.len + 1);
                                git_SHA1_Update(&ctx, two.buf ? two.buf : "",
                                            two.len + 1);
                        }
                        strbuf_reset(&one);
                        strbuf_reset(&two);
                } else if (hunk == RR_SIDE_1)
                        strbuf_addbuf(&one, &buf);
                else if (hunk == RR_ORIGINAL)
                        ; /* discard */
                else if (hunk == RR_SIDE_2)
                        strbuf_addbuf(&two, &buf);
                else
                        rerere_io_putstr(buf.buf, io);
                continue;
        bad:
                hunk = 99; /* force error exit */
                break;
        }
        strbuf_release(&one);
        strbuf_release(&two);
        strbuf_release(&buf);

        if (sha1)
                git_SHA1_Final(sha1, &ctx);
        if (hunk != RR_CONTEXT)
                return -1;
        return hunk_no;
}

/*
 * Scan the path for conflicts, do the "handle_path()" thing above, and
 * return the number of conflict hunks found.
 */
static int handle_file(const char *path, unsigned char *sha1, const char *output)
{
        int hunk_no = 0;
        struct rerere_io_file io;
        int marker_size = ll_merge_marker_size(path);

        memset(&io, 0, sizeof(io));
        io.io.getline = rerere_file_getline;
        io.input = fopen(path, "r");
        io.io.wrerror = 0;
        if (!io.input)
                return error("Could not open %s", path);

        if (output) {
                io.io.output = fopen(output, "w");
                if (!io.io.output) {
                        fclose(io.input);
                        return error("Could not write %s", output);
                }
        }

        hunk_no = handle_path(sha1, (struct rerere_io *)&io, marker_size);

        fclose(io.input);
        if (io.io.wrerror)
                error("There were errors while writing %s (%s)",
                      path, strerror(io.io.wrerror));
        if (io.io.output && fclose(io.io.output))
                io.io.wrerror = error("Failed to flush %s: %s",
                                      path, strerror(errno));

        if (hunk_no < 0) {
                if (output)
                        unlink_or_warn(output);
                return error("Could not parse conflict hunks in %s", path);
        }
        if (io.io.wrerror)
                return -1;
        return hunk_no;
}

/*
 * Subclass of rerere_io that reads from an in-core buffer that is a
 * strbuf
 */
struct rerere_io_mem {
        struct rerere_io io;
        struct strbuf input;
};

/*
 * ... and its getline() method implementation
 */
static int rerere_mem_getline(struct strbuf *sb, struct rerere_io *io_)
{
        struct rerere_io_mem *io = (struct rerere_io_mem *)io_;
        char *ep;
        size_t len;

        strbuf_release(sb);
        if (!io->input.len)
                return -1;
        ep = memchr(io->input.buf, '\n', io->input.len);
        if (!ep)
                ep = io->input.buf + io->input.len;
        else if (*ep == '\n')
                ep++;
        len = ep - io->input.buf;
        strbuf_add(sb, io->input.buf, len);
        strbuf_remove(&io->input, 0, len);
        return 0;
}

static int handle_cache(const char *path, unsigned char *sha1, const char *output)
{
        mmfile_t mmfile[3] = {{NULL}};
        mmbuffer_t result = {NULL, 0};
        const struct cache_entry *ce;
        int pos, len, i, hunk_no;
        struct rerere_io_mem io;
        int marker_size = ll_merge_marker_size(path);

        /*
         * Reproduce the conflicted merge in-core
         */
        len = strlen(path);
        pos = cache_name_pos(path, len);
        if (0 <= pos)
                return -1;
        pos = -pos - 1;

        while (pos < active_nr) {
                enum object_type type;
                unsigned long size;

                ce = active_cache[pos++];
                if (ce_namelen(ce) != len || memcmp(ce->name, path, len))
                        break;
                i = ce_stage(ce) - 1;
                if (!mmfile[i].ptr) {
                        mmfile[i].ptr = read_sha1_file(ce->sha1, &type, &size);
                        mmfile[i].size = size;
                }
        }
        for (i = 0; i < 3; i++)
                if (!mmfile[i].ptr && !mmfile[i].size)
                        mmfile[i].ptr = xstrdup("");

        /*
         * NEEDSWORK: handle conflicts from merges with
         * merge.renormalize set, too
         */
        ll_merge(&result, path, &mmfile[0], NULL,
                 &mmfile[1], "ours",
                 &mmfile[2], "theirs", NULL);
        for (i = 0; i < 3; i++)
                free(mmfile[i].ptr);

        memset(&io, 0, sizeof(io));
        io.io.getline = rerere_mem_getline;
        if (output)
                io.io.output = fopen(output, "w");
        else
                io.io.output = NULL;
        strbuf_init(&io.input, 0);
        strbuf_attach(&io.input, result.ptr, result.size, result.size);

        /*
         * Grab the conflict ID and optionally write the original
         * contents with conflict markers out.
         */
        hunk_no = handle_path(sha1, (struct rerere_io *)&io, marker_size);
        strbuf_release(&io.input);
        if (io.io.output)
                fclose(io.io.output);
        return hunk_no;
}

/*
 * Look at a cache entry at "i" and see if it is not conflicting,
 * conflicting and we are willing to handle, or conflicting and
 * we are unable to handle, and return the determination in *type.
 * Return the cache index to be looked at next, by skipping the
 * stages we have already looked at in this invocation of this
 * function.
 */
static int check_one_conflict(int i, int *type)
{
        const struct cache_entry *e = active_cache[i];

        if (!ce_stage(e)) {
                *type = RESOLVED;
                return i + 1;
        }

        *type = PUNTED;
        while (ce_stage(active_cache[i]) == 1)
                i++;

        /* Only handle regular files with both stages #2 and #3 */
        if (i + 1 < active_nr) {
                const struct cache_entry *e2 = active_cache[i];
                const struct cache_entry *e3 = active_cache[i + 1];
                if (ce_stage(e2) == 2 &&
                    ce_stage(e3) == 3 &&
                    ce_same_name(e, e3) &&
                    S_ISREG(e2->ce_mode) &&
                    S_ISREG(e3->ce_mode))
                        *type = THREE_STAGED;
        }

        /* Skip the entries with the same name */
        while (i < active_nr && ce_same_name(e, active_cache[i]))
                i++;
        return i;
}

/*
 * Scan the index and find paths that have conflicts that rerere can
 * handle, i.e. the ones that has both stages #2 and #3.
 *
 * NEEDSWORK: we do not record or replay a previous "resolve by
 * deletion" for a delete-modify conflict, as that is inherently risky
 * without knowing what modification is being discarded.  The only
 * safe case, i.e. both side doing the deletion and modification that
 * are identical to the previous round, might want to be handled,
 * though.
 */
static int find_conflict(struct string_list *conflict)
{
        int i;
        if (read_cache() < 0)
                return error("Could not read index");

        for (i = 0; i < active_nr;) {
                int conflict_type;
                const struct cache_entry *e = active_cache[i];
                i = check_one_conflict(i, &conflict_type);
                if (conflict_type == THREE_STAGED)
                        string_list_insert(conflict, (const char *)e->name);
        }
        return 0;
}

/*
 * The merge_rr list is meant to hold outstanding conflicted paths
 * that rerere could handle.  Abuse the list by adding other types of
 * entries to allow the caller to show "rerere remaining".
 *
 * - Conflicted paths that rerere does not handle are added
 * - Conflicted paths that have been resolved are marked as such
 *   by storing RERERE_RESOLVED to .util field (where conflict ID
 *   is expected to be stored).
 *
 * Do *not* write MERGE_RR file out after calling this function.
 *
 * NEEDSWORK: we may want to fix the caller that implements "rerere
 * remaining" to do this without abusing merge_rr.
 */
int rerere_remaining(struct string_list *merge_rr)
{
        int i;
        if (setup_rerere(merge_rr, RERERE_READONLY))
                return 0;
        if (read_cache() < 0)
                return error("Could not read index");

        for (i = 0; i < active_nr;) {
                int conflict_type;
                const struct cache_entry *e = active_cache[i];
                i = check_one_conflict(i, &conflict_type);
                if (conflict_type == PUNTED)
                        string_list_insert(merge_rr, (const char *)e->name);
                else if (conflict_type == RESOLVED) {
                        struct string_list_item *it;
                        it = string_list_lookup(merge_rr, (const char *)e->name);
                        if (it != NULL) {
                                free_rerere_id(it);
                                it->util = RERERE_RESOLVED;
                        }
                }
        }
        return 0;
}

/*
 * Find the conflict identified by "id"; the change between its
 * "preimage" (i.e. a previous contents with conflict markers) and its
 * "postimage" (i.e. the corresponding contents with conflicts
 * resolved) may apply cleanly to the contents stored in "path", i.e.
 * the conflict this time around.
 *
 * Returns 0 for successful replay of recorded resolution, or non-zero
 * for failure.
 */
static int merge(const struct rerere_id *id, const char *path)
{
        FILE *f;
        int ret;
        mmfile_t cur = {NULL, 0}, base = {NULL, 0}, other = {NULL, 0};
        mmbuffer_t result = {NULL, 0};

        /*
         * Normalize the conflicts in path and write it out to
         * "thisimage" temporary file.
         */
        if (handle_file(path, NULL, rerere_path(id, "thisimage")) < 0) {
                ret = 1;
                goto out;
        }

        if (read_mmfile(&cur, rerere_path(id, "thisimage")) ||
            read_mmfile(&base, rerere_path(id, "preimage")) ||
            read_mmfile(&other, rerere_path(id, "postimage"))) {
                ret = 1;
                goto out;
        }

        /*
         * A three-way merge. Note that this honors user-customizable
         * low-level merge driver settings.
         */
        ret = ll_merge(&result, path, &base, NULL, &cur, "", &other, "", NULL);
        if (ret)
                goto out;

        /*
         * A successful replay of recorded resolution.
         * Mark that "postimage" was used to help gc.
         */
        if (utime(rerere_path(id, "postimage"), NULL) < 0)
                warning("failed utime() on %s: %s",
                        rerere_path(id, "postimage"),
                        strerror(errno));

        /* Update "path" with the resolution */
        f = fopen(path, "w");
        if (!f)
                return error("Could not open %s: %s", path,
                             strerror(errno));
        if (fwrite(result.ptr, result.size, 1, f) != 1)
                error("Could not write %s: %s", path, strerror(errno));
        if (fclose(f))
                return error("Writing %s failed: %s", path,
                             strerror(errno));

out:
        free(cur.ptr);
        free(base.ptr);
        free(other.ptr);
        free(result.ptr);

        return ret;
}

static struct lock_file index_lock;

static void update_paths(struct string_list *update)
{
        int i;

        hold_locked_index(&index_lock, 1);

        for (i = 0; i < update->nr; i++) {
                struct string_list_item *item = &update->items[i];
                if (add_file_to_cache(item->string, 0))
                        exit(128);
                fprintf(stderr, "Staged '%s' using previous resolution.\n",
                        item->string);
        }

        if (active_cache_changed) {
                if (write_locked_index(&the_index, &index_lock, COMMIT_LOCK))
                        die("Unable to write new index file");
        } else
                rollback_lock_file(&index_lock);
}

/*
 * The path indicated by rr_item may still have conflict for which we
 * have a recorded resolution, in which case replay it and optionally
 * update it.  Or it may have been resolved by the user and we may
 * only have the preimage for that conflict, in which case the result
 * needs to be recorded as a resolution in a postimage file.
 */
static void do_rerere_one_path(struct string_list_item *rr_item,
                               struct string_list *update)
{
        const char *path = rr_item->string;
        const struct rerere_id *id = rr_item->util;

        /* Is there a recorded resolution we could attempt to apply? */
        if (has_rerere_resolution(id)) {
                if (merge(id, path))
                        return; /* failed to replay */

                if (rerere_autoupdate)
                        string_list_insert(update, path);
                else
                        fprintf(stderr,
                                "Resolved '%s' using previous resolution.\n",
                                path);
        } else if (!handle_file(path, NULL, NULL)) {
                /* The user has resolved it. */
                copy_file(rerere_path(id, "postimage"), path, 0666);
                fprintf(stderr, "Recorded resolution for '%s'.\n", path);
        } else {
                return;
        }
        free_rerere_id(rr_item);
        rr_item->util = NULL;
}

static int do_plain_rerere(struct string_list *rr, int fd)
{
        struct string_list conflict = STRING_LIST_INIT_DUP;
        struct string_list update = STRING_LIST_INIT_DUP;
        int i;

        find_conflict(&conflict);

        /*
         * MERGE_RR records paths with conflicts immediately after
         * merge failed.  Some of the conflicted paths might have been
         * hand resolved in the working tree since then, but the
         * initial run would catch all and register their preimages.
         */
        for (i = 0; i < conflict.nr; i++) {
                struct rerere_id *id;
                unsigned char sha1[20];
                const char *path = conflict.items[i].string;
                int ret;

                if (string_list_has_string(rr, path))
                        continue;

                /*
                 * Ask handle_file() to scan and assign a
                 * conflict ID.  No need to write anything out
                 * yet.
                 */
                ret = handle_file(path, sha1, NULL);
                if (ret < 1)
                        continue;

                id = new_rerere_id(sha1);
                string_list_insert(rr, path)->util = id;

                /*
                 * If the directory does not exist, create
                 * it.  mkdir_in_gitdir() will fail with
                 * EEXIST if there already is one.
                 *
                 * NEEDSWORK: make sure "gc" does not remove
                 * preimage without removing the directory.
                 */
                if (mkdir_in_gitdir(rerere_path(id, NULL)))
                        continue;

                /*
                 * We are the first to encounter this
                 * conflict.  Ask handle_file() to write the
                 * normalized contents to the "preimage" file.
                 */
                handle_file(path, NULL, rerere_path(id, "preimage"));
                fprintf(stderr, "Recorded preimage for '%s'\n", path);
        }

        for (i = 0; i < rr->nr; i++)
                do_rerere_one_path(&rr->items[i], &update);

        if (update.nr)
                update_paths(&update);

        return write_rr(rr, fd);
}

static void git_rerere_config(void)
{
        git_config_get_bool("rerere.enabled", &rerere_enabled);
        git_config_get_bool("rerere.autoupdate", &rerere_autoupdate);
        git_config(git_default_config, NULL);
}

static GIT_PATH_FUNC(git_path_rr_cache, "rr-cache")

static int is_rerere_enabled(void)
{
        int rr_cache_exists;

        if (!rerere_enabled)
                return 0;

        rr_cache_exists = is_directory(git_path_rr_cache());
        if (rerere_enabled < 0)
                return rr_cache_exists;

        if (!rr_cache_exists && mkdir_in_gitdir(git_path_rr_cache()))
                die("Could not create directory %s", git_path_rr_cache());
        return 1;
}

int setup_rerere(struct string_list *merge_rr, int flags)
{
        int fd;

        git_rerere_config();
        if (!is_rerere_enabled())
                return -1;

        if (flags & (RERERE_AUTOUPDATE|RERERE_NOAUTOUPDATE))
                rerere_autoupdate = !!(flags & RERERE_AUTOUPDATE);
        if (flags & RERERE_READONLY)
                fd = 0;
        else
                fd = hold_lock_file_for_update(&write_lock, git_path_merge_rr(),
                                               LOCK_DIE_ON_ERROR);
        read_rr(merge_rr);
        return fd;
}

/*
 * The main entry point that is called internally from codepaths that
 * perform mergy operations, possibly leaving conflicted index entries
 * and working tree files.
 */
int rerere(int flags)
{
        struct string_list merge_rr = STRING_LIST_INIT_DUP;
        int fd;

        fd = setup_rerere(&merge_rr, flags);
        if (fd < 0)
                return 0;
        return do_plain_rerere(&merge_rr, fd);
}

static int rerere_forget_one_path(const char *path, struct string_list *rr)
{
        const char *filename;
        struct rerere_id *id;
        unsigned char sha1[20];
        int ret;
        struct string_list_item *item;

        /*
         * Recreate the original conflict from the stages in the
         * index and compute the conflict ID
         */
        ret = handle_cache(path, sha1, NULL);
        if (ret < 1)
                return error("Could not parse conflict hunks in '%s'", path);

        /* Nuke the recorded resolution for the conflict */
        id = new_rerere_id(sha1);
        filename = rerere_path(id, "postimage");
        if (unlink(filename))
                return (errno == ENOENT
                        ? error("no remembered resolution for %s", path)
                        : error("cannot unlink %s: %s", filename, strerror(errno)));

        /*
         * Update the preimage so that the user can resolve the
         * conflict in the working tree, run us again to record
         * the postimage.
         */
        handle_cache(path, sha1, rerere_path(id, "preimage"));
        fprintf(stderr, "Updated preimage for '%s'\n", path);

        /*
         * And remember that we can record resolution for this
         * conflict when the user is done.
         */
        item = string_list_insert(rr, path);
        free_rerere_id(item);
        item->util = id;
        fprintf(stderr, "Forgot resolution for %s\n", path);
        return 0;
}

int rerere_forget(struct pathspec *pathspec)
{
        int i, fd;
        struct string_list conflict = STRING_LIST_INIT_DUP;
        struct string_list merge_rr = STRING_LIST_INIT_DUP;

        if (read_cache() < 0)
                return error("Could not read index");

        fd = setup_rerere(&merge_rr, RERERE_NOAUTOUPDATE);
        if (fd < 0)
                return 0;

        /*
         * The paths may have been resolved (incorrectly);
         * recover the original conflicted state and then
         * find the conflicted paths.
         */
        unmerge_cache(pathspec);
        find_conflict(&conflict);
        for (i = 0; i < conflict.nr; i++) {
                struct string_list_item *it = &conflict.items[i];
                if (!match_pathspec(pathspec, it->string,
                                    strlen(it->string), 0, NULL, 0))
                        continue;
                rerere_forget_one_path(it->string, &merge_rr);
        }
        return write_rr(&merge_rr, fd);
}

/*
 * Garbage collection support
 */

/*
 * Note that this is not reentrant but is used only one-at-a-time
 * so it does not matter right now.
 */
static struct rerere_id *dirname_to_id(const char *name)
{
        static struct rerere_id id;
        strcpy(id.hex, name);
        return &id;
}

static time_t rerere_created_at(const char *dir_name)
{
        struct stat st;
        struct rerere_id *id = dirname_to_id(dir_name);

        return stat(rerere_path(id, "preimage"), &st) ? (time_t) 0 : st.st_mtime;
}

static time_t rerere_last_used_at(const char *dir_name)
{
        struct stat st;
        struct rerere_id *id = dirname_to_id(dir_name);

        return stat(rerere_path(id, "postimage"), &st) ? (time_t) 0 : st.st_mtime;
}

/*
 * Remove the recorded resolution for a given conflict ID
 */
static void unlink_rr_item(struct rerere_id *id)
{
        unlink(rerere_path(id, "thisimage"));
        unlink(rerere_path(id, "preimage"));
        unlink(rerere_path(id, "postimage"));
        /*
         * NEEDSWORK: what if this rmdir() fails?  Wouldn't we then
         * assume that we already have preimage recorded in
         * do_plain_rerere()?
         */
        rmdir(rerere_path(id, NULL));
}

void rerere_gc(struct string_list *rr)
{
        struct string_list to_remove = STRING_LIST_INIT_DUP;
        DIR *dir;
        struct dirent *e;
        int i, cutoff;
        time_t now = time(NULL), then;
        int cutoff_noresolve = 15;
        int cutoff_resolve = 60;

        if (setup_rerere(rr, 0) < 0)
                return;

        git_config_get_int("gc.rerereresolved", &cutoff_resolve);
        git_config_get_int("gc.rerereunresolved", &cutoff_noresolve);
        git_config(git_default_config, NULL);
        dir = opendir(git_path("rr-cache"));
        if (!dir)
                die_errno("unable to open rr-cache directory");
        /* Collect stale conflict IDs ... */
        while ((e = readdir(dir))) {
                if (is_dot_or_dotdot(e->d_name))
                        continue;

                then = rerere_last_used_at(e->d_name);
                if (then) {
                        cutoff = cutoff_resolve;
                } else {
                        then = rerere_created_at(e->d_name);
                        if (!then)
                                continue;
                        cutoff = cutoff_noresolve;
                }
                if (then < now - cutoff * 86400)
                        string_list_append(&to_remove, e->d_name);
        }
        closedir(dir);
        /* ... and then remove them one-by-one */
        for (i = 0; i < to_remove.nr; i++)
                unlink_rr_item(dirname_to_id(to_remove.items[i].string));
        string_list_clear(&to_remove, 0);
        rollback_lock_file(&write_lock);
}

/*
 * During a conflict resolution, after "rerere" recorded the
 * preimages, abandon them if the user did not resolve them or
 * record their resolutions.  And drop $GIT_DIR/MERGE_RR.
 *
 * NEEDSWORK: shouldn't we be calling this from "reset --hard"?
 */
void rerere_clear(struct string_list *merge_rr)
{
        int i;

        if (setup_rerere(merge_rr, 0) < 0)
                return;

        for (i = 0; i < merge_rr->nr; i++) {
                struct rerere_id *id = merge_rr->items[i].util;
                if (!has_rerere_resolution(id))
                        unlink_rr_item(id);
        }
        unlink_or_warn(git_path_merge_rr());
        rollback_lock_file(&write_lock);
}