rerere: do use multiple variants

[thirdparty/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"
#include "sha1-lookup.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 char *merge_rr_path;

static int rerere_dir_nr;
static int rerere_dir_alloc;

#define RR_HAS_POSTIMAGE 1
#define RR_HAS_PREIMAGE 2
static struct rerere_dir {
	unsigned char sha1[20];
	int status_alloc, status_nr;
	unsigned char *status;
} **rerere_dir;

static void free_rerere_dirs(void)
{
	int i;
	for (i = 0; i < rerere_dir_nr; i++) {
		free(rerere_dir[i]->status);
		free(rerere_dir[i]);
	}
	free(rerere_dir);
	rerere_dir_nr = rerere_dir_alloc = 0;
	rerere_dir = NULL;
}

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 sha1_to_hex(id->collection->sha1);
}

static void fit_variant(struct rerere_dir *rr_dir, int variant)
{
	variant++;
	ALLOC_GROW(rr_dir->status, variant, rr_dir->status_alloc);
	if (rr_dir->status_nr < variant) {
		memset(rr_dir->status + rr_dir->status_nr,
		       '\0', variant - rr_dir->status_nr);
		rr_dir->status_nr = variant;
	}
}

static void assign_variant(struct rerere_id *id)
{
	int variant;
	struct rerere_dir *rr_dir = id->collection;

	variant = id->variant;
	if (variant < 0) {
		for (variant = 0; variant < rr_dir->status_nr; variant++)
			if (!rr_dir->status[variant])
				break;
	}
	fit_variant(rr_dir, variant);
	id->variant = variant;
}

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

	if (id->variant <= 0)
		return git_path("rr-cache/%s/%s", rerere_id_hex(id), file);

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

static int is_rr_file(const char *name, const char *filename, int *variant)
{
	const char *suffix;
	char *ep;

	if (!strcmp(name, filename)) {
		*variant = 0;
		return 1;
	}
	if (!skip_prefix(name, filename, &suffix) || *suffix != '.')
		return 0;

	errno = 0;
	*variant = strtol(suffix + 1, &ep, 10);
	if (errno || *ep)
		return 0;
	return 1;
}

static void scan_rerere_dir(struct rerere_dir *rr_dir)
{
	struct dirent *de;
	DIR *dir = opendir(git_path("rr-cache/%s", sha1_to_hex(rr_dir->sha1)));

	if (!dir)
		return;
	while ((de = readdir(dir)) != NULL) {
		int variant;

		if (is_rr_file(de->d_name, "postimage", &variant)) {
			fit_variant(rr_dir, variant);
			rr_dir->status[variant] |= RR_HAS_POSTIMAGE;
		} else if (is_rr_file(de->d_name, "preimage", &variant)) {
			fit_variant(rr_dir, variant);
			rr_dir->status[variant] |= RR_HAS_PREIMAGE;
		}
	}
	closedir(dir);
}

static const unsigned char *rerere_dir_sha1(size_t i, void *table)
{
	struct rerere_dir **rr_dir = table;
	return rr_dir[i]->sha1;
}

static struct rerere_dir *find_rerere_dir(const char *hex)
{
	unsigned char sha1[20];
	struct rerere_dir *rr_dir;
	int pos;

	if (get_sha1_hex(hex, sha1))
		return NULL; /* BUG */
	pos = sha1_pos(sha1, rerere_dir, rerere_dir_nr, rerere_dir_sha1);
	if (pos < 0) {
		rr_dir = xmalloc(sizeof(*rr_dir));
		hashcpy(rr_dir->sha1, sha1);
		rr_dir->status = NULL;
		rr_dir->status_nr = 0;
		rr_dir->status_alloc = 0;
		pos = -1 - pos;

		/* Make sure the array is big enough ... */
		ALLOC_GROW(rerere_dir, rerere_dir_nr + 1, rerere_dir_alloc);
		/* ... and add it in. */
		rerere_dir_nr++;
		memmove(rerere_dir + pos + 1, rerere_dir + pos,
			(rerere_dir_nr - pos - 1) * sizeof(*rerere_dir));
		rerere_dir[pos] = rr_dir;
		scan_rerere_dir(rr_dir);
	}
	return rerere_dir[pos];
}

static int has_rerere_resolution(const struct rerere_id *id)
{
	const int both = RR_HAS_POSTIMAGE|RR_HAS_PREIMAGE;
	int variant = id->variant;

	if (variant < 0)
		return 0;
	return ((id->collection->status[variant] & both) == both);
}

static struct rerere_id *new_rerere_id_hex(char *hex)
{
	struct rerere_id *id = xmalloc(sizeof(*id));
	id->collection = find_rerere_dir(hex);
	id->variant = -1; /* not known yet */
	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(merge_rr_path, "r");

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

		/* 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] != '.') {
			variant = 0;
			path = buf.buf + 40;
		} else {
			errno = 0;
			variant = strtol(buf.buf + 41, &path, 10);
			if (errno)
				die("corrupt MERGE_RR");
		}
		if (*(path++) != '\t')
			die("corrupt MERGE_RR");
		buf.buf[40] = '\0';
		id = new_rerere_id_hex(buf.buf);
		id->variant = variant;
		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;
		assert(id->variant >= 0);
		if (0 < id->variant)
			strbuf_addf(&buf, "%s.%d\t%s%c",
				    rerere_id_hex(id), id->variant,
				    rr->items[i].string, 0);
		else
			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 (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);
}

static void remove_variant(struct rerere_id *id)
{
	unlink_or_warn(rerere_path(id, "postimage"));
	unlink_or_warn(rerere_path(id, "preimage"));
	id->collection->status[id->variant] = 0;
}

/*
 * 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;
	struct rerere_id *id = rr_item->util;
	struct rerere_dir *rr_dir = id->collection;
	int variant;

	variant = id->variant;

	/* Has the user resolved it already? */
	if (variant >= 0) {
		if (!handle_file(path, NULL, NULL)) {
			copy_file(rerere_path(id, "postimage"), path, 0666);
			id->collection->status[variant] |= RR_HAS_POSTIMAGE;
			fprintf(stderr, "Recorded resolution for '%s'.\n", path);
			free_rerere_id(rr_item);
			rr_item->util = NULL;
			return;
		}
		/*
		 * There may be other variants that can cleanly
		 * replay.  Try them and update the variant number for
		 * this one.
		 */
	}

	/* Does any existing resolution apply cleanly? */
	for (variant = 0; variant < rr_dir->status_nr; variant++) {
		const int both = RR_HAS_PREIMAGE | RR_HAS_POSTIMAGE;
		struct rerere_id vid = *id;

		if ((rr_dir->status[variant] & both) != both)
			continue;

		vid.variant = variant;
		if (merge(&vid, path))
			continue; /* failed to replay */

		/*
		 * If there already is a different variant that applies
		 * cleanly, there is no point maintaining our own variant.
		 */
		if (0 <= id->variant && id->variant != variant)
			remove_variant(id);

		if (rerere_autoupdate)
			string_list_insert(update, path);
		else
			fprintf(stderr,
				"Resolved '%s' using previous resolution.\n",
				path);
		free_rerere_id(rr_item);
		rr_item->util = NULL;
		return;
	}

	/* None of the existing one applies; we need a new variant */
	assign_variant(id);

	variant = id->variant;
	handle_file(path, NULL, rerere_path(id, "preimage"));
	if (id->collection->status[variant] & RR_HAS_POSTIMAGE) {
		const char *path = rerere_path(id, "postimage");
		if (unlink(path))
			die_errno("cannot unlink stray '%s'", path);
		id->collection->status[variant] &= ~RR_HAS_POSTIMAGE;
	}
	id->collection->status[variant] |= RR_HAS_PREIMAGE;
	fprintf(stderr, "Recorded preimage for '%s'\n", path);
}

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;

		/* Ensure that the directory exists. */
		mkdir_in_gitdir(rerere_path(id, NULL));
	}

	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 int is_rerere_enabled(void)
{
	const char *rr_cache;
	int rr_cache_exists;

	if (!rerere_enabled)
		return 0;

	rr_cache = git_path("rr-cache");
	rr_cache_exists = is_directory(rr_cache);
	if (rerere_enabled < 0)
		return rr_cache_exists;

	if (!rr_cache_exists && mkdir_in_gitdir(rr_cache))
		die("Could not create directory %s", 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);
	merge_rr_path = git_pathdup("MERGE_RR");
	fd = hold_lock_file_for_update(&write_lock, merge_rr_path,
				       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, status;

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

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);
	id->variant = 0; /* for now */
	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);

	/*
	 * 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;
	id.collection = find_rerere_dir(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;

	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);
}

/*
 * 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;

	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"));
}