1 files changed, 425 insertions, 162 deletions

diff --git a/rerere.c b/rerere.c
index 94aea9a..403c700 100644
--- a/rerere.c
+++ b/rerere.c
@@ -20,47 +20,74 @@ static int rerere_enabled = -1;
 /* automatically update cleanly resolved paths to the index */
 static int rerere_autoupdate;
 
-static char *merge_rr_path;
+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 char *hex, const char *file)
+const char *rerere_path(const struct rerere_id *id, const char *file)
 {
-	return git_path("rr-cache/%s/%s", hex, 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 char *hex)
+static int has_rerere_resolution(const struct rerere_id *id)
 {
 	struct stat st;
-	return !stat(rerere_path(hex, "postimage"), &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)
 {
-	unsigned char sha1[20];
-	char buf[PATH_MAX];
-	FILE *in = fopen(merge_rr_path, "r");
+	struct strbuf buf = STRBUF_INIT;
+	FILE *in = fopen(git_path_merge_rr(), "r");
+
 	if (!in)
 		return;
-	while (fread(buf, 40, 1, in) == 1) {
-		int i;
-		char *name;
-		if (get_sha1_hex(buf, sha1))
+	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");
-		buf[40] = '\0';
-		name = xstrdup(buf);
-		if (fgetc(in) != '\t')
+
+		if (buf.buf[40] != '\t')
 			die("corrupt MERGE_RR");
-		for (i = 0; i < sizeof(buf); i++) {
-			int c = fgetc(in);
-			if (c < 0)
-				die("corrupt MERGE_RR");
-			buf[i] = c;
-			if (c == 0)
-				 break;
-		}
-		if (i == sizeof(buf))
-			die("filename too long");
-		string_list_insert(rr, buf)->util = name;
+		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);
 }
 
@@ -70,22 +97,42 @@ static int write_rr(struct string_list *rr, int out_fd)
 {
 	int i;
 	for (i = 0; i < rr->nr; i++) {
-		const char *path;
-		int length;
-		if (!rr->items[i].util)
+		struct strbuf buf = STRBUF_INIT;
+		struct rerere_id *id;
+
+		assert(rr->items[i].util != RERERE_RESOLVED);
+
+		id = rr->items[i].util;
+		if (!id)
 			continue;
-		path = rr->items[i].string;
-		length = strlen(path) + 1;
-		if (write_in_full(out_fd, rr->items[i].util, 40) != 40 ||
-		    write_str_in_full(out_fd, "\t") != 1 ||
-		    write_in_full(out_fd, path, length) != length)
+		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)
@@ -99,31 +146,34 @@ static inline void ferr_puts(const char *s, FILE *fp, int *err)
 	ferr_write(s, strlen(s), fp, err);
 }
 
-struct rerere_io {
-	int (*getline)(struct strbuf *, struct rerere_io *);
-	FILE *output;
-	int wrerror;
-	/* some more stuff */
-};
-
 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) {
+		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);
@@ -140,19 +190,42 @@ static void rerere_io_putmem(const char *mem, size_t sz, struct rerere_io *io)
 		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');
 }
 
-static int is_cmarker(char *buf, int marker_char, int marker_size, int want_sp)
+/*
+ * 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;
@@ -161,6 +234,21 @@ static int is_cmarker(char *buf, int marker_char, int marker_size, int want_sp)
 	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;
@@ -175,19 +263,19 @@ static int handle_path(unsigned char *sha1, struct rerere_io *io, int marker_siz
 		git_SHA1_Init(&ctx);
 
 	while (!io->getline(&buf, io)) {
-		if (is_cmarker(buf.buf, '<', marker_size, 1)) {
+		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, 0)) {
+		} 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, 0)) {
+		} 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, 1)) {
+		} else if (is_cmarker(buf.buf, '>', marker_size)) {
 			if (hunk != RR_SIDE_2)
 				goto bad;
 			if (strbuf_cmp(&one, &two) > 0)
@@ -231,6 +319,10 @@ static int handle_path(unsigned char *sha1, struct rerere_io *io, int marker_siz
 	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;
@@ -272,11 +364,18 @@ static int handle_file(const char *path, unsigned char *sha1, const char *output
 	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_;
@@ -315,24 +414,23 @@ static int handle_cache(const char *path, unsigned char *sha1, const char *outpu
 		return -1;
 	pos = -pos - 1;
 
-	for (i = 0; i < 3; i++) {
+	while (pos < active_nr) {
 		enum object_type type;
 		unsigned long size;
-		int j;
 
-		if (active_nr <= pos)
-			break;
 		ce = active_cache[pos++];
 		if (ce_namelen(ce) != len || memcmp(ce->name, path, len))
-			continue;
-		j = ce_stage(ce) - 1;
-		mmfile[j].ptr = read_sha1_file(ce->sha1, &type, &size);
-		mmfile[j].size = size;
+			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++) {
+	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
@@ -352,6 +450,10 @@ static int handle_cache(const char *path, unsigned char *sha1, const char *outpu
 	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)
@@ -359,6 +461,14 @@ static int handle_cache(const char *path, unsigned char *sha1, const char *outpu
 	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];
@@ -369,10 +479,8 @@ static int check_one_conflict(int i, int *type)
 	}
 
 	*type = PUNTED;
-	if (ce_stage(e) == 1) {
-		if (active_nr <= ++i)
-			return i + 1;
-	}
+	while (ce_stage(active_cache[i]) == 1)
+		i++;
 
 	/* Only handle regular files with both stages #2 and #3 */
 	if (i + 1 < active_nr) {
@@ -392,6 +500,17 @@ static int check_one_conflict(int i, int *type)
 	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;
@@ -408,9 +527,26 @@ static int find_conflict(struct string_list *conflict)
 	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");
 
@@ -424,7 +560,7 @@ int rerere_remaining(struct string_list *merge_rr)
 			struct string_list_item *it;
 			it = string_list_lookup(merge_rr, (const char *)e->name);
 			if (it != NULL) {
-				free(it->util);
+				free_rerere_id(it);
 				it->util = RERERE_RESOLVED;
 			}
 		}
@@ -432,39 +568,66 @@ int rerere_remaining(struct string_list *merge_rr)
 	return 0;
 }
 
-static int merge(const char *name, const char *path)
+/*
+ * 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};
 
-	if (handle_file(path, NULL, rerere_path(name, "thisimage")) < 0)
-		return 1;
+	/*
+	 * 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(name, "thisimage")) ||
-			read_mmfile(&base, rerere_path(name, "preimage")) ||
-			read_mmfile(&other, rerere_path(name, "postimage"))) {
+	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) {
-		FILE *f;
-
-		if (utime(rerere_path(name, "postimage"), NULL) < 0)
-			warning("failed utime() on %s: %s",
-					rerere_path(name, "postimage"),
-					strerror(errno));
-		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));
-	}
+	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);
@@ -487,6 +650,8 @@ static void update_paths(struct string_list *update)
 		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) {
@@ -496,6 +661,41 @@ static void update_paths(struct string_list *update)
 		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;
@@ -505,65 +705,55 @@ static int do_plain_rerere(struct string_list *rr, int fd)
 	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.
+	 * 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;
-		if (!string_list_has_string(rr, path)) {
-			unsigned char sha1[20];
-			char *hex;
-			int ret;
-			ret = handle_file(path, sha1, NULL);
-			if (ret < 1)
-				continue;
-			hex = xstrdup(sha1_to_hex(sha1));
-			string_list_insert(rr, path)->util = hex;
-			if (mkdir_in_gitdir(git_path("rr-cache/%s", hex)))
-				continue;
-			handle_file(path, NULL, rerere_path(hex, "preimage"));
-			fprintf(stderr, "Recorded preimage for '%s'\n", path);
-		}
-	}
+		int ret;
 
-	/*
-	 * Now some of the paths that had conflicts earlier might have been
-	 * hand resolved.  Others may be similar to a conflict already that
-	 * was resolved before.
-	 */
+		if (string_list_has_string(rr, path))
+			continue;
 
-	for (i = 0; i < rr->nr; i++) {
-		int ret;
-		const char *path = rr->items[i].string;
-		const char *name = (const char *)rr->items[i].util;
-
-		if (has_rerere_resolution(name)) {
-			if (!merge(name, path)) {
-				const char *msg;
-				if (rerere_autoupdate) {
-					string_list_insert(&update, path);
-					msg = "Staged '%s' using previous resolution.\n";
-				} else
-					msg = "Resolved '%s' using previous resolution.\n";
-				fprintf(stderr, msg, path);
-				goto mark_resolved;
-			}
-		}
+		/*
+		 * 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;
 
-		/* Let's see if we have resolved it. */
-		ret = handle_file(path, NULL, NULL);
-		if (ret)
+		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;
 
-		fprintf(stderr, "Recorded resolution for '%s'.\n", path);
-		copy_file(rerere_path(name, "postimage"), path, 0666);
-	mark_resolved:
-		rr->items[i].util = NULL;
+		/*
+		 * 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);
 
@@ -577,21 +767,21 @@ static void git_rerere_config(void)
 	git_config(git_default_config, NULL);
 }
 
+static GIT_PATH_FUNC(git_path_rr_cache, "rr-cache")
+
 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);
+	rr_cache_exists = is_directory(git_path_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);
+	if (!rr_cache_exists && mkdir_in_gitdir(git_path_rr_cache()))
+		die("Could not create directory %s", git_path_rr_cache());
 	return 1;
 }
 
@@ -605,13 +795,20 @@ int setup_rerere(struct string_list *merge_rr, int flags)
 
 	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);
+	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;
@@ -626,25 +823,42 @@ int rerere(int flags)
 static int rerere_forget_one_path(const char *path, struct string_list *rr)
 {
 	const char *filename;
-	char *hex;
+	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);
-	hex = xstrdup(sha1_to_hex(sha1));
-	filename = rerere_path(hex, "postimage");
+
+	/* 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)));
 
-	handle_cache(path, sha1, rerere_path(hex, "preimage"));
+	/*
+	 * 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);
 
-
-	string_list_insert(rr, path)->util = hex;
+	/*
+	 * 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;
 }
@@ -662,6 +876,11 @@ int rerere_forget(struct pathspec *pathspec)
 	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++) {
@@ -674,24 +893,51 @@ int rerere_forget(struct pathspec *pathspec)
 	return write_rr(&merge_rr, fd);
 }
 
-static time_t rerere_created_at(const char *name)
+/*
+ * 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;
-	return stat(rerere_path(name, "preimage"), &st) ? (time_t) 0 : st.st_mtime;
+	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 *name)
+static time_t rerere_last_used_at(const char *dir_name)
 {
 	struct stat st;
-	return stat(rerere_path(name, "postimage"), &st) ? (time_t) 0 : st.st_mtime;
+	struct rerere_id *id = dirname_to_id(dir_name);
+
+	return stat(rerere_path(id, "postimage"), &st) ? (time_t) 0 : st.st_mtime;
 }
 
-static void unlink_rr_item(const char *name)
+/*
+ * Remove the recorded resolution for a given conflict ID
+ */
+static void unlink_rr_item(struct rerere_id *id)
 {
-	unlink(rerere_path(name, "thisimage"));
-	unlink(rerere_path(name, "preimage"));
-	unlink(rerere_path(name, "postimage"));
-	rmdir(git_path("rr-cache/%s", name));
+	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)
@@ -704,12 +950,16 @@ void rerere_gc(struct string_list *rr)
 	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;
@@ -727,19 +977,32 @@ void rerere_gc(struct string_list *rr)
 			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(to_remove.items[i].string);
+		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++) {
-		const char *name = (const char *)merge_rr->items[i].util;
-		if (!has_rerere_resolution(name))
-			unlink_rr_item(name);
+		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"));
+	unlink_or_warn(git_path_merge_rr());
+	rollback_lock_file(&write_lock);
 }