22 files changed, 545 insertions, 42 deletions

diff --git a/Documentation/config/pack.txt b/Documentation/config/pack.txt
index 837f1b1679..3da4ea98e2 100644
--- a/Documentation/config/pack.txt
+++ b/Documentation/config/pack.txt
@@ -133,3 +133,10 @@ pack.writeBitmapHashCache::
 	between an older, bitmapped pack and objects that have been
 	pushed since the last gc). The downside is that it consumes 4
 	bytes per object of disk space. Defaults to true.
+
+pack.writeReverseIndex::
+	When true, git will write a corresponding .rev file (see:
+	link:../technical/pack-format.html[Documentation/technical/pack-format.txt])
+	for each new packfile that it writes in all places except for
+	linkgit:git-fast-import[1] and in the bulk checkin mechanism.
+	Defaults to false.
diff --git a/Documentation/git-index-pack.txt b/Documentation/git-index-pack.txt
index af0c26232c..69ba904d44 100644
--- a/Documentation/git-index-pack.txt
+++ b/Documentation/git-index-pack.txt
@@ -9,17 +9,18 @@ git-index-pack - Build pack index file for an existing packed archive
 SYNOPSIS
 --------
 [verse]
-'git index-pack' [-v] [-o <index-file>] <pack-file>
+'git index-pack' [-v] [-o <index-file>] [--[no-]rev-index] <pack-file>
 'git index-pack' --stdin [--fix-thin] [--keep] [-v] [-o <index-file>]
-                 [<pack-file>]
+		  [--[no-]rev-index] [<pack-file>]
 
 
 DESCRIPTION
 -----------
 Reads a packed archive (.pack) from the specified file, and
-builds a pack index file (.idx) for it.  The packed archive
-together with the pack index can then be placed in the
-objects/pack/ directory of a Git repository.
+builds a pack index file (.idx) for it. Optionally writes a
+reverse-index (.rev) for the specified pack. The packed
+archive together with the pack index can then be placed in
+the objects/pack/ directory of a Git repository.
 
 
 OPTIONS
@@ -35,6 +36,13 @@ OPTIONS
 	fails if the name of packed archive does not end
 	with .pack).
 
+--[no-]rev-index::
+	When this flag is provided, generate a reverse index
+	(a `.rev` file) corresponding to the given pack. If
+	`--verify` is given, ensure that the existing
+	reverse index is correct. Takes precedence over
+	`pack.writeReverseIndex`.
+
 --stdin::
 	When this flag is provided, the pack is read from stdin
 	instead and a copy is then written to <pack-file>. If
diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
index 96d2fc589f..8833b71c8b 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -274,6 +274,26 @@ Pack file entry: <+
 
     Index checksum of all of the above.
 
+== pack-*.rev files have the format:
+
+  - A 4-byte magic number '0x52494458' ('RIDX').
+
+  - A 4-byte version identifier (= 1).
+
+  - A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).
+
+  - A table of index positions (one per packed object, num_objects in
+    total, each a 4-byte unsigned integer in network order), sorted by
+    their corresponding offsets in the packfile.
+
+  - A trailer, containing a:
+
+    checksum of the corresponding packfile, and
+
+    a checksum of all of the above.
+
+All 4-byte numbers are in network order.
+
 == multi-pack-index (MIDX) files have the following format:
 
 The multi-pack-index files refer to multiple pack-files and loose objects.
diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 557bd2f348..54f74c4874 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -17,7 +17,7 @@
 #include "promisor-remote.h"
 
 static const char index_pack_usage[] =
-"git index-pack [-v] [-o <index-file>] [--keep | --keep=<msg>] [--verify] [--strict] (<pack-file> | --stdin [--fix-thin] [<pack-file>])";
+"git index-pack [-v] [-o <index-file>] [--keep | --keep=<msg>] [--[no-]rev-index] [--verify] [--strict] (<pack-file> | --stdin [--fix-thin] [<pack-file>])";
 
 struct object_entry {
 	struct pack_idx_entry idx;
@@ -1436,15 +1436,15 @@ static void fix_unresolved_deltas(struct hashfile *f)
 	free(sorted_by_pos);
 }
 
-static const char *derive_filename(const char *pack_name, const char *suffix,
-				   struct strbuf *buf)
+static const char *derive_filename(const char *pack_name, const char *strip,
+				   const char *suffix, struct strbuf *buf)
 {
 	size_t len;
-	if (!strip_suffix(pack_name, ".pack", &len))
-		die(_("packfile name '%s' does not end with '.pack'"),
-		    pack_name);
+	if (!strip_suffix(pack_name, strip, &len) || !len ||
+	    pack_name[len - 1] != '.')
+		die(_("packfile name '%s' does not end with '.%s'"),
+		    pack_name, strip);
 	strbuf_add(buf, pack_name, len);
-	strbuf_addch(buf, '.');
 	strbuf_addstr(buf, suffix);
 	return buf->buf;
 }
@@ -1459,7 +1459,7 @@ static void write_special_file(const char *suffix, const char *msg,
 	int msg_len = strlen(msg);
 
 	if (pack_name)
-		filename = derive_filename(pack_name, suffix, &name_buf);
+		filename = derive_filename(pack_name, "pack", suffix, &name_buf);
 	else
 		filename = odb_pack_name(&name_buf, hash, suffix);
 
@@ -1484,12 +1484,14 @@ static void write_special_file(const char *suffix, const char *msg,
 
 static void final(const char *final_pack_name, const char *curr_pack_name,
 		  const char *final_index_name, const char *curr_index_name,
+		  const char *final_rev_index_name, const char *curr_rev_index_name,
 		  const char *keep_msg, const char *promisor_msg,
 		  unsigned char *hash)
 {
 	const char *report = "pack";
 	struct strbuf pack_name = STRBUF_INIT;
 	struct strbuf index_name = STRBUF_INIT;
+	struct strbuf rev_index_name = STRBUF_INIT;
 	int err;
 
 	if (!from_stdin) {
@@ -1524,6 +1526,16 @@ static void final(const char *final_pack_name, const char *curr_pack_name,
 	} else
 		chmod(final_index_name, 0444);
 
+	if (curr_rev_index_name) {
+		if (final_rev_index_name != curr_rev_index_name) {
+			if (!final_rev_index_name)
+				final_rev_index_name = odb_pack_name(&rev_index_name, hash, "rev");
+			if (finalize_object_file(curr_rev_index_name, final_rev_index_name))
+				die(_("cannot store reverse index file"));
+		} else
+			chmod(final_rev_index_name, 0444);
+	}
+
 	if (do_fsck_object) {
 		struct packed_git *p;
 		p = add_packed_git(final_index_name, strlen(final_index_name), 0);
@@ -1553,6 +1565,7 @@ static void final(const char *final_pack_name, const char *curr_pack_name,
 		}
 	}
 
+	strbuf_release(&rev_index_name);
 	strbuf_release(&index_name);
 	strbuf_release(&pack_name);
 }
@@ -1578,6 +1591,12 @@ static int git_index_pack_config(const char *k, const char *v, void *cb)
 		}
 		return 0;
 	}
+	if (!strcmp(k, "pack.writereverseindex")) {
+		if (git_config_bool(k, v))
+			opts->flags |= WRITE_REV;
+		else
+			opts->flags &= ~WRITE_REV;
+	}
 	return git_default_config(k, v, cb);
 }
 
@@ -1695,12 +1714,14 @@ static void show_pack_info(int stat_only)
 
 int cmd_index_pack(int argc, const char **argv, const char *prefix)
 {
-	int i, fix_thin_pack = 0, verify = 0, stat_only = 0;
+	int i, fix_thin_pack = 0, verify = 0, stat_only = 0, rev_index;
 	const char *curr_index;
-	const char *index_name = NULL, *pack_name = NULL;
+	const char *curr_rev_index = NULL;
+	const char *index_name = NULL, *pack_name = NULL, *rev_index_name = NULL;
 	const char *keep_msg = NULL;
 	const char *promisor_msg = NULL;
 	struct strbuf index_name_buf = STRBUF_INIT;
+	struct strbuf rev_index_name_buf = STRBUF_INIT;
 	struct pack_idx_entry **idx_objects;
 	struct pack_idx_option opts;
 	unsigned char pack_hash[GIT_MAX_RAWSZ];
@@ -1727,6 +1748,11 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 	if (prefix && chdir(prefix))
 		die(_("Cannot come back to cwd"));
 
+	if (git_env_bool(GIT_TEST_WRITE_REV_INDEX, 0))
+		rev_index = 1;
+	else
+		rev_index = !!(opts.flags & (WRITE_REV_VERIFY | WRITE_REV));
+
 	for (i = 1; i < argc; i++) {
 		const char *arg = argv[i];
 
@@ -1805,6 +1831,10 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 				if (hash_algo == GIT_HASH_UNKNOWN)
 					die(_("unknown hash algorithm '%s'"), arg);
 				repo_set_hash_algo(the_repository, hash_algo);
+			} else if (!strcmp(arg, "--rev-index")) {
+				rev_index = 1;
+			} else if (!strcmp(arg, "--no-rev-index")) {
+				rev_index = 0;
 			} else
 				usage(index_pack_usage);
 			continue;
@@ -1824,7 +1854,16 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 	if (from_stdin && hash_algo)
 		die(_("--object-format cannot be used with --stdin"));
 	if (!index_name && pack_name)
-		index_name = derive_filename(pack_name, "idx", &index_name_buf);
+		index_name = derive_filename(pack_name, "pack", "idx", &index_name_buf);
+
+	opts.flags &= ~(WRITE_REV | WRITE_REV_VERIFY);
+	if (rev_index) {
+		opts.flags |= verify ? WRITE_REV_VERIFY : WRITE_REV;
+		if (index_name)
+			rev_index_name = derive_filename(index_name,
+							 "idx", "rev",
+							 &rev_index_name_buf);
+	}
 
 	if (verify) {
 		if (!index_name)
@@ -1878,11 +1917,16 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 	for (i = 0; i < nr_objects; i++)
 		idx_objects[i] = &objects[i].idx;
 	curr_index = write_idx_file(index_name, idx_objects, nr_objects, &opts, pack_hash);
+	if (rev_index)
+		curr_rev_index = write_rev_file(rev_index_name, idx_objects,
+						nr_objects, pack_hash,
+						opts.flags);
 	free(idx_objects);
 
 	if (!verify)
 		final(pack_name, curr_pack,
 		      index_name, curr_index,
+		      rev_index_name, curr_rev_index,
 		      keep_msg, promisor_msg,
 		      pack_hash);
 	else
@@ -1893,10 +1937,13 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 
 	free(objects);
 	strbuf_release(&index_name_buf);
+	strbuf_release(&rev_index_name_buf);
 	if (pack_name == NULL)
 		free((void *) curr_pack);
 	if (index_name == NULL)
 		free((void *) curr_index);
+	if (rev_index_name == NULL)
+		free((void *) curr_rev_index);
 
 	/*
 	 * Let the caller know this pack is not self contained
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 13cde5896a..6d62aaf59a 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -2953,6 +2953,13 @@ static int git_pack_config(const char *k, const char *v, void *cb)
 			    pack_idx_opts.version);
 		return 0;
 	}
+	if (!strcmp(k, "pack.writereverseindex")) {
+		if (git_config_bool(k, v))
+			pack_idx_opts.flags |= WRITE_REV;
+		else
+			pack_idx_opts.flags &= ~WRITE_REV;
+		return 0;
+	}
 	if (!strcmp(k, "uploadpack.blobpackfileuri")) {
 		struct configured_exclusion *ex = xmalloc(sizeof(*ex));
 		const char *oid_end, *pack_end;
@@ -3592,6 +3599,8 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 
 	reset_pack_idx_option(&pack_idx_opts);
 	git_config(git_pack_config, NULL);
+	if (git_env_bool(GIT_TEST_WRITE_REV_INDEX, 0))
+		pack_idx_opts.flags |= WRITE_REV;
 
 	progress = isatty(2);
 	argc = parse_options(argc, argv, prefix, pack_objects_options,
diff --git a/builtin/repack.c b/builtin/repack.c
index 2158b48f4c..01440de2d5 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -209,6 +209,7 @@ static struct {
 } exts[] = {
 	{".pack"},
 	{".idx"},
+	{".rev", 1},
 	{".bitmap", 1},
 	{".promisor", 1},
 };
diff --git a/ci/run-build-and-tests.sh b/ci/run-build-and-tests.sh
index 50e0b90073..a66b5e8c75 100755
--- a/ci/run-build-and-tests.sh
+++ b/ci/run-build-and-tests.sh
@@ -24,6 +24,7 @@ linux-gcc)
 	export GIT_TEST_MULTI_PACK_INDEX=1
 	export GIT_TEST_ADD_I_USE_BUILTIN=1
 	export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=master
+	export GIT_TEST_WRITE_REV_INDEX=1
 	make test
 	;;
 linux-clang)
diff --git a/object-store.h b/object-store.h
index c4fc9dd74e..541dab0858 100644
--- a/object-store.h
+++ b/object-store.h
@@ -85,6 +85,9 @@ struct packed_git {
 		 multi_pack_index:1;
 	unsigned char hash[GIT_MAX_RAWSZ];
 	struct revindex_entry *revindex;
+	const uint32_t *revindex_data;
+	const uint32_t *revindex_map;
+	size_t revindex_size;
 	/* something like ".git/objects/pack/xxxxx.pack" */
 	char pack_name[FLEX_ARRAY]; /* more */
 };
diff --git a/pack-revindex.c b/pack-revindex.c
index 5e69bc7372..83fe4de773 100644
--- a/pack-revindex.c
+++ b/pack-revindex.c
@@ -2,6 +2,7 @@
 #include "pack-revindex.h"
 #include "object-store.h"
 #include "packfile.h"
+#include "config.h"
 
 struct revindex_entry {
 	off_t offset;
@@ -164,16 +165,133 @@ static void create_pack_revindex(struct packed_git *p)
 	sort_revindex(p->revindex, num_ent, p->pack_size);
 }
 
-int load_pack_revindex(struct packed_git *p)
+static int create_pack_revindex_in_memory(struct packed_git *p)
 {
-	if (!p->revindex) {
-		if (open_pack_index(p))
-			return -1;
-		create_pack_revindex(p);
-	}
+	if (git_env_bool(GIT_TEST_REV_INDEX_DIE_IN_MEMORY, 0))
+		die("dying as requested by '%s'",
+		    GIT_TEST_REV_INDEX_DIE_IN_MEMORY);
+	if (open_pack_index(p))
+		return -1;
+	create_pack_revindex(p);
 	return 0;
 }
 
+static char *pack_revindex_filename(struct packed_git *p)
+{
+	size_t len;
+	if (!strip_suffix(p->pack_name, ".pack", &len))
+		BUG("pack_name does not end in .pack");
+	return xstrfmt("%.*s.rev", (int)len, p->pack_name);
+}
+
+#define RIDX_HEADER_SIZE (12)
+#define RIDX_MIN_SIZE (RIDX_HEADER_SIZE + (2 * the_hash_algo->rawsz))
+
+struct revindex_header {
+	uint32_t signature;
+	uint32_t version;
+	uint32_t hash_id;
+};
+
+static int load_revindex_from_disk(char *revindex_name,
+				   uint32_t num_objects,
+				   const uint32_t **data_p, size_t *len_p)
+{
+	int fd, ret = 0;
+	struct stat st;
+	void *data = NULL;
+	size_t revindex_size;
+	struct revindex_header *hdr;
+
+	fd = git_open(revindex_name);
+
+	if (fd < 0) {
+		ret = -1;
+		goto cleanup;
+	}
+	if (fstat(fd, &st)) {
+		ret = error_errno(_("failed to read %s"), revindex_name);
+		goto cleanup;
+	}
+
+	revindex_size = xsize_t(st.st_size);
+
+	if (revindex_size < RIDX_MIN_SIZE) {
+		ret = error(_("reverse-index file %s is too small"), revindex_name);
+		goto cleanup;
+	}
+
+	if (revindex_size - RIDX_MIN_SIZE != st_mult(sizeof(uint32_t), num_objects)) {
+		ret = error(_("reverse-index file %s is corrupt"), revindex_name);
+		goto cleanup;
+	}
+
+	data = xmmap(NULL, revindex_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	hdr = data;
+
+	if (ntohl(hdr->signature) != RIDX_SIGNATURE) {
+		ret = error(_("reverse-index file %s has unknown signature"), revindex_name);
+		goto cleanup;
+	}
+	if (ntohl(hdr->version) != 1) {
+		ret = error(_("reverse-index file %s has unsupported version %"PRIu32),
+			    revindex_name, ntohl(hdr->version));
+		goto cleanup;
+	}
+	if (!(ntohl(hdr->hash_id) == 1 || ntohl(hdr->hash_id) == 2)) {
+		ret = error(_("reverse-index file %s has unsupported hash id %"PRIu32),
+			    revindex_name, ntohl(hdr->hash_id));
+		goto cleanup;
+	}
+
+cleanup:
+	if (ret) {
+		if (data)
+			munmap(data, revindex_size);
+	} else {
+		*len_p = revindex_size;
+		*data_p = (const uint32_t *)data;
+	}
+
+	close(fd);
+	return ret;
+}
+
+static int load_pack_revindex_from_disk(struct packed_git *p)
+{
+	char *revindex_name;
+	int ret;
+	if (open_pack_index(p))
+		return -1;
+
+	revindex_name = pack_revindex_filename(p);
+
+	ret = load_revindex_from_disk(revindex_name,
+				      p->num_objects,
+				      &p->revindex_map,
+				      &p->revindex_size);
+	if (ret)
+		goto cleanup;
+
+	p->revindex_data = (const uint32_t *)((const char *)p->revindex_map + RIDX_HEADER_SIZE);
+
+cleanup:
+	free(revindex_name);
+	return ret;
+}
+
+int load_pack_revindex(struct packed_git *p)
+{
+	if (p->revindex || p->revindex_data)
+		return 0;
+
+	if (!load_pack_revindex_from_disk(p))
+		return 0;
+	else if (!create_pack_revindex_in_memory(p))
+		return 0;
+	return -1;
+}
+
 int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos)
 {
 	unsigned lo, hi;
@@ -203,18 +321,28 @@ int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos)
 
 uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos)
 {
-	if (!p->revindex)
+	if (!(p->revindex || p->revindex_data))
 		BUG("pack_pos_to_index: reverse index not yet loaded");
 	if (p->num_objects <= pos)
 		BUG("pack_pos_to_index: out-of-bounds object at %"PRIu32, pos);
-	return p->revindex[pos].nr;
+
+	if (p->revindex)
+		return p->revindex[pos].nr;
+	else
+		return get_be32(p->revindex_data + pos);
 }
 
 off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos)
 {
-	if (!p->revindex)
+	if (!(p->revindex || p->revindex_data))
 		BUG("pack_pos_to_index: reverse index not yet loaded");
 	if (p->num_objects < pos)
 		BUG("pack_pos_to_offset: out-of-bounds object at %"PRIu32, pos);
-	return p->revindex[pos].offset;
+
+	if (p->revindex)
+		return p->revindex[pos].offset;
+	else if (pos == p->num_objects)
+		return p->pack_size - the_hash_algo->rawsz;
+	else
+		return nth_packed_object_offset(p, pack_pos_to_index(p, pos));
 }
diff --git a/pack-revindex.h b/pack-revindex.h
index 6e0320b08b..ba7c82c125 100644
--- a/pack-revindex.h
+++ b/pack-revindex.h
@@ -16,11 +16,21 @@
  *   can be found
  */
 
+
+#define RIDX_SIGNATURE 0x52494458 /* "RIDX" */
+#define RIDX_VERSION 1
+
+#define GIT_TEST_WRITE_REV_INDEX "GIT_TEST_WRITE_REV_INDEX"
+#define GIT_TEST_REV_INDEX_DIE_IN_MEMORY "GIT_TEST_REV_INDEX_DIE_IN_MEMORY"
+
 struct packed_git;
 
 /*
  * load_pack_revindex populates the revindex's internal data-structures for the
  * given pack, returning zero on success and a negative value otherwise.
+ *
+ * If a '.rev' file is present it is mmap'd, and pointers are assigned into it
+ * (instead of using the in-memory variant).
  */
 int load_pack_revindex(struct packed_git *p);
 
@@ -55,7 +65,9 @@ uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos);
  * If the reverse index has not yet been loaded, or the position is out of
  * bounds, this function aborts.
  *
- * This function runs in constant time.
+ * This function runs in constant time under both in-memory and on-disk reverse
+ * indexes, but an additional step is taken to consult the corresponding .idx
+ * file when using the on-disk format.
  */
 off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos);
 
diff --git a/pack-write.c b/pack-write.c
index e9bb3fd949..680c36755d 100644
--- a/pack-write.c
+++ b/pack-write.c
@@ -167,6 +167,113 @@ const char *write_idx_file(const char *index_name, struct pack_idx_entry **objec
 	return index_name;
 }
 
+static int pack_order_cmp(const void *va, const void *vb, void *ctx)
+{
+	struct pack_idx_entry **objects = ctx;
+
+	off_t oa = objects[*(uint32_t*)va]->offset;
+	off_t ob = objects[*(uint32_t*)vb]->offset;
+
+	if (oa < ob)
+		return -1;
+	if (oa > ob)
+		return 1;
+	return 0;
+}
+
+static void write_rev_header(struct hashfile *f)
+{
+	uint32_t oid_version;
+	switch (hash_algo_by_ptr(the_hash_algo)) {
+	case GIT_HASH_SHA1:
+		oid_version = 1;
+		break;
+	case GIT_HASH_SHA256:
+		oid_version = 2;
+		break;
+	default:
+		die("write_rev_header: unknown hash version");
+	}
+
+	hashwrite_be32(f, RIDX_SIGNATURE);
+	hashwrite_be32(f, RIDX_VERSION);
+	hashwrite_be32(f, oid_version);
+}
+
+static void write_rev_index_positions(struct hashfile *f,
+				      struct pack_idx_entry **objects,
+				      uint32_t nr_objects)
+{
+	uint32_t *pack_order;
+	uint32_t i;
+
+	ALLOC_ARRAY(pack_order, nr_objects);
+	for (i = 0; i < nr_objects; i++)
+		pack_order[i] = i;
+	QSORT_S(pack_order, nr_objects, pack_order_cmp, objects);
+
+	for (i = 0; i < nr_objects; i++)
+		hashwrite_be32(f, pack_order[i]);
+
+	free(pack_order);
+}
+
+static void write_rev_trailer(struct hashfile *f, const unsigned char *hash)
+{
+	hashwrite(f, hash, the_hash_algo->rawsz);
+}
+
+const char *write_rev_file(const char *rev_name,
+			   struct pack_idx_entry **objects,
+			   uint32_t nr_objects,
+			   const unsigned char *hash,
+			   unsigned flags)
+{
+	struct hashfile *f;
+	int fd;
+
+	if ((flags & WRITE_REV) && (flags & WRITE_REV_VERIFY))
+		die(_("cannot both write and verify reverse index"));
+
+	if (flags & WRITE_REV) {
+		if (!rev_name) {
+			struct strbuf tmp_file = STRBUF_INIT;
+			fd = odb_mkstemp(&tmp_file, "pack/tmp_rev_XXXXXX");
+			rev_name = strbuf_detach(&tmp_file, NULL);
+		} else {
+			unlink(rev_name);
+			fd = open(rev_name, O_CREAT|O_EXCL|O_WRONLY, 0600);
+			if (fd < 0)
+				die_errno("unable to create '%s'", rev_name);
+		}
+		f = hashfd(fd, rev_name);
+	} else if (flags & WRITE_REV_VERIFY) {
+		struct stat statbuf;
+		if (stat(rev_name, &statbuf)) {
+			if (errno == ENOENT) {
+				/* .rev files are optional */
+				return NULL;
+			} else
+				die_errno(_("could not stat: %s"), rev_name);
+		}
+		f = hashfd_check(rev_name);
+	} else
+		return NULL;
+
+	write_rev_header(f);
+
+	write_rev_index_positions(f, objects, nr_objects);
+	write_rev_trailer(f, hash);
+
+	if (rev_name && adjust_shared_perm(rev_name) < 0)
+		die(_("failed to make %s readable"), rev_name);
+
+	finalize_hashfile(f, NULL, CSUM_HASH_IN_STREAM | CSUM_CLOSE |
+				    ((flags & WRITE_IDX_VERIFY) ? 0 : CSUM_FSYNC));
+
+	return rev_name;
+}
+
 off_t write_pack_header(struct hashfile *f, uint32_t nr_entries)
 {
 	struct pack_header hdr;
@@ -342,7 +449,7 @@ void finish_tmp_packfile(struct strbuf *name_buffer,
 			 struct pack_idx_option *pack_idx_opts,
 			 unsigned char hash[])
 {
-	const char *idx_tmp_name;
+	const char *idx_tmp_name, *rev_tmp_name = NULL;
 	int basename_len = name_buffer->len;
 
 	if (adjust_shared_perm(pack_tmp_name))
@@ -353,6 +460,9 @@ void finish_tmp_packfile(struct strbuf *name_buffer,
 	if (adjust_shared_perm(idx_tmp_name))
 		die_errno("unable to make temporary index file readable");
 
+	rev_tmp_name = write_rev_file(NULL, written_list, nr_written, hash,
+				      pack_idx_opts->flags);
+
 	strbuf_addf(name_buffer, "%s.pack", hash_to_hex(hash));
 
 	if (rename(pack_tmp_name, name_buffer->buf))
@@ -366,6 +476,14 @@ void finish_tmp_packfile(struct strbuf *name_buffer,
 
 	strbuf_setlen(name_buffer, basename_len);
 
+	if (rev_tmp_name) {
+		strbuf_addf(name_buffer, "%s.rev", hash_to_hex(hash));
+		if (rename(rev_tmp_name, name_buffer->buf))
+			die_errno("unable to rename temporary reverse-index file");
+	}
+
+	strbuf_setlen(name_buffer, basename_len);
+
 	free((void *)idx_tmp_name);
 }
 
diff --git a/pack.h b/pack.h
index 9ae640f417..afdcf8f5c7 100644
--- a/pack.h
+++ b/pack.h
@@ -42,6 +42,8 @@ struct pack_idx_option {
 	/* flag bits */
 #define WRITE_IDX_VERIFY 01 /* verify only, do not write the idx file */
 #define WRITE_IDX_STRICT 02
+#define WRITE_REV 04
+#define WRITE_REV_VERIFY 010
 
 	uint32_t version;
 	uint32_t off32_limit;
@@ -91,6 +93,8 @@ struct ref;
 
 void write_promisor_file(const char *promisor_name, struct ref **sought, int nr_sought);
 
+const char *write_rev_file(const char *rev_name, struct pack_idx_entry **objects, uint32_t nr_objects, const unsigned char *hash, unsigned flags);
+
 /*
  * The "hdr" output buffer should be at least this big, which will handle sizes
  * up to 2^67.
diff --git a/packfile.c b/packfile.c
index 4b938b4372..1fec12ac5f 100644
--- a/packfile.c
+++ b/packfile.c
@@ -324,11 +324,21 @@ void close_pack_index(struct packed_git *p)
 	}
 }
 
+void close_pack_revindex(struct packed_git *p) {
+	if (!p->revindex_map)
+		return;
+
+	munmap((void *)p->revindex_map, p->revindex_size);
+	p->revindex_map = NULL;
+	p->revindex_data = NULL;
+}
+
 void close_pack(struct packed_git *p)
 {
 	close_pack_windows(p);
 	close_pack_fd(p);
 	close_pack_index(p);
+	close_pack_revindex(p);
 }
 
 void close_object_store(struct raw_object_store *o)
@@ -351,7 +361,7 @@ void close_object_store(struct raw_object_store *o)
 
 void unlink_pack_path(const char *pack_name, int force_delete)
 {
-	static const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"};
+	static const char *exts[] = {".pack", ".idx", ".rev", ".keep", ".bitmap", ".promisor"};
 	int i;
 	struct strbuf buf = STRBUF_INIT;
 	size_t plen;
@@ -853,6 +863,7 @@ static void prepare_pack(const char *full_name, size_t full_name_len,
 	if (!strcmp(file_name, "multi-pack-index"))
 		return;
 	if (ends_with(file_name, ".idx") ||
+	    ends_with(file_name, ".rev") ||
 	    ends_with(file_name, ".pack") ||
 	    ends_with(file_name, ".bitmap") ||
 	    ends_with(file_name, ".keep") ||
diff --git a/packfile.h b/packfile.h
index a58fc738e0..4cfec9e8d3 100644
--- a/packfile.h
+++ b/packfile.h
@@ -90,6 +90,7 @@ uint32_t get_pack_fanout(struct packed_git *p, uint32_t value);
 
 unsigned char *use_pack(struct packed_git *, struct pack_window **, off_t, unsigned long *);
 void close_pack_windows(struct packed_git *);
+void close_pack_revindex(struct packed_git *);
 void close_pack(struct packed_git *);
 void close_object_store(struct raw_object_store *o);
 void unuse_pack(struct pack_window **);
diff --git a/t/README b/t/README
index 1d80fb7004..af706e0cd6 100644
--- a/t/README
+++ b/t/README
@@ -433,6 +433,9 @@ GIT_TEST_DEFAULT_HASH=<hash-algo> specifies which hash algorithm to
 use in the test scripts. Recognized values for <hash-algo> are "sha1"
 and "sha256".
 
+GIT_TEST_WRITE_REV_INDEX=<boolean>, when true enables the
+'pack.writeReverseIndex' setting.
+
 Naming Tests
 ------------
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 297de502a9..2fc3aadbd1 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -710,8 +710,9 @@ test_expect_success 'expire respects .keep files' '
 		PACKA=$(ls .git/objects/pack/a-pack*\.pack | sed s/\.pack\$//) &&
 		touch $PACKA.keep &&
 		git multi-pack-index expire &&
-		ls -S .git/objects/pack/a-pack* | grep $PACKA >a-pack-files &&
-		test_line_count = 3 a-pack-files &&
+		test_path_is_file $PACKA.idx &&
+		test_path_is_file $PACKA.keep &&
+		test_path_is_file $PACKA.pack &&
 		test-tool read-midx .git/objects | grep idx >midx-list &&
 		test_line_count = 2 midx-list
 	)
diff --git a/t/t5325-reverse-index.sh b/t/t5325-reverse-index.sh
new file mode 100755
index 0000000000..da453f68d6
--- /dev/null
+++ b/t/t5325-reverse-index.sh
@@ -0,0 +1,120 @@
+#!/bin/sh
+
+test_description='on-disk reverse index'
+. ./test-lib.sh
+
+# The below tests want control over the 'pack.writeReverseIndex' setting
+# themselves to assert various combinations of it with other options.
+sane_unset GIT_TEST_WRITE_REV_INDEX
+
+packdir=.git/objects/pack
+
+test_expect_success 'setup' '
+	test_commit base &&
+
+	pack=$(git pack-objects --all $packdir/pack) &&
+	rev=$packdir/pack-$pack.rev &&
+
+	test_path_is_missing $rev
+'
+
+test_index_pack () {
+	rm -f $rev &&
+	conf=$1 &&
+	shift &&
+	# remove the index since Windows won't overwrite an existing file
+	rm $packdir/pack-$pack.idx &&
+	git -c pack.writeReverseIndex=$conf index-pack "$@" \
+		$packdir/pack-$pack.pack
+}
+
+test_expect_success 'index-pack with pack.writeReverseIndex' '
+	test_index_pack "" &&
+	test_path_is_missing $rev &&
+
+	test_index_pack false &&
+	test_path_is_missing $rev &&
+
+	test_index_pack true &&
+	test_path_is_file $rev
+'
+
+test_expect_success 'index-pack with --[no-]rev-index' '
+	for conf in "" true false
+	do
+		test_index_pack "$conf" --rev-index &&
+		test_path_exists $rev &&
+
+		test_index_pack "$conf" --no-rev-index &&
+		test_path_is_missing $rev
+	done
+'
+
+test_expect_success 'index-pack can verify reverse indexes' '
+	test_when_finished "rm -f $rev" &&
+	test_index_pack true &&
+
+	test_path_is_file $rev &&
+	git index-pack --rev-index --verify $packdir/pack-$pack.pack &&
+
+	# Intentionally corrupt the reverse index.
+	chmod u+w $rev &&
+	printf "xxxx" | dd of=$rev bs=1 count=4 conv=notrunc &&
+
+	test_must_fail git index-pack --rev-index --verify \
+		$packdir/pack-$pack.pack 2>err &&
+	grep "validation error" err
+'
+
+test_expect_success 'index-pack infers reverse index name with -o' '
+	git index-pack --rev-index -o other.idx $packdir/pack-$pack.pack &&
+	test_path_is_file other.idx &&
+	test_path_is_file other.rev
+'
+
+test_expect_success 'pack-objects respects pack.writeReverseIndex' '
+	test_when_finished "rm -fr pack-1-*" &&
+
+	git -c pack.writeReverseIndex= pack-objects --all pack-1 &&
+	test_path_is_missing pack-1-*.rev &&
+
+	git -c pack.writeReverseIndex=false pack-objects --all pack-1 &&
+	test_path_is_missing pack-1-*.rev &&
+
+	git -c pack.writeReverseIndex=true pack-objects --all pack-1 &&
+	test_path_is_file pack-1-*.rev
+'
+
+test_expect_success 'reverse index is not generated when available on disk' '
+	test_index_pack true &&
+	test_path_is_file $rev &&
+
+	git rev-parse HEAD >tip &&
+	GIT_TEST_REV_INDEX_DIE_IN_MEMORY=1 git cat-file \
+		--batch-check="%(objectsize:disk)" <tip
+'
+
+test_expect_success 'revindex in-memory vs on-disk' '
+	git init repo &&
+	test_when_finished "rm -fr repo" &&
+	(
+		cd repo &&
+
+		test_commit commit &&
+
+		git rev-list --objects --no-object-names --all >objects &&
+
+		git -c pack.writeReverseIndex=false repack -ad &&
+		test_path_is_missing $packdir/pack-*.rev &&
+		git cat-file --batch-check="%(objectsize:disk) %(objectname)" \
+			<objects >in-core &&
+
+		git -c pack.writeReverseIndex=true repack -ad &&
+		test_path_is_file $packdir/pack-*.rev &&
+		git cat-file --batch-check="%(objectsize:disk) %(objectname)" \
+			<objects >on-disk &&
+
+		test_cmp on-disk in-core
+	)
+'
+test_done
diff --git a/t/t5604-clone-reference.sh b/t/t5604-clone-reference.sh
index 5d682706ae..e845d621f6 100755
--- a/t/t5604-clone-reference.sh
+++ b/t/t5604-clone-reference.sh
@@ -329,7 +329,7 @@ test_expect_success SYMLINKS 'clone repo with symlinked or unknown files at obje
 	for raw in $(ls T*.raw)
 	do
 		sed -e "s!/../!/Y/!; s![0-9a-f]\{38,\}!Z!" -e "/commit-graph/d" \
-		    -e "/multi-pack-index/d" <$raw >$raw.de-sha-1 &&
+		    -e "/multi-pack-index/d" -e "/rev/d" <$raw >$raw.de-sha-1 &&
 		sort $raw.de-sha-1 >$raw.de-sha || return 1
 	done &&
 
diff --git a/t/t5702-protocol-v2.sh b/t/t5702-protocol-v2.sh
index 3d994e0b1b..e8f0b4a299 100755
--- a/t/t5702-protocol-v2.sh
+++ b/t/t5702-protocol-v2.sh
@@ -851,8 +851,10 @@ test_expect_success 'part of packfile response provided as URI' '
 	test -f h2found &&
 
 	# Ensure that there are exactly 6 files (3 .pack and 3 .idx).
-	ls http_child/.git/objects/pack/* >filelist &&
-	test_line_count = 6 filelist
+	ls http_child/.git/objects/pack/*.pack >packlist &&
+	ls http_child/.git/objects/pack/*.idx >idxlist &&
+	test_line_count = 3 idxlist &&
+	test_line_count = 3 packlist
 '
 
 test_expect_success 'fetching with valid packfile URI but invalid hash fails' '
@@ -905,8 +907,10 @@ test_expect_success 'packfile-uri with transfer.fsckobjects' '
 		clone "$HTTPD_URL/smart/http_parent" http_child &&
 
 	# Ensure that there are exactly 4 files (2 .pack and 2 .idx).
-	ls http_child/.git/objects/pack/* >filelist &&
-	test_line_count = 4 filelist
+	ls http_child/.git/objects/pack/*.pack >packlist &&
+	ls http_child/.git/objects/pack/*.idx >idxlist &&
+	test_line_count = 2 idxlist &&
+	test_line_count = 2 packlist
 '
 
 test_expect_success 'packfile-uri with transfer.fsckobjects fails on bad object' '
diff --git a/t/t6500-gc.sh b/t/t6500-gc.sh
index 4a3b8f48ac..f76586f808 100755
--- a/t/t6500-gc.sh
+++ b/t/t6500-gc.sh
@@ -106,17 +106,17 @@ test_expect_success 'auto gc with too many loose objects does not attempt to cre
 	test_commit "$(test_oid obj2)" &&
 	# Our first gc will create a pack; our second will create a second pack
 	git gc --auto &&
-	ls .git/objects/pack | sort >existing_packs &&
+	ls .git/objects/pack/pack-*.pack | sort >existing_packs &&
 	test_commit "$(test_oid obj3)" &&
 	test_commit "$(test_oid obj4)" &&
 
 	git gc --auto 2>err &&
 	test_i18ngrep ! "^warning:" err &&
-	ls .git/objects/pack/ | sort >post_packs &&
+	ls .git/objects/pack/pack-*.pack | sort >post_packs &&
 	comm -1 -3 existing_packs post_packs >new &&
 	comm -2 -3 existing_packs post_packs >del &&
 	test_line_count = 0 del && # No packs are deleted
-	test_line_count = 2 new # There is one new pack and its .idx
+	test_line_count = 1 new # There is one new pack
 '
 
 test_expect_success 'gc --no-quiet' '
diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh
index 3d17e932a0..8f1caf8025 100755
--- a/t/t9300-fast-import.sh
+++ b/t/t9300-fast-import.sh
@@ -1632,7 +1632,10 @@ test_expect_success 'O: blank lines not necessary after other commands' '
 	INPUT_END
 
 	git fast-import <input &&
-	test 8 = $(find .git/objects/pack -type f | grep -v multi-pack-index | wc -l) &&
+	ls -la .git/objects/pack/pack-*.pack >packlist &&
+	ls -la .git/objects/pack/pack-*.pack >idxlist &&
+	test_line_count = 4 idxlist &&
+	test_line_count = 4 packlist &&
 	test $(git rev-parse refs/tags/O3-2nd) = $(git rev-parse O3^) &&
 	git log --reverse --pretty=oneline O3 | sed s/^.*z// >actual &&
 	test_cmp expect actual
diff --git a/tmp-objdir.c b/tmp-objdir.c
index 42ed4db5d3..b8d880e362 100644
--- a/tmp-objdir.c
+++ b/tmp-objdir.c
@@ -185,9 +185,11 @@ static int pack_copy_priority(const char *name)
 		return 1;
 	if (ends_with(name, ".pack"))
 		return 2;
-	if (ends_with(name, ".idx"))
+	if (ends_with(name, ".rev"))
 		return 3;
-	return 4;
+	if (ends_with(name, ".idx"))
+		return 4;
+	return 5;
 }
 
 static int pack_copy_cmp(const char *a, const char *b)