Merge branch 'tb/reverse-midx'

An on-disk reverse-index to map the in-pack location of an object
back to its object name across multiple packfiles is introduced.

* tb/reverse-midx:
  midx.c: improve cache locality in midx_pack_order_cmp()
  pack-revindex: write multi-pack reverse indexes
  pack-write.c: extract 'write_rev_file_order'
  pack-revindex: read multi-pack reverse indexes
  Documentation/technical: describe multi-pack reverse indexes
  midx: make some functions non-static
  midx: keep track of the checksum
  midx: don't free midx_name early
  midx: allow marking a pack as preferred
  t/helper/test-read-midx.c: add '--show-objects'
  builtin/multi-pack-index.c: display usage on unrecognized command
  builtin/multi-pack-index.c: don't enter bogus cmd_mode
  builtin/multi-pack-index.c: split sub-commands
  builtin/multi-pack-index.c: define common usage with a macro
  builtin/multi-pack-index.c: don't handle 'progress' separately
  builtin/multi-pack-index.c: inline 'flags' with options

14 files changed, 733 insertions, 68 deletions

diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
index eb0caa0439..ffd601bc17 100644
--- a/Documentation/git-multi-pack-index.txt
+++ b/Documentation/git-multi-pack-index.txt
@@ -9,7 +9,8 @@ git-multi-pack-index - Write and verify multi-pack-indexes
 SYNOPSIS
 --------
 [verse]
-'git multi-pack-index' [--object-dir=<dir>] [--[no-]progress] <subcommand>
+'git multi-pack-index' [--object-dir=<dir>] [--[no-]progress]
+	[--preferred-pack=<pack>] <subcommand>
 
 DESCRIPTION
 -----------
@@ -30,7 +31,16 @@ OPTIONS
 The following subcommands are available:
 
 write::
-	Write a new MIDX file.
+	Write a new MIDX file. The following options are available for
+	the `write` sub-command:
++
+--
+	--preferred-pack=<pack>::
+		Optionally specify the tie-breaking pack used when
+		multiple packs contain the same object. If not given,
+		ties are broken in favor of the pack with the lowest
+		mtime.
+--
 
 verify::
 	Verify the contents of the MIDX file.
diff --git a/Documentation/technical/multi-pack-index.txt b/Documentation/technical/multi-pack-index.txt
index e8e377a59f..fb688976c4 100644
--- a/Documentation/technical/multi-pack-index.txt
+++ b/Documentation/technical/multi-pack-index.txt
@@ -43,8 +43,9 @@ Design Details
   a change in format.
 
 - The MIDX keeps only one record per object ID. If an object appears
-  in multiple packfiles, then the MIDX selects the copy in the most-
-  recently modified packfile.
+  in multiple packfiles, then the MIDX selects the copy in the
+  preferred packfile, otherwise selecting from the most-recently
+  modified packfile.
 
 - If there exist packfiles in the pack directory not registered in
   the MIDX, then those packfiles are loaded into the `packed_git`
diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
index 1faa949bf6..8d2f42f29e 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -379,3 +379,86 @@ CHUNK DATA:
 TRAILER:
 
 	Index checksum of the above contents.
+
+== multi-pack-index reverse indexes
+
+Similar to the pack-based reverse index, the multi-pack index can also
+be used to generate a reverse index.
+
+Instead of mapping between offset, pack-, and index position, this
+reverse index maps between an object's position within the MIDX, and
+that object's position within a pseudo-pack that the MIDX describes
+(i.e., the ith entry of the multi-pack reverse index holds the MIDX
+position of ith object in pseudo-pack order).
+
+To clarify the difference between these orderings, consider a multi-pack
+reachability bitmap (which does not yet exist, but is what we are
+building towards here). Each bit needs to correspond to an object in the
+MIDX, and so we need an efficient mapping from bit position to MIDX
+position.
+
+One solution is to let bits occupy the same position in the oid-sorted
+index stored by the MIDX. But because oids are effectively random, their
+resulting reachability bitmaps would have no locality, and thus compress
+poorly. (This is the reason that single-pack bitmaps use the pack
+ordering, and not the .idx ordering, for the same purpose.)
+
+So we'd like to define an ordering for the whole MIDX based around
+pack ordering, which has far better locality (and thus compresses more
+efficiently). We can think of a pseudo-pack created by the concatenation
+of all of the packs in the MIDX. E.g., if we had a MIDX with three packs
+(a, b, c), with 10, 15, and 20 objects respectively, we can imagine an
+ordering of the objects like:
+
+    |a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19|
+
+where the ordering of the packs is defined by the MIDX's pack list,
+and then the ordering of objects within each pack is the same as the
+order in the actual packfile.
+
+Given the list of packs and their counts of objects, you can
+naïvely reconstruct that pseudo-pack ordering (e.g., the object at
+position 27 must be (c,1) because packs "a" and "b" consumed 25 of the
+slots). But there's a catch. Objects may be duplicated between packs, in
+which case the MIDX only stores one pointer to the object (and thus we'd
+want only one slot in the bitmap).
+
+Callers could handle duplicates themselves by reading objects in order
+of their bit-position, but that's linear in the number of objects, and
+much too expensive for ordinary bitmap lookups. Building a reverse index
+solves this, since it is the logical inverse of the index, and that
+index has already removed duplicates. But, building a reverse index on
+the fly can be expensive. Since we already have an on-disk format for
+pack-based reverse indexes, let's reuse it for the MIDX's pseudo-pack,
+too.
+
+Objects from the MIDX are ordered as follows to string together the
+pseudo-pack. Let `pack(o)` return the pack from which `o` was selected
+by the MIDX, and define an ordering of packs based on their numeric ID
+(as stored by the MIDX). Let `offset(o)` return the object offset of `o`
+within `pack(o)`. Then, compare `o1` and `o2` as follows:
+
+  - If one of `pack(o1)` and `pack(o2)` is preferred and the other
+    is not, then the preferred one sorts first.
++
+(This is a detail that allows the MIDX bitmap to determine which
+pack should be used by the pack-reuse mechanism, since it can ask
+the MIDX for the pack containing the object at bit position 0).
+
+  - If `pack(o1) ≠ pack(o2)`, then sort the two objects in descending
+    order based on the pack ID.
+
+  - Otherwise, `pack(o1) = pack(o2)`, and the objects are sorted in
+    pack-order (i.e., `o1` sorts ahead of `o2` exactly when `offset(o1)
+    < offset(o2)`).
+
+In short, a MIDX's pseudo-pack is the de-duplicated concatenation of
+objects in packs stored by the MIDX, laid out in pack order, and the
+packs arranged in MIDX order (with the preferred pack coming first).
+
+Finally, note that the MIDX's reverse index is not stored as a chunk in
+the multi-pack-index itself. This is done because the reverse index
+includes the checksum of the pack or MIDX to which it belongs, which
+makes it impossible to write in the MIDX. To avoid races when rewriting
+the MIDX, a MIDX reverse index includes the MIDX's checksum in its
+filename (e.g., `multi-pack-index-xyz.rev`).
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
index 5bf88cd2a8..5d3ea445fd 100644
--- a/builtin/multi-pack-index.c
+++ b/builtin/multi-pack-index.c
@@ -4,67 +4,181 @@
 #include "parse-options.h"
 #include "midx.h"
 #include "trace2.h"
+#include "object-store.h"
 
+#define BUILTIN_MIDX_WRITE_USAGE \
+	N_("git multi-pack-index [<options>] write [--preferred-pack=<pack>]")
+
+#define BUILTIN_MIDX_VERIFY_USAGE \
+	N_("git multi-pack-index [<options>] verify")
+
+#define BUILTIN_MIDX_EXPIRE_USAGE \
+	N_("git multi-pack-index [<options>] expire")
+
+#define BUILTIN_MIDX_REPACK_USAGE \
+	N_("git multi-pack-index [<options>] repack [--batch-size=<size>]")
+
+static char const * const builtin_multi_pack_index_write_usage[] = {
+	BUILTIN_MIDX_WRITE_USAGE,
+	NULL
+};
+static char const * const builtin_multi_pack_index_verify_usage[] = {
+	BUILTIN_MIDX_VERIFY_USAGE,
+	NULL
+};
+static char const * const builtin_multi_pack_index_expire_usage[] = {
+	BUILTIN_MIDX_EXPIRE_USAGE,
+	NULL
+};
+static char const * const builtin_multi_pack_index_repack_usage[] = {
+	BUILTIN_MIDX_REPACK_USAGE,
+	NULL
+};
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [<options>] (write|verify|expire|repack --batch-size=<size>)"),
+	BUILTIN_MIDX_WRITE_USAGE,
+	BUILTIN_MIDX_VERIFY_USAGE,
+	BUILTIN_MIDX_EXPIRE_USAGE,
+	BUILTIN_MIDX_REPACK_USAGE,
 	NULL
 };
 
 static struct opts_multi_pack_index {
 	const char *object_dir;
+	const char *preferred_pack;
 	unsigned long batch_size;
-	int progress;
+	unsigned flags;
 } opts;
 
-int cmd_multi_pack_index(int argc, const char **argv,
-			 const char *prefix)
+static struct option common_opts[] = {
+	OPT_FILENAME(0, "object-dir", &opts.object_dir,
+	  N_("object directory containing set of packfile and pack-index pairs")),
+	OPT_BIT(0, "progress", &opts.flags, N_("force progress reporting"), MIDX_PROGRESS),
+	OPT_END(),
+};
+
+static struct option *add_common_options(struct option *prev)
 {
-	unsigned flags = 0;
+	return parse_options_concat(common_opts, prev);
+}
+
+static int cmd_multi_pack_index_write(int argc, const char **argv)
+{
+	struct option *options;
+	static struct option builtin_multi_pack_index_write_options[] = {
+		OPT_STRING(0, "preferred-pack", &opts.preferred_pack,
+			   N_("preferred-pack"),
+			   N_("pack for reuse when computing a multi-pack bitmap")),
+		OPT_END(),
+	};
+
+	options = add_common_options(builtin_multi_pack_index_write_options);
+
+	trace2_cmd_mode(argv[0]);
 
-	static struct option builtin_multi_pack_index_options[] = {
-		OPT_FILENAME(0, "object-dir", &opts.object_dir,
-		  N_("object directory containing set of packfile and pack-index pairs")),
-		OPT_BOOL(0, "progress", &opts.progress, N_("force progress reporting")),
+	argc = parse_options(argc, argv, NULL,
+			     options, builtin_multi_pack_index_write_usage,
+			     PARSE_OPT_KEEP_UNKNOWN);
+	if (argc)
+		usage_with_options(builtin_multi_pack_index_write_usage,
+				   options);
+
+	FREE_AND_NULL(options);
+
+	return write_midx_file(opts.object_dir, opts.preferred_pack,
+			       opts.flags);
+}
+
+static int cmd_multi_pack_index_verify(int argc, const char **argv)
+{
+	struct option *options = common_opts;
+
+	trace2_cmd_mode(argv[0]);
+
+	argc = parse_options(argc, argv, NULL,
+			     options, builtin_multi_pack_index_verify_usage,
+			     PARSE_OPT_KEEP_UNKNOWN);
+	if (argc)
+		usage_with_options(builtin_multi_pack_index_verify_usage,
+				   options);
+
+	return verify_midx_file(the_repository, opts.object_dir, opts.flags);
+}
+
+static int cmd_multi_pack_index_expire(int argc, const char **argv)
+{
+	struct option *options = common_opts;
+
+	trace2_cmd_mode(argv[0]);
+
+	argc = parse_options(argc, argv, NULL,
+			     options, builtin_multi_pack_index_expire_usage,
+			     PARSE_OPT_KEEP_UNKNOWN);
+	if (argc)
+		usage_with_options(builtin_multi_pack_index_expire_usage,
+				   options);
+
+	return expire_midx_packs(the_repository, opts.object_dir, opts.flags);
+}
+
+static int cmd_multi_pack_index_repack(int argc, const char **argv)
+{
+	struct option *options;
+	static struct option builtin_multi_pack_index_repack_options[] = {
 		OPT_MAGNITUDE(0, "batch-size", &opts.batch_size,
 		  N_("during repack, collect pack-files of smaller size into a batch that is larger than this size")),
 		OPT_END(),
 	};
 
+	options = add_common_options(builtin_multi_pack_index_repack_options);
+
+	trace2_cmd_mode(argv[0]);
+
+	argc = parse_options(argc, argv, NULL,
+			     options,
+			     builtin_multi_pack_index_repack_usage,
+			     PARSE_OPT_KEEP_UNKNOWN);
+	if (argc)
+		usage_with_options(builtin_multi_pack_index_repack_usage,
+				   options);
+
+	FREE_AND_NULL(options);
+
+	return midx_repack(the_repository, opts.object_dir,
+			   (size_t)opts.batch_size, opts.flags);
+}
+
+int cmd_multi_pack_index(int argc, const char **argv,
+			 const char *prefix)
+{
+	struct option *builtin_multi_pack_index_options = common_opts;
+
 	git_config(git_default_config, NULL);
 
-	opts.progress = isatty(2);
+	if (isatty(2))
+		opts.flags |= MIDX_PROGRESS;
 	argc = parse_options(argc, argv, prefix,
 			     builtin_multi_pack_index_options,
-			     builtin_multi_pack_index_usage, 0);
+			     builtin_multi_pack_index_usage,
+			     PARSE_OPT_STOP_AT_NON_OPTION);
 
 	if (!opts.object_dir)
 		opts.object_dir = get_object_directory();
-	if (opts.progress)
-		flags |= MIDX_PROGRESS;
 
 	if (argc == 0)
+		goto usage;
+
+	if (!strcmp(argv[0], "repack"))
+		return cmd_multi_pack_index_repack(argc, argv);
+	else if (!strcmp(argv[0], "write"))
+		return cmd_multi_pack_index_write(argc, argv);
+	else if (!strcmp(argv[0], "verify"))
+		return cmd_multi_pack_index_verify(argc, argv);
+	else if (!strcmp(argv[0], "expire"))
+		return cmd_multi_pack_index_expire(argc, argv);
+	else {
+usage:
+		error(_("unrecognized subcommand: %s"), argv[0]);
 		usage_with_options(builtin_multi_pack_index_usage,
 				   builtin_multi_pack_index_options);
-
-	if (argc > 1) {
-		die(_("too many arguments"));
-		return 1;
 	}
-
-	trace2_cmd_mode(argv[0]);
-
-	if (!strcmp(argv[0], "repack"))
-		return midx_repack(the_repository, opts.object_dir,
-			(size_t)opts.batch_size, flags);
-	if (opts.batch_size)
-		die(_("--batch-size option is only for 'repack' subcommand"));
-
-	if (!strcmp(argv[0], "write"))
-		return write_midx_file(opts.object_dir, flags);
-	if (!strcmp(argv[0], "verify"))
-		return verify_midx_file(the_repository, opts.object_dir, flags);
-	if (!strcmp(argv[0], "expire"))
-		return expire_midx_packs(the_repository, opts.object_dir, flags);
-
-	die(_("unrecognized subcommand: %s"), argv[0]);
 }
diff --git a/builtin/repack.c b/builtin/repack.c
index 6ce2556c9e..2847fdfbab 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -721,7 +721,7 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	remove_temporary_files();
 
 	if (git_env_bool(GIT_TEST_MULTI_PACK_INDEX, 0))
-		write_midx_file(get_object_directory(), 0);
+		write_midx_file(get_object_directory(), NULL, 0);
 
 	string_list_clear(&names, 0);
 	string_list_clear(&rollback, 0);
diff --git a/midx.c b/midx.c
index becfafe65e..9e86583172 100644
--- a/midx.c
+++ b/midx.c
@@ -12,6 +12,7 @@
 #include "run-command.h"
 #include "repository.h"
 #include "chunk-format.h"
+#include "pack.h"
 
 #define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
 #define MIDX_VERSION 1
@@ -47,11 +48,22 @@ static uint8_t oid_version(void)
 	}
 }
 
+static const unsigned char *get_midx_checksum(struct multi_pack_index *m)
+{
+	return m->data + m->data_len - the_hash_algo->rawsz;
+}
+
 static char *get_midx_filename(const char *object_dir)
 {
 	return xstrfmt("%s/pack/multi-pack-index", object_dir);
 }
 
+char *get_midx_rev_filename(struct multi_pack_index *m)
+{
+	return xstrfmt("%s/pack/multi-pack-index-%s.rev",
+		       m->object_dir, hash_to_hex(get_midx_checksum(m)));
+}
+
 static int midx_read_oid_fanout(const unsigned char *chunk_start,
 				size_t chunk_size, void *data)
 {
@@ -239,7 +251,7 @@ struct object_id *nth_midxed_object_oid(struct object_id *oid,
 	return oid;
 }
 
-static off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos)
+off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos)
 {
 	const unsigned char *offset_data;
 	uint32_t offset32;
@@ -258,7 +270,7 @@ static off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos)
 	return offset32;
 }
 
-static uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos)
+uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos)
 {
 	return get_be32(m->chunk_object_offsets +
 			(off_t)pos * MIDX_CHUNK_OFFSET_WIDTH);
@@ -431,6 +443,14 @@ static int pack_info_compare(const void *_a, const void *_b)
 	return strcmp(a->pack_name, b->pack_name);
 }
 
+static int idx_or_pack_name_cmp(const void *_va, const void *_vb)
+{
+	const char *pack_name = _va;
+	const struct pack_info *compar = _vb;
+
+	return cmp_idx_or_pack_name(pack_name, compar->pack_name);
+}
+
 struct write_midx_context {
 	struct pack_info *info;
 	uint32_t nr;
@@ -443,8 +463,11 @@ struct write_midx_context {
 	uint32_t entries_nr;
 
 	uint32_t *pack_perm;
+	uint32_t *pack_order;
 	unsigned large_offsets_needed:1;
 	uint32_t num_large_offsets;
+
+	int preferred_pack_idx;
 };
 
 static void add_pack_to_midx(const char *full_path, size_t full_path_len,
@@ -489,6 +512,7 @@ struct pack_midx_entry {
 	uint32_t pack_int_id;
 	time_t pack_mtime;
 	uint64_t offset;
+	unsigned preferred : 1;
 };
 
 static int midx_oid_compare(const void *_a, const void *_b)
@@ -500,6 +524,12 @@ static int midx_oid_compare(const void *_a, const void *_b)
 	if (cmp)
 		return cmp;
 
+	/* Sort objects in a preferred pack first when multiple copies exist. */
+	if (a->preferred > b->preferred)
+		return -1;
+	if (a->preferred < b->preferred)
+		return 1;
+
 	if (a->pack_mtime > b->pack_mtime)
 		return -1;
 	else if (a->pack_mtime < b->pack_mtime)
@@ -527,7 +557,8 @@ static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
 static void fill_pack_entry(uint32_t pack_int_id,
 			    struct packed_git *p,
 			    uint32_t cur_object,
-			    struct pack_midx_entry *entry)
+			    struct pack_midx_entry *entry,
+			    int preferred)
 {
 	if (nth_packed_object_id(&entry->oid, p, cur_object) < 0)
 		die(_("failed to locate object %d in packfile"), cur_object);
@@ -536,6 +567,7 @@ static void fill_pack_entry(uint32_t pack_int_id,
 	entry->pack_mtime = p->mtime;
 
 	entry->offset = nth_packed_object_offset(p, cur_object);
+	entry->preferred = !!preferred;
 }
 
 /*
@@ -552,7 +584,8 @@ static void fill_pack_entry(uint32_t pack_int_id,
 static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 						  struct pack_info *info,
 						  uint32_t nr_packs,
-						  uint32_t *nr_objects)
+						  uint32_t *nr_objects,
+						  int preferred_pack)
 {
 	uint32_t cur_fanout, cur_pack, cur_object;
 	uint32_t alloc_fanout, alloc_objects, total_objects = 0;
@@ -589,12 +622,17 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 				nth_midxed_pack_midx_entry(m,
 							   &entries_by_fanout[nr_fanout],
 							   cur_object);
+				if (nth_midxed_pack_int_id(m, cur_object) == preferred_pack)
+					entries_by_fanout[nr_fanout].preferred = 1;
+				else
+					entries_by_fanout[nr_fanout].preferred = 0;
 				nr_fanout++;
 			}
 		}
 
 		for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++) {
 			uint32_t start = 0, end;
+			int preferred = cur_pack == preferred_pack;
 
 			if (cur_fanout)
 				start = get_pack_fanout(info[cur_pack].p, cur_fanout - 1);
@@ -602,7 +640,11 @@ static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
 
 			for (cur_object = start; cur_object < end; cur_object++) {
 				ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
-				fill_pack_entry(cur_pack, info[cur_pack].p, cur_object, &entries_by_fanout[nr_fanout]);
+				fill_pack_entry(cur_pack,
+						info[cur_pack].p,
+						cur_object,
+						&entries_by_fanout[nr_fanout],
+						preferred);
 				nr_fanout++;
 			}
 		}
@@ -776,10 +818,80 @@ static int write_midx_large_offsets(struct hashfile *f,
 	return 0;
 }
 
+struct midx_pack_order_data {
+	uint32_t nr;
+	uint32_t pack;
+	off_t offset;
+};
+
+static int midx_pack_order_cmp(const void *va, const void *vb)
+{
+	const struct midx_pack_order_data *a = va, *b = vb;
+	if (a->pack < b->pack)
+		return -1;
+	else if (a->pack > b->pack)
+		return 1;
+	else if (a->offset < b->offset)
+		return -1;
+	else if (a->offset > b->offset)
+		return 1;
+	else
+		return 0;
+}
+
+static uint32_t *midx_pack_order(struct write_midx_context *ctx)
+{
+	struct midx_pack_order_data *data;
+	uint32_t *pack_order;
+	uint32_t i;
+
+	ALLOC_ARRAY(data, ctx->entries_nr);
+	for (i = 0; i < ctx->entries_nr; i++) {
+		struct pack_midx_entry *e = &ctx->entries[i];
+		data[i].nr = i;
+		data[i].pack = ctx->pack_perm[e->pack_int_id];
+		if (!e->preferred)
+			data[i].pack |= (1U << 31);
+		data[i].offset = e->offset;
+	}
+
+	QSORT(data, ctx->entries_nr, midx_pack_order_cmp);
+
+	ALLOC_ARRAY(pack_order, ctx->entries_nr);
+	for (i = 0; i < ctx->entries_nr; i++)
+		pack_order[i] = data[i].nr;
+	free(data);
+
+	return pack_order;
+}
+
+static void write_midx_reverse_index(char *midx_name, unsigned char *midx_hash,
+				     struct write_midx_context *ctx)
+{
+	struct strbuf buf = STRBUF_INIT;
+	const char *tmp_file;
+
+	strbuf_addf(&buf, "%s-%s.rev", midx_name, hash_to_hex(midx_hash));
+
+	tmp_file = write_rev_file_order(NULL, ctx->pack_order, ctx->entries_nr,
+					midx_hash, WRITE_REV);
+
+	if (finalize_object_file(tmp_file, buf.buf))
+		die(_("cannot store reverse index file"));
+
+	strbuf_release(&buf);
+}
+
+static void clear_midx_files_ext(struct repository *r, const char *ext,
+				 unsigned char *keep_hash);
+
 static int write_midx_internal(const char *object_dir, struct multi_pack_index *m,
-			       struct string_list *packs_to_drop, unsigned flags)
+			       struct string_list *packs_to_drop,
+			       const char *preferred_pack_name,
+			       unsigned flags)
 {
 	char *midx_name;
+	unsigned char midx_hash[GIT_MAX_RAWSZ];
 	uint32_t i;
 	struct hashfile *f = NULL;
 	struct lock_file lk;
@@ -828,7 +940,19 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 	if (ctx.m && ctx.nr == ctx.m->num_packs && !packs_to_drop)
 		goto cleanup;
 
-	ctx.entries = get_sorted_entries(ctx.m, ctx.info, ctx.nr, &ctx.entries_nr);
+	ctx.preferred_pack_idx = -1;
+	if (preferred_pack_name) {
+		for (i = 0; i < ctx.nr; i++) {
+			if (!cmp_idx_or_pack_name(preferred_pack_name,
+						  ctx.info[i].pack_name)) {
+				ctx.preferred_pack_idx = i;
+				break;
+			}
+		}
+	}
+
+	ctx.entries = get_sorted_entries(ctx.m, ctx.info, ctx.nr, &ctx.entries_nr,
+					 ctx.preferred_pack_idx);
 
 	ctx.large_offsets_needed = 0;
 	for (i = 0; i < ctx.entries_nr; i++) {
@@ -889,13 +1013,30 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 			pack_name_concat_len += strlen(ctx.info[i].pack_name) + 1;
 	}
 
+	/* Check that the preferred pack wasn't expired (if given). */
+	if (preferred_pack_name) {
+		struct pack_info *preferred = bsearch(preferred_pack_name,
+						      ctx.info, ctx.nr,
+						      sizeof(*ctx.info),
+						      idx_or_pack_name_cmp);
+
+		if (!preferred)
+			warning(_("unknown preferred pack: '%s'"),
+				preferred_pack_name);
+		else {
+			uint32_t perm = ctx.pack_perm[preferred->orig_pack_int_id];
+			if (perm == PACK_EXPIRED)
+				warning(_("preferred pack '%s' is expired"),
+					preferred_pack_name);
+		}
+	}
+
 	if (pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
 		pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
 					(pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
 
 	hold_lock_file_for_update(&lk, midx_name, LOCK_DIE_ON_ERROR);
 	f = hashfd(get_lock_file_fd(&lk), get_lock_file_path(&lk));
-	FREE_AND_NULL(midx_name);
 
 	if (ctx.m)
 		close_midx(ctx.m);
@@ -927,8 +1068,16 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 	write_midx_header(f, get_num_chunks(cf), ctx.nr - dropped_packs);
 	write_chunkfile(cf, &ctx);
 
-	finalize_hashfile(f, NULL, CSUM_FSYNC | CSUM_HASH_IN_STREAM);
+	finalize_hashfile(f, midx_hash, CSUM_FSYNC | CSUM_HASH_IN_STREAM);
 	free_chunkfile(cf);
+
+	if (flags & MIDX_WRITE_REV_INDEX)
+		ctx.pack_order = midx_pack_order(&ctx);
+
+	if (flags & MIDX_WRITE_REV_INDEX)
+		write_midx_reverse_index(midx_name, midx_hash, &ctx);
+	clear_midx_files_ext(the_repository, ".rev", midx_hash);
+
 	commit_lock_file(&lk);
 
 cleanup:
@@ -943,13 +1092,55 @@ cleanup:
 	free(ctx.info);
 	free(ctx.entries);
 	free(ctx.pack_perm);
+	free(ctx.pack_order);
 	free(midx_name);
 	return result;
 }
 
-int write_midx_file(const char *object_dir, unsigned flags)
+int write_midx_file(const char *object_dir,
+		    const char *preferred_pack_name,
+		    unsigned flags)
 {
-	return write_midx_internal(object_dir, NULL, NULL, flags);
+	return write_midx_internal(object_dir, NULL, NULL, preferred_pack_name,
+				   flags);
+}
+
+struct clear_midx_data {
+	char *keep;
+	const char *ext;
+};
+
+static void clear_midx_file_ext(const char *full_path, size_t full_path_len,
+				const char *file_name, void *_data)
+{
+	struct clear_midx_data *data = _data;
+
+	if (!(starts_with(file_name, "multi-pack-index-") &&
+	      ends_with(file_name, data->ext)))
+		return;
+	if (data->keep && !strcmp(data->keep, file_name))
+		return;
+
+	if (unlink(full_path))
+		die_errno(_("failed to remove %s"), full_path);
+}
+
+static void clear_midx_files_ext(struct repository *r, const char *ext,
+				 unsigned char *keep_hash)
+{
+	struct clear_midx_data data;
+	memset(&data, 0, sizeof(struct clear_midx_data));
+
+	if (keep_hash)
+		data.keep = xstrfmt("multi-pack-index-%s%s",
+				    hash_to_hex(keep_hash), ext);
+	data.ext = ext;
+
+	for_each_file_in_pack_dir(r->objects->odb->path,
+				  clear_midx_file_ext,
+				  &data);
+
+	free(data.keep);
 }
 
 void clear_midx_file(struct repository *r)
@@ -964,6 +1155,8 @@ void clear_midx_file(struct repository *r)
 	if (remove_path(midx))
 		die(_("failed to clear multi-pack-index at %s"), midx);
 
+	clear_midx_files_ext(r, ".rev", NULL);
+
 	free(midx);
 }
 
@@ -1184,7 +1377,7 @@ int expire_midx_packs(struct repository *r, const char *object_dir, unsigned fla
 	free(count);
 
 	if (packs_to_drop.nr)
-		result = write_midx_internal(object_dir, m, &packs_to_drop, flags);
+		result = write_midx_internal(object_dir, m, &packs_to_drop, NULL, flags);
 
 	string_list_clear(&packs_to_drop, 0);
 	return result;
@@ -1373,7 +1566,7 @@ int midx_repack(struct repository *r, const char *object_dir, size_t batch_size,
 		goto cleanup;
 	}
 
-	result = write_midx_internal(object_dir, m, NULL, flags);
+	result = write_midx_internal(object_dir, m, NULL, NULL, flags);
 	m = NULL;
 
 cleanup:
diff --git a/midx.h b/midx.h
index b18cf53bc4..8684cf0fef 100644
--- a/midx.h
+++ b/midx.h
@@ -15,6 +15,10 @@ struct multi_pack_index {
 	const unsigned char *data;
 	size_t data_len;
 
+	const uint32_t *revindex_data;
+	const uint32_t *revindex_map;
+	size_t revindex_len;
+
 	uint32_t signature;
 	unsigned char version;
 	unsigned char hash_len;
@@ -36,10 +40,15 @@ struct multi_pack_index {
 };
 
 #define MIDX_PROGRESS     (1 << 0)
+#define MIDX_WRITE_REV_INDEX (1 << 1)
+
+char *get_midx_rev_filename(struct multi_pack_index *m);
 
 struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local);
 int prepare_midx_pack(struct repository *r, struct multi_pack_index *m, uint32_t pack_int_id);
 int bsearch_midx(const struct object_id *oid, struct multi_pack_index *m, uint32_t *result);
+off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos);
+uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos);
 struct object_id *nth_midxed_object_oid(struct object_id *oid,
 					struct multi_pack_index *m,
 					uint32_t n);
@@ -47,7 +56,7 @@ int fill_midx_entry(struct repository *r, const struct object_id *oid, struct pa
 int midx_contains_pack(struct multi_pack_index *m, const char *idx_or_pack_name);
 int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, int local);
 
-int write_midx_file(const char *object_dir, unsigned flags);
+int write_midx_file(const char *object_dir, const char *preferred_pack_name, unsigned flags);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(struct repository *r, const char *object_dir, unsigned flags);
 int expire_midx_packs(struct repository *r, const char *object_dir, unsigned flags);
diff --git a/pack-revindex.c b/pack-revindex.c
index 4262530449..0e4a31d9db 100644
--- a/pack-revindex.c
+++ b/pack-revindex.c
@@ -3,6 +3,7 @@
 #include "object-store.h"
 #include "packfile.h"
 #include "config.h"
+#include "midx.h"
 
 struct revindex_entry {
 	off_t offset;
@@ -293,6 +294,43 @@ int load_pack_revindex(struct packed_git *p)
 	return -1;
 }
 
+int load_midx_revindex(struct multi_pack_index *m)
+{
+	char *revindex_name;
+	int ret;
+	if (m->revindex_data)
+		return 0;
+
+	revindex_name = get_midx_rev_filename(m);
+
+	ret = load_revindex_from_disk(revindex_name,
+				      m->num_objects,
+				      &m->revindex_map,
+				      &m->revindex_len);
+	if (ret)
+		goto cleanup;
+
+	m->revindex_data = (const uint32_t *)((const char *)m->revindex_map + RIDX_HEADER_SIZE);
+
+cleanup:
+	free(revindex_name);
+	return ret;
+}
+
+int close_midx_revindex(struct multi_pack_index *m)
+{
+	if (!m || !m->revindex_map)
+		return 0;
+
+	munmap((void*)m->revindex_map, m->revindex_len);
+
+	m->revindex_map = NULL;
+	m->revindex_data = NULL;
+	m->revindex_len = 0;
+
+	return 0;
+}
+
 int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos)
 {
 	unsigned lo, hi;
@@ -347,3 +385,91 @@ off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos)
 	else
 		return nth_packed_object_offset(p, pack_pos_to_index(p, pos));
 }
+
+uint32_t pack_pos_to_midx(struct multi_pack_index *m, uint32_t pos)
+{
+	if (!m->revindex_data)
+		BUG("pack_pos_to_midx: reverse index not yet loaded");
+	if (m->num_objects <= pos)
+		BUG("pack_pos_to_midx: out-of-bounds object at %"PRIu32, pos);
+	return get_be32(m->revindex_data + pos);
+}
+
+struct midx_pack_key {
+	uint32_t pack;
+	off_t offset;
+
+	uint32_t preferred_pack;
+	struct multi_pack_index *midx;
+};
+
+static int midx_pack_order_cmp(const void *va, const void *vb)
+{
+	const struct midx_pack_key *key = va;
+	struct multi_pack_index *midx = key->midx;
+
+	uint32_t versus = pack_pos_to_midx(midx, (uint32_t*)vb - (const uint32_t *)midx->revindex_data);
+	uint32_t versus_pack = nth_midxed_pack_int_id(midx, versus);
+	off_t versus_offset;
+
+	uint32_t key_preferred = key->pack == key->preferred_pack;
+	uint32_t versus_preferred = versus_pack == key->preferred_pack;
+
+	/*
+	 * First, compare the preferred-ness, noting that the preferred pack
+	 * comes first.
+	 */
+	if (key_preferred && !versus_preferred)
+		return -1;
+	else if (!key_preferred && versus_preferred)
+		return 1;
+
+	/* Then, break ties first by comparing the pack IDs. */
+	if (key->pack < versus_pack)
+		return -1;
+	else if (key->pack > versus_pack)
+		return 1;
+
+	/* Finally, break ties by comparing offsets within a pack. */
+	versus_offset = nth_midxed_offset(midx, versus);
+	if (key->offset < versus_offset)
+		return -1;
+	else if (key->offset > versus_offset)
+		return 1;
+
+	return 0;
+}
+
+int midx_to_pack_pos(struct multi_pack_index *m, uint32_t at, uint32_t *pos)
+{
+	struct midx_pack_key key;
+	uint32_t *found;
+
+	if (!m->revindex_data)
+		BUG("midx_to_pack_pos: reverse index not yet loaded");
+	if (m->num_objects <= at)
+		BUG("midx_to_pack_pos: out-of-bounds object at %"PRIu32, at);
+
+	key.pack = nth_midxed_pack_int_id(m, at);
+	key.offset = nth_midxed_offset(m, at);
+	key.midx = m;
+	/*
+	 * The preferred pack sorts first, so determine its identifier by
+	 * looking at the first object in pseudo-pack order.
+	 *
+	 * Note that if no --preferred-pack is explicitly given when writing a
+	 * multi-pack index, then whichever pack has the lowest identifier
+	 * implicitly is preferred (and includes all its objects, since ties are
+	 * broken first by pack identifier).
+	 */
+	key.preferred_pack = nth_midxed_pack_int_id(m, pack_pos_to_midx(m, 0));
+
+	found = bsearch(&key, m->revindex_data, m->num_objects,
+			sizeof(*m->revindex_data), midx_pack_order_cmp);
+
+	if (!found)
+		return error("bad offset for revindex");
+
+	*pos = found - m->revindex_data;
+	return 0;
+}
diff --git a/pack-revindex.h b/pack-revindex.h
index ba7c82c125..479b8f2f9c 100644
--- a/pack-revindex.h
+++ b/pack-revindex.h
@@ -14,6 +14,20 @@
  *
  * - offset: the byte offset within the .pack file at which the object contents
  *   can be found
+ *
+ * The revindex can also be used with a multi-pack index (MIDX). In this
+ * setting:
+ *
+ *   - index position refers to an object's numeric position within the MIDX
+ *
+ *   - pack position refers to an object's position within a non-existent pack
+ *     described by the MIDX. The pack structure is described in
+ *     Documentation/technical/pack-format.txt.
+ *
+ *     It is effectively a concatanation of all packs in the MIDX (ordered by
+ *     their numeric ID within the MIDX) in their original order within each
+ *     pack), removing duplicates, and placing the preferred pack (if any)
+ *     first.
  */
 
 
@@ -24,6 +38,7 @@
 #define GIT_TEST_REV_INDEX_DIE_IN_MEMORY "GIT_TEST_REV_INDEX_DIE_IN_MEMORY"
 
 struct packed_git;
+struct multi_pack_index;
 
 /*
  * load_pack_revindex populates the revindex's internal data-structures for the
@@ -35,6 +50,22 @@ struct packed_git;
 int load_pack_revindex(struct packed_git *p);
 
 /*
+ * load_midx_revindex loads the '.rev' file corresponding to the given
+ * multi-pack index by mmap-ing it and assigning pointers in the
+ * multi_pack_index to point at it.
+ *
+ * A negative number is returned on error.
+ */
+int load_midx_revindex(struct multi_pack_index *m);
+
+/*
+ * Frees resources associated with a multi-pack reverse index.
+ *
+ * A negative number is returned on error.
+ */
+int close_midx_revindex(struct multi_pack_index *m);
+
+/*
  * offset_to_pack_pos converts an object offset to a pack position. This
  * function returns zero on success, and a negative number otherwise. The
  * parameter 'pos' is usable only on success.
@@ -71,4 +102,26 @@ uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos);
  */
 off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos);
 
+/*
+ * pack_pos_to_midx converts the object at position "pos" within the MIDX
+ * pseudo-pack into a MIDX position.
+ *
+ * If the reverse index has not yet been loaded, or the position is out of
+ * bounds, this function aborts.
+ *
+ * This function runs in time O(log N) with the number of objects in the MIDX.
+ */
+uint32_t pack_pos_to_midx(struct multi_pack_index *m, uint32_t pos);
+
+/*
+ * midx_to_pack_pos converts from the MIDX-relative position at "at" to the
+ * corresponding pack position.
+ *
+ * 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.
+ */
+int midx_to_pack_pos(struct multi_pack_index *midx, uint32_t at, uint32_t *pos);
+
 #endif
diff --git a/pack-write.c b/pack-write.c
index 2ca85a9d16..f1fc3ecafa 100644
--- a/pack-write.c
+++ b/pack-write.c
@@ -201,21 +201,12 @@ static void write_rev_header(struct hashfile *f)
 }
 
 static void write_rev_index_positions(struct hashfile *f,
-				      struct pack_idx_entry **objects,
+				      uint32_t *pack_order,
 				      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)
@@ -229,6 +220,29 @@ const char *write_rev_file(const char *rev_name,
 			   const unsigned char *hash,
 			   unsigned flags)
 {
+	uint32_t *pack_order;
+	uint32_t i;
+	const char *ret;
+
+	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);
+
+	ret = write_rev_file_order(rev_name, pack_order, nr_objects, hash,
+				   flags);
+
+	free(pack_order);
+
+	return ret;
+}
+
+const char *write_rev_file_order(const char *rev_name,
+				 uint32_t *pack_order,
+				 uint32_t nr_objects,
+				 const unsigned char *hash,
+				 unsigned flags)
+{
 	struct hashfile *f;
 	int fd;
 
@@ -262,7 +276,7 @@ const char *write_rev_file(const char *rev_name,
 
 	write_rev_header(f);
 
-	write_rev_index_positions(f, objects, nr_objects);
+	write_rev_index_positions(f, pack_order, nr_objects);
 	write_rev_trailer(f, hash);
 
 	if (rev_name && adjust_shared_perm(rev_name) < 0)
diff --git a/pack.h b/pack.h
index 857cbd5bd4..fa13954526 100644
--- a/pack.h
+++ b/pack.h
@@ -94,6 +94,7 @@ 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);
+const char *write_rev_file_order(const char *rev_name, uint32_t *pack_order, uint32_t nr_objects, const unsigned char *hash, unsigned flags);
 
 /*
  * The "hdr" output buffer should be at least this big, which will handle sizes
diff --git a/packfile.c b/packfile.c
index 6661f3325a..8668345d93 100644
--- a/packfile.c
+++ b/packfile.c
@@ -862,6 +862,9 @@ static void prepare_pack(const char *full_name, size_t full_name_len,
 
 	if (!strcmp(file_name, "multi-pack-index"))
 		return;
+	if (starts_with(file_name, "multi-pack-index") &&
+	    ends_with(file_name, ".rev"))
+		return;
 	if (ends_with(file_name, ".idx") ||
 	    ends_with(file_name, ".rev") ||
 	    ends_with(file_name, ".pack") ||
diff --git a/t/helper/test-read-midx.c b/t/helper/test-read-midx.c
index 2430880f78..7c2eb11a8e 100644
--- a/t/helper/test-read-midx.c
+++ b/t/helper/test-read-midx.c
@@ -4,7 +4,7 @@
 #include "repository.h"
 #include "object-store.h"
 
-static int read_midx_file(const char *object_dir)
+static int read_midx_file(const char *object_dir, int show_objects)
 {
 	uint32_t i;
 	struct multi_pack_index *m;
@@ -43,13 +43,29 @@ static int read_midx_file(const char *object_dir)
 
 	printf("object-dir: %s\n", m->object_dir);
 
+	if (show_objects) {
+		struct object_id oid;
+		struct pack_entry e;
+
+		for (i = 0; i < m->num_objects; i++) {
+			nth_midxed_object_oid(&oid, m, i);
+			fill_midx_entry(the_repository, &oid, &e, m);
+
+			printf("%s %"PRIu64"\t%s\n",
+			       oid_to_hex(&oid), e.offset, e.p->pack_name);
+		}
+		return 0;
+	}
+
 	return 0;
 }
 
 int cmd__read_midx(int argc, const char **argv)
 {
-	if (argc != 2)
-		usage("read-midx <object-dir>");
+	if (!(argc == 2 || argc == 3))
+		usage("read-midx [--show-objects] <object-dir>");
 
-	return read_midx_file(argv[1]);
+	if (!strcmp(argv[1], "--show-objects"))
+		return read_midx_file(argv[2], 1);
+	return read_midx_file(argv[1], 0);
 }
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index b4afab1dfc..5641d158df 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -234,6 +234,48 @@ test_expect_success 'warn on improper hash version' '
 	)
 '
 
+test_expect_success 'midx picks objects from preferred pack' '
+	test_when_finished rm -rf preferred.git &&
+	git init --bare preferred.git &&
+	(
+		cd preferred.git &&
+
+		a=$(echo "a" | git hash-object -w --stdin) &&
+		b=$(echo "b" | git hash-object -w --stdin) &&
+		c=$(echo "c" | git hash-object -w --stdin) &&
+
+		# Set up two packs, duplicating the object "B" at different
+		# offsets.
+		#
+		# Note that the "BC" pack (the one we choose as preferred) sorts
+		# lexically after the "AB" pack, meaning that omitting the
+		# --preferred-pack argument would cause this test to fail (since
+		# the MIDX code would select the copy of "b" in the "AB" pack).
+		git pack-objects objects/pack/test-AB <<-EOF &&
+		$a
+		$b
+		EOF
+		bc=$(git pack-objects objects/pack/test-BC <<-EOF
+		$b
+		$c
+		EOF
+		) &&
+
+		git multi-pack-index --object-dir=objects \
+			write --preferred-pack=test-BC-$bc.idx 2>err &&
+		test_must_be_empty err &&
+
+		test-tool read-midx --show-objects objects >out &&
+
+		ofs=$(git show-index <objects/pack/test-BC-$bc.idx | grep $b |
+			cut -d" " -f1) &&
+		printf "%s %s\tobjects/pack/test-BC-%s.pack\n" \
+			"$b" "$ofs" "$bc" >expect &&
+		grep ^$b out >actual &&
+
+		test_cmp expect actual
+	)
+'
 
 test_expect_success 'verify multi-pack-index success' '
 	git multi-pack-index verify --object-dir=$objdir