multi-pack-index: prepare 'repack' subcommand

In an environment where the multi-pack-index is useful, it is due
to many pack-files and an inability to repack the object store
into a single pack-file. However, it is likely that many of these
pack-files are rather small, and could be repacked into a slightly
larger pack-file without too much effort. It may also be important
to ensure the object store is highly available and the repack
operation does not interrupt concurrent git commands.

Introduce a 'repack' subcommand to 'git multi-pack-index' that
takes a '--batch-size' option. The subcommand will inspect the
multi-pack-index for referenced pack-files whose size is smaller
than the batch size, until collecting a list of pack-files whose
sizes sum to larger than the batch size. Then, a new pack-file
will be created containing the objects from those pack-files that
are referenced by the multi-pack-index. The resulting pack is
likely to actually be smaller than the batch size due to
compression and the fact that there may be objects in the pack-
files that have duplicate copies in other pack-files.

The current change introduces the command-line arguments, and we
add a test that ensures we parse these options properly. Since
we specify a small batch size, we will guarantee that future
implementations do not change the list of pack-files.

In addition, we hard-code the modified times of the packs in
the pack directory to ensure the list of packs sorted by modified
time matches the order if sorted by size (ascending). This will
be important in a future test.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-multi-pack-index.txt

@@ -36,6 +36,23 @@ expire::
 	have no objects referenced by the MIDX. Rewrite the MIDX file
 	afterward to remove all references to these pack-files.
 
+repack::
+	Create a new pack-file containing objects in small pack-files
+	referenced by the multi-pack-index. If the size given by the
+	`--batch-size=<size>` argument is zero, then create a pack
+	containing all objects referenced by the multi-pack-index. For
+	a non-zero batch size, Select the pack-files by examining packs
+	from oldest-to-newest, computing the "expected size" by counting
+	the number of objects in the pack referenced by the
+	multi-pack-index, then divide by the total number of objects in
+	the pack and multiply by the pack size. We select packs with
+	expected size below the batch size until the set of packs have
+	total expected size at least the batch size. If the total size
+	does not reach the batch size, then do nothing. If a new pack-
+	file is created, rewrite the multi-pack-index to reference the
+	new pack-file. A later run of 'git multi-pack-index expire' will
+	delete the pack-files that were part of this batch.
+
 
 EXAMPLES
 --------

builtin/multi-pack-index.c

@@ -6,12 +6,13 @@
 #include "trace2.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire|repack --batch-size=<size>)"),
 	NULL
 };
 
 static struct opts_multi_pack_index {
 	const char *object_dir;
+	unsigned long batch_size;
 } opts;
 
 int cmd_multi_pack_index(int argc, const char **argv,
@@ -20,6 +21,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 	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_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(),
 	};
 
@@ -43,6 +46,11 @@ int cmd_multi_pack_index(int argc, const char **argv,
 
 	trace2_cmd_mode(argv[0]);
 
+	if (!strcmp(argv[0], "repack"))
+		return midx_repack(the_repository, opts.object_dir, (size_t)opts.batch_size);
+	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);
 	if (!strcmp(argv[0], "verify"))
@@ -50,5 +58,5 @@ int cmd_multi_pack_index(int argc, const char **argv,
 	if (!strcmp(argv[0], "expire"))
 		return expire_midx_packs(the_repository, opts.object_dir);
 
-	die(_("unrecognized verb: %s"), argv[0]);
+	die(_("unrecognized subcommand: %s"), argv[0]);
 }

midx.c

@@ -1226,3 +1226,8 @@ int expire_midx_packs(struct repository *r, const char *object_dir)
 	string_list_clear(&packs_to_drop, 0);
 	return result;
 }
+
+int midx_repack(struct repository *r, const char *object_dir, size_t batch_size)
+{
+	return 0;
+}

midx.h

@@ -51,6 +51,7 @@ int write_midx_file(const char *object_dir);
 void clear_midx_file(struct repository *r);
 int verify_midx_file(struct repository *r, const char *object_dir);
 int expire_midx_packs(struct repository *r, const char *object_dir);
+int midx_repack(struct repository *r, const char *object_dir, size_t batch_size);
 
 void close_midx(struct multi_pack_index *m);
 

t/t5319-multi-pack-index.sh

@@ -398,7 +398,8 @@ test_expect_success 'setup expire tests' '
 		git pack-objects --revs .git/objects/pack/pack-E <<-EOF &&
 		refs/heads/E
 		EOF
-		git multi-pack-index write
+		git multi-pack-index write &&
+		cp -r .git/objects/pack .git/objects/pack-backup
 	)
 '
 
@@ -432,4 +433,21 @@ test_expect_success 'expire removes unreferenced packs' '
 	)
 '
 
+test_expect_success 'repack with minimum size does not alter existing packs' '
+	(
+		cd dup &&
+		rm -rf .git/objects/pack &&
+		mv .git/objects/pack-backup .git/objects/pack &&
+		touch -m -t 201901010000 .git/objects/pack/pack-D* &&
+		touch -m -t 201901010001 .git/objects/pack/pack-C* &&
+		touch -m -t 201901010002 .git/objects/pack/pack-B* &&
+		touch -m -t 201901010003 .git/objects/pack/pack-A* &&
+		ls .git/objects/pack >expect &&
+		MINSIZE=$(ls -l .git/objects/pack/*pack | awk "{print \$5;}" | sort -n | head -n 1) &&
+		git multi-pack-index repack --batch-size=$MINSIZE &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done