multi-pack-index: prepare for 'expire' subcommand

The multi-pack-index tracks objects in a collection of pack-files.
Only one copy of each object is indexed, using the modified time
of the pack-files to determine tie-breakers. It is possible to
have a pack-file with no referenced objects because all objects
have a duplicate in a newer pack-file.

Introduce a new 'expire' subcommand to the multi-pack-index builtin.
This subcommand will delete these unused pack-files and rewrite the
multi-pack-index to no longer refer to those files. More details
about the specifics will follow as the method is implemented.

Add a test that verifies the 'expire' subcommand is correctly wired,
but will still be valid when the verb is implemented. Specifically,
create a set of packs that should all have referenced objects and
should not be removed during an 'expire' operation. The packs are
created carefully to ensure they have a specific order when sorted
by size. This will be important in a later test.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

Documentation/git-multi-pack-index.txt

@@ -31,6 +31,11 @@ write::
 verify::
 	Verify the contents of the MIDX file.
 
+expire::
+	Delete the pack-files that are tracked 	by the MIDX file, but
+	have no objects referenced by the MIDX. Rewrite the MIDX file
+	afterward to remove all references to these pack-files.
+
 
 EXAMPLES
 --------

builtin/multi-pack-index.c

@@ -6,7 +6,7 @@
 #include "trace2.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire)"),
 	NULL
 };
 
@@ -47,6 +47,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return write_midx_file(opts.object_dir);
 	if (!strcmp(argv[0], "verify"))
 		return verify_midx_file(the_repository, opts.object_dir);
+	if (!strcmp(argv[0], "expire"))
+		return expire_midx_packs(the_repository, opts.object_dir);
 
 	die(_("unrecognized verb: %s"), argv[0]);
 }

midx.c

@@ -1140,3 +1140,8 @@ int verify_midx_file(struct repository *r, const char *object_dir)
 
 	return verify_midx_error;
 }
+
+int expire_midx_packs(struct repository *r, const char *object_dir)
+{
+	return 0;
+}

midx.h

@@ -50,6 +50,7 @@ int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, i
 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);
 
 void close_midx(struct multi_pack_index *m);
 

t/t5319-multi-pack-index.sh

@@ -363,4 +363,53 @@ test_expect_success 'verify incorrect 64-bit offset' '
 		"incorrect object offset"
 '
 
+test_expect_success 'setup expire tests' '
+	mkdir dup &&
+	(
+		cd dup &&
+		git init &&
+		test-tool genrandom "data" 4096 >large_file.txt &&
+		git update-index --add large_file.txt &&
+		for i in $(test_seq 1 20)
+		do
+			test_commit $i
+		done &&
+		git branch A HEAD &&
+		git branch B HEAD~8 &&
+		git branch C HEAD~13 &&
+		git branch D HEAD~16 &&
+		git branch E HEAD~18 &&
+		git pack-objects --revs .git/objects/pack/pack-A <<-EOF &&
+		refs/heads/A
+		^refs/heads/B
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-B <<-EOF &&
+		refs/heads/B
+		^refs/heads/C
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-C <<-EOF &&
+		refs/heads/C
+		^refs/heads/D
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-D <<-EOF &&
+		refs/heads/D
+		^refs/heads/E
+		EOF
+		git pack-objects --revs .git/objects/pack/pack-E <<-EOF &&
+		refs/heads/E
+		EOF
+		git multi-pack-index write
+	)
+'
+
+test_expect_success 'expire does not remove any packs' '
+	(
+		cd dup &&
+		ls .git/objects/pack >expect &&
+		git multi-pack-index expire &&
+		ls .git/objects/pack >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done