pack-objects: enable multi-pack reuse via `feature.experimental`

Now that multi-pack reuse is supported, enable it via the
feature.experimental configuration in addition to the classic
`pack.allowPackReuse`.

This will allow more users to experiment with the new behavior who might
not otherwise be aware of the existing `pack.allowPackReuse`
configuration option.

The enum with values NO_PACK_REUSE, SINGLE_PACK_REUSE, and
MULTI_PACK_REUSE is defined statically in builtin/pack-objects.c's
compilation unit. We could hoist that enum into a scope visible from the
repository_settings struct, and then use that enum value in
pack-objects. Instead, define a single int that indicates what
pack-objects's default value should be to avoid additional unnecessary
code movement.

Though `feature.experimental` implies `pack.allowPackReuse=multi`, this
can still be overridden by explicitly setting the latter configuration
to either "single" or "false". Tests covering all of these cases are
showin t5332.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

5 files changed, 23 insertions, 0 deletions

diff --git a/Documentation/config/feature.txt b/Documentation/config/feature.txt
index bf9546fca4..f061b64b74 100644
--- a/Documentation/config/feature.txt
+++ b/Documentation/config/feature.txt
@@ -17,6 +17,9 @@ skipping more commits at a time, reducing the number of round trips.
 +
 * `pack.useBitmapBoundaryTraversal=true` may improve bitmap traversal times by
 walking fewer objects.
++
+* `pack.allowPackReuse=multi` may improve the time it takes to create a pack by
+reusing objects from multiple packs instead of just one.
 
 feature.manyFiles::
 	Enable config options that optimize for repos with many files in the
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index d8c2128a97..329aeac804 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -4396,6 +4396,8 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 		prepare_repo_settings(the_repository);
 		if (sparse < 0)
 			sparse = the_repository->settings.pack_use_sparse;
+		if (the_repository->settings.pack_use_multi_pack_reuse)
+			allow_pack_reuse = MULTI_PACK_REUSE;
 	}
 
 	reset_pack_idx_option(&pack_idx_opts);
diff --git a/repo-settings.c b/repo-settings.c
index 30cd478762..a0b590bc6c 100644
--- a/repo-settings.c
+++ b/repo-settings.c
@@ -43,6 +43,7 @@ void prepare_repo_settings(struct repository *r)
 	if (experimental) {
 		r->settings.fetch_negotiation_algorithm = FETCH_NEGOTIATION_SKIPPING;
 		r->settings.pack_use_bitmap_boundary_traversal = 1;
+		r->settings.pack_use_multi_pack_reuse = 1;
 	}
 	if (manyfiles) {
 		r->settings.index_version = 4;
diff --git a/repository.h b/repository.h
index 7a250a6605..21949c5a17 100644
--- a/repository.h
+++ b/repository.h
@@ -39,6 +39,7 @@ struct repo_settings {
 	int sparse_index;
 	int pack_read_reverse_index;
 	int pack_use_bitmap_boundary_traversal;
+	int pack_use_multi_pack_reuse;
 
 	/*
 	 * Does this repository have core.useReplaceRefs=true (on by
diff --git a/t/t5332-multi-pack-reuse.sh b/t/t5332-multi-pack-reuse.sh
index d516062f84..b925a81d37 100755
--- a/t/t5332-multi-pack-reuse.sh
+++ b/t/t5332-multi-pack-reuse.sh
@@ -58,6 +58,22 @@ test_expect_success 'preferred pack is reused for single-pack reuse' '
 	test_pack_objects_reused_all 3 1
 '
 
+test_expect_success 'multi-pack reuse is disabled by default' '
+	test_pack_objects_reused_all 3 1
+'
+
+test_expect_success 'feature.experimental implies multi-pack reuse' '
+	test_config feature.experimental true &&
+
+	test_pack_objects_reused_all 6 2
+'
+
+test_expect_success 'multi-pack reuse can be disabled with feature.experimental' '
+	test_config feature.experimental true &&
+	test_config pack.allowPackReuse single &&
+
+	test_pack_objects_reused_all 3 1
+'
 
 test_expect_success 'enable multi-pack reuse' '
 	git config pack.allowPackReuse multi