config: let feature.experimental imply gc.cruftPacks=true

We are interested in exploring whether gc.cruftPacks=true should become
the default value.

To determine whether it is safe to do so, let's encourage more users to
try it out.

Users who have set feature.experimental=true have already volunteered to
try new and possibly-breaking config changes, so let's try this new
default with that set of users.

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

Documentation/config/feature.txt

@@ -14,6 +14,9 @@ feature.experimental::
 +
 * `fetch.negotiationAlgorithm=skipping` may improve fetch negotiation times by
 skipping more commits at a time, reducing the number of round trips.
++
+* `gc.cruftPacks=true` reduces disk space used by unreachable objects during
+garbage collection, preventing loose object explosions.
 
 feature.manyFiles::
 	Enable config options that optimize for repos with many files in the

builtin/gc.c

@@ -42,7 +42,7 @@ static const char * const builtin_gc_usage[] = {
 
 static int pack_refs = 1;
 static int prune_reflogs = 1;
-static int cruft_packs = 0;
+static int cruft_packs = -1;
 static int aggressive_depth = 50;
 static int aggressive_window = 250;
 static int gc_auto_threshold = 6700;
@@ -600,6 +600,10 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 	if (prune_expire && parse_expiry_date(prune_expire, &dummy))
 		die(_("failed to parse prune expiry value %s"), prune_expire);
 
+	prepare_repo_settings(the_repository);
+	if (cruft_packs < 0)
+		cruft_packs = the_repository->settings.gc_cruft_packs;
+
 	if (aggressive) {
 		strvec_push(&repack, "-f");
 		if (aggressive_depth > 0)
@@ -711,7 +715,6 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 		clean_pack_garbage();
 	}
 
-	prepare_repo_settings(the_repository);
 	if (the_repository->settings.gc_write_commit_graph == 1)
 		write_commit_graph_reachable(the_repository->objects->odb,
 					     !quiet && !daemonized ? COMMIT_GRAPH_WRITE_PROGRESS : 0,

repo-settings.c

@@ -43,6 +43,7 @@ void prepare_repo_settings(struct repository *r)
 	/* Defaults modified by feature.* */
 	if (experimental) {
 		r->settings.fetch_negotiation_algorithm = FETCH_NEGOTIATION_SKIPPING;
+		r->settings.gc_cruft_packs = 1;
 	}
 	if (manyfiles) {
 		r->settings.index_version = 4;

repository.h

@@ -33,6 +33,7 @@ struct repo_settings {
 	int commit_graph_generation_version;
 	int commit_graph_read_changed_paths;
 	int gc_write_commit_graph;
+	int gc_cruft_packs;
 	int fetch_write_commit_graph;
 	int command_requires_full_index;
 	int sparse_index;

t/t6500-gc.sh

@@ -221,6 +221,11 @@ assert_cruft_packs () {
 	done <packs
 }
 
+assert_no_cruft_packs () {
+	find .git/objects/pack -name "*.mtimes" >mtimes &&
+	test_must_be_empty mtimes
+}
+
 test_expect_success 'gc --cruft generates a cruft pack' '
 	test_when_finished "rm -fr crufts" &&
 	git init crufts &&
@@ -245,6 +250,54 @@ test_expect_success 'gc.cruftPacks=true generates a cruft pack' '
 	)
 '
 
+test_expect_success 'feature.experimental=true generates a cruft pack' '
+	git init crufts &&
+	test_when_finished "rm -fr crufts" &&
+	(
+		cd crufts &&
+
+		prepare_cruft_history &&
+		git -c feature.experimental=true gc &&
+		assert_cruft_packs
+	)
+'
+
+test_expect_success 'feature.experimental=false allows explicit cruft packs' '
+	git init crufts &&
+	test_when_finished "rm -fr crufts" &&
+	(
+		cd crufts &&
+
+		prepare_cruft_history &&
+		git -c gc.cruftPacks=true -c feature.experimental=false gc &&
+		assert_cruft_packs
+	)
+'
+
+test_expect_success 'feature.experimental=true can be overridden' '
+	git init crufts &&
+	test_when_finished "rm -fr crufts" &&
+	(
+		cd crufts &&
+
+		prepare_cruft_history &&
+		git -c feature.expiremental=true -c gc.cruftPacks=false gc &&
+		assert_no_cruft_packs
+	)
+'
+
+test_expect_success 'feature.experimental=false avoids cruft packs by default' '
+	git init crufts &&
+	test_when_finished "rm -fr crufts" &&
+	(
+		cd crufts &&
+
+		prepare_cruft_history &&
+		git -c feature.experimental=false gc &&
+		assert_no_cruft_packs
+	)
+'
+
 run_and_wait_for_auto_gc () {
 	# We read stdout from gc for the side effect of waiting until the
 	# background gc process exits, closing its fd 9.  Furthermore, the