pack-objects: enable --path-walk via config

Users may want to enable the --path-walk option for 'git pack-objects' by
default, especially underneath commands like 'git push' or 'git repack'.

This should be limited to client repositories, since the --path-walk option
disables bitmap walks, so would be bad to include in Git servers when
serving fetches and clones. There is potential that it may be helpful to
consider when repacking the repository, to take advantage of improved deltas
across historical versions of the same files.

Much like how "pack.useSparse" was introduced and included in
"feature.experimental" before being enabled by default, use the repository
settings infrastructure to make the new "pack.usePathWalk" config enabled by
"feature.experimental" and "feature.manyFiles".

In order to test that this config works, add a new trace2 region around
the path walk code that can be checked by a 'git push' command.

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

Documentation/config/feature.adoc

@@ -20,6 +20,10 @@ 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.
++
+* `pack.usePathWalk` may speed up packfile creation and make the packfiles be
+significantly smaller in the presence of certain filename collisions with Git's
+default name-hash.
 
 feature.manyFiles::
 	Enable config options that optimize for repos with many files in the

Documentation/config/pack.adoc

@@ -155,6 +155,10 @@ pack.useSparse::
 	commits contain certain types of direct renames. Default is
 	`true`.
 
+pack.usePathWalk::
+	Enable the `--path-walk` option by default for `git pack-objects`
+	processes. See linkgit:git-pack-objects[1] for full details.
+
 pack.preferBitmapTips::
 	When selecting which commits will receive bitmaps, prefer a
 	commit at the tip of any reference that is a suffix of any value

builtin/pack-objects.c

@@ -44,6 +44,7 @@
 #include "blob.h"
 #include "tree.h"
 #include "path-walk.h"
+#include "trace2.h"
 
 /*
  * Objects we are going to pack are collected in the `to_pack` structure.
@@ -4283,6 +4284,7 @@ static void get_object_list_path_walk(struct rev_info *revs)
 {
 	struct path_walk_info info = PATH_WALK_INFO_INIT;
 	unsigned int processed = 0;
+	int result;
 
 	info.revs = revs;
 	info.path_fn = add_objects_by_path;
@@ -4296,7 +4298,11 @@ static void get_object_list_path_walk(struct rev_info *revs)
 	 */
 	info.prune_all_uninteresting = sparse;
 
-	if (walk_objects_by_path(&info))
+	trace2_region_enter("pack-objects", "path-walk", revs->repo);
+	result = walk_objects_by_path(&info);
+	trace2_region_leave("pack-objects", "path-walk", revs->repo);
+
+	if (result)
 		die(_("failed to pack objects via path-walk"));
 }
 
@@ -4652,6 +4658,9 @@ int cmd_pack_objects(int argc,
 		if (use_bitmap_index > 0 ||
 		    !use_internal_rev_list)
 			path_walk = 0;
+		else if (the_repository->gitdir &&
+			 the_repository->settings.pack_use_path_walk)
+			path_walk = 1;
 		else
 			path_walk = git_env_bool("GIT_TEST_PACK_PATH_WALK", 0);
 	}

repo-settings.c

@@ -47,11 +47,13 @@ void prepare_repo_settings(struct repository *r)
 		r->settings.fetch_negotiation_algorithm = FETCH_NEGOTIATION_SKIPPING;
 		r->settings.pack_use_bitmap_boundary_traversal = 1;
 		r->settings.pack_use_multi_pack_reuse = 1;
+		r->settings.pack_use_path_walk = 1;
 	}
 	if (manyfiles) {
 		r->settings.index_version = 4;
 		r->settings.index_skip_hash = 1;
 		r->settings.core_untracked_cache = UNTRACKED_CACHE_WRITE;
+		r->settings.pack_use_path_walk = 1;
 	}
 
 	/* Commit graph config or default, does not cascade (simple) */
@@ -66,6 +68,7 @@ void prepare_repo_settings(struct repository *r)
 
 	/* Boolean config or default, does not cascade (simple)  */
 	repo_cfg_bool(r, "pack.usesparse", &r->settings.pack_use_sparse, 1);
+	repo_cfg_bool(r, "pack.usepathwalk", &r->settings.pack_use_path_walk, 0);
 	repo_cfg_bool(r, "core.multipackindex", &r->settings.core_multi_pack_index, 1);
 	repo_cfg_bool(r, "index.sparse", &r->settings.sparse_index, 0);
 	repo_cfg_bool(r, "index.skiphash", &r->settings.index_skip_hash, r->settings.index_skip_hash);

repo-settings.h

@@ -56,6 +56,7 @@ struct repo_settings {
 	enum untracked_cache_setting core_untracked_cache;
 
 	int pack_use_sparse;
+	int pack_use_path_walk;
 	enum fetch_negotiation_setting fetch_negotiation_algorithm;
 
 	int core_multi_pack_index;

t/t5516-fetch-push.sh

@@ -1907,4 +1907,14 @@ test_expect_success 'push with config push.useBitmaps' '
 		--thin --delta-base-offset -q --no-use-bitmap-index <false
 '
 
+test_expect_success 'push with config pack.usePathWalk=true' '
+	mk_test testrepo heads/main &&
+	git checkout main &&
+	test_config pack.usePathWalk true &&
+	GIT_TRACE2_EVENT="$(pwd)/path-walk.txt" \
+	git push --quiet testrepo main:test &&
+
+	test_region pack-objects path-walk path-walk.txt
+'
+
 test_done