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".

Signed-off-by: Derrick Stolee <stolee@gmail.com>

Author: derrickstolee

Diff for: Documentation/config/feature.txt

@@ -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

Diff for: Documentation/config/pack.txt

@@ -155,6 +155,14 @@ pack.useSparse::
 	commits contain certain types of direct renames. Default is
 	`true`.
 
+pack.usePathWalk::
+	When true, git will default to using the '--path-walk' option in
+	'git pack-objects' when the '--revs' option is present. This
+	algorithm groups objects by path to maximize the ability to
+	compute delta chains across historical versions of the same
+	object. This may disable other options, such as using bitmaps to
+	enumerate objects.
+
 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

Diff for: builtin/pack-objects.c

@@ -4597,6 +4597,9 @@ 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);
 	}

Diff for: 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);

Diff for: repo-settings.h

@@ -53,6 +53,7 @@ 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;