pack-objects: add --path-walk option

In order to more easily compute delta bases among objects that appear at the
exact same path, add a --path-walk option to 'git pack-objects'.

This option will use the path-walk API instead of the object walk given by
the revision machinery. Since objects will be provided in batches
representing a common path, those objects can be tested for delta bases
immediately instead of waiting for a sort of the full object list by
name-hash. This has multiple benefits, including avoiding collisions by
name-hash.

The objects marked as UNINTERESTING are included in these batches, so we
are guaranteeing some locality to find good delta bases.

After the individual passes are done on a per-path basis, the default
name-hash is used to find other opportunistic delta bases that did not
match exactly by the full path name.

RFC TODO: It is important to note that this option is inherently
incompatible with using a bitmap index. This walk probably also does not
work with other advanced features, such as delta islands.

Getting ahead of myself, this option compares well with --full-name-hash
when the packfile is large enough, but also performs at least as well as
the default in all cases that I've seen.

RFC TODO: this should probably be recording the batch locations to another
list so they could be processed in a second phase using threads.

RFC TODO: list some examples of how this outperforms previous pack-objects
strategies. (This is coming in later commits that include performance
test changes.)

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

Documentation/git-pack-objects.adoc

@@ -16,7 +16,7 @@ SYNOPSIS
 	[--cruft] [--cruft-expiration=<time>]
 	[--stdout [--filter=<filter-spec>] | <base-name>]
 	[--shallow] [--keep-true-parents] [--[no-]sparse]
-	[--name-hash-version=<n>] < <object-list>
+	[--name-hash-version=<n>] [--path-walk] < <object-list>
 
 
 DESCRIPTION
@@ -375,6 +375,16 @@ many different directories. At the moment, this version is not allowed
 when writing reachability bitmap files with `--write-bitmap-index` and it
 will be automatically changed to version `1`.
 
+--path-walk::
+	By default, `git pack-objects` walks objects in an order that
+	presents trees and blobs in an order unrelated to the path they
+	appear relative to a commit's root tree. The `--path-walk` option
+	enables a different walking algorithm that organizes trees and
+	blobs by path. This has the potential to improve delta compression
+	especially in the presence of filenames that cause collisions in
+	Git's default name-hash algorithm. Due to changing how the objects
+	are walked, this option is not compatible with `--delta-islands`,
+	`--shallow`, or `--filter`.
 
 DELTA ISLANDS
 -------------

Documentation/technical/api-path-walk.adoc

@@ -70,3 +70,4 @@ Examples
 See example usages in:
 	`t/helper/test-path-walk.c`,
 	`builtin/backfill.c`
+	`builtin/pack-objects.c`

builtin/pack-objects.c

@@ -41,6 +41,9 @@
 #include "promisor-remote.h"
 #include "pack-mtimes.h"
 #include "parse-options.h"
+#include "blob.h"
+#include "tree.h"
+#include "path-walk.h"
 
 /*
  * Objects we are going to pack are collected in the `to_pack` structure.
@@ -217,6 +220,7 @@ static int delta_search_threads;
 static int pack_to_stdout;
 static int sparse;
 static int thin;
+static int path_walk;
 static int num_preferred_base;
 static struct progress *progress_state;
 
@@ -4188,6 +4192,105 @@ static void mark_bitmap_preferred_tips(void)
 	}
 }
 
+static inline int is_oid_interesting(struct repository *repo,
+				     struct object_id *oid)
+{
+	struct object *o = lookup_object(repo, oid);
+	return o && !(o->flags & UNINTERESTING);
+}
+
+static int add_objects_by_path(const char *path,
+			       struct oid_array *oids,
+			       enum object_type type,
+			       void *data)
+{
+	struct object_entry **delta_list;
+	size_t oe_start = to_pack.nr_objects;
+	size_t oe_end;
+	unsigned int sub_list_size;
+	unsigned int *processed = data;
+
+	/*
+	 * First, add all objects to the packing data, including the ones
+	 * marked UNINTERESTING (translated to 'exclude') as they can be
+	 * used as delta bases.
+	 */
+	for (size_t i = 0; i < oids->nr; i++) {
+		int exclude;
+		struct object_info oi = OBJECT_INFO_INIT;
+		struct object_id *oid = &oids->oid[i];
+
+		/* Skip objects that do not exist locally. */
+		if (exclude_promisor_objects &&
+		    oid_object_info_extended(the_repository, oid, &oi,
+					     OBJECT_INFO_FOR_PREFETCH) < 0)
+			continue;
+
+		exclude = !is_oid_interesting(the_repository, oid);
+
+		if (exclude && !thin)
+			continue;
+
+		add_object_entry(oid, type, path, exclude);
+	}
+
+	oe_end = to_pack.nr_objects;
+
+	/* We can skip delta calculations if it is a no-op. */
+	if (oe_end == oe_start || !window)
+		return 0;
+
+	sub_list_size = 0;
+	ALLOC_ARRAY(delta_list, oe_end - oe_start);
+
+	for (size_t i = 0; i < oe_end - oe_start; i++) {
+		struct object_entry *entry = to_pack.objects + oe_start + i;
+
+		if (!should_attempt_deltas(entry))
+			continue;
+
+		delta_list[sub_list_size++] = entry;
+	}
+
+	/*
+	 * Find delta bases among this list of objects that all match the same
+	 * path. This causes the delta compression to be interleaved in the
+	 * object walk, which can lead to confusing progress indicators. This is
+	 * also incompatible with threaded delta calculations. In the future,
+	 * consider creating a list of regions in the full to_pack.objects array
+	 * that could be picked up by the threaded delta computation.
+	 */
+	if (sub_list_size && window) {
+		QSORT(delta_list, sub_list_size, type_size_sort);
+		find_deltas(delta_list, &sub_list_size, window, depth, processed);
+	}
+
+	free(delta_list);
+	return 0;
+}
+
+static void get_object_list_path_walk(struct rev_info *revs)
+{
+	struct path_walk_info info = PATH_WALK_INFO_INIT;
+	unsigned int processed = 0;
+
+	info.revs = revs;
+	info.path_fn = add_objects_by_path;
+	info.path_fn_data = &processed;
+	revs->tag_objects = 1;
+
+	/*
+	 * Allow the --[no-]sparse option to be interesting here, if only
+	 * for testing purposes. Paths with no interesting objects will not
+	 * contribute to the resulting pack, but only create noisy preferred
+	 * base objects.
+	 */
+	info.prune_all_uninteresting = sparse;
+
+	if (walk_objects_by_path(&info))
+		die(_("failed to pack objects via path-walk"));
+}
+
 static void get_object_list(struct rev_info *revs, int ac, const char **av)
 {
 	struct setup_revision_opt s_r_opt = {
@@ -4234,7 +4337,7 @@ static void get_object_list(struct rev_info *revs, int ac, const char **av)
 
 	warn_on_object_refname_ambiguity = save_warning;
 
-	if (use_bitmap_index && !get_object_list_from_bitmap(revs))
+	if (use_bitmap_index && !path_walk && !get_object_list_from_bitmap(revs))
 		return;
 
 	if (use_delta_islands)
@@ -4243,15 +4346,19 @@ static void get_object_list(struct rev_info *revs, int ac, const char **av)
 	if (write_bitmap_index)
 		mark_bitmap_preferred_tips();
 
-	if (prepare_revision_walk(revs))
-		die(_("revision walk setup failed"));
-	mark_edges_uninteresting(revs, show_edge, sparse);
-
 	if (!fn_show_object)
 		fn_show_object = show_object;
-	traverse_commit_list(revs,
-			     show_commit, fn_show_object,
-			     NULL);
+
+	if (path_walk) {
+		get_object_list_path_walk(revs);
+	} else {
+		if (prepare_revision_walk(revs))
+			die(_("revision walk setup failed"));
+		mark_edges_uninteresting(revs, show_edge, sparse);
+		traverse_commit_list(revs,
+				show_commit, fn_show_object,
+				NULL);
+	}
 
 	if (unpack_unreachable_expiration) {
 		revs->ignore_missing_links = 1;
@@ -4461,6 +4568,8 @@ int cmd_pack_objects(int argc,
 			 N_("use the sparse reachability algorithm")),
 		OPT_BOOL(0, "thin", &thin,
 			 N_("create thin packs")),
+		OPT_BOOL(0, "path-walk", &path_walk,
+			 N_("use the path-walk API to walk objects when possible")),
 		OPT_BOOL(0, "shallow", &shallow,
 			 N_("create packs suitable for shallow fetches")),
 		OPT_BOOL(0, "honor-pack-keep", &ignore_packed_keep_on_disk,
@@ -4546,7 +4655,27 @@ int cmd_pack_objects(int argc,
 		window = 0;
 
 	strvec_push(&rp, "pack-objects");
-	if (thin) {
+
+	if (path_walk && filter_options.choice) {
+		warning(_("cannot use --filter with --path-walk"));
+		path_walk = 0;
+	}
+	if (path_walk && use_delta_islands) {
+		warning(_("cannot use delta islands with --path-walk"));
+		path_walk = 0;
+	}
+	if (path_walk && shallow) {
+		warning(_("cannot use --shallow with --path-walk"));
+		path_walk = 0;
+	}
+	if (path_walk) {
+		strvec_push(&rp, "--boundary");
+		 /*
+		  * We must disable the bitmaps because we are removing
+		  * the --objects / --objects-edge[-aggressive] options.
+		  */
+		use_bitmap_index = 0;
+	} else if (thin) {
 		use_internal_rev_list = 1;
 		strvec_push(&rp, shallow
 				? "--objects-edge-aggressive"

t/t5300-pack-object.sh

@@ -723,4 +723,21 @@ test_expect_success '--name-hash-version=2 and --write-bitmap-index are incompat
 	! test_grep "currently, --write-bitmap-index requires --name-hash-version=1" err
 '
 
+# Basic "repack everything" test
+test_expect_success '--path-walk pack everything' '
+	git -C server rev-parse HEAD >in &&
+	git -C server pack-objects --stdout --revs --path-walk <in >out.pack &&
+	git -C server index-pack --stdin <out.pack
+'
+
+# Basic "thin pack" test
+test_expect_success '--path-walk thin pack' '
+	cat >in <<-EOF &&
+	$(git -C server rev-parse HEAD)
+	^$(git -C server rev-parse HEAD~2)
+	EOF
+	git -C server pack-objects --thin --stdout --revs --path-walk <in >out.pack &&
+	git -C server index-pack --fix-thin --stdin <out.pack
+'
+
 test_done