path-walk: support blob size limit filter

Extend the path-walk API to handle the 'blob:limit=<size>' object
filter natively. This filter omits blobs whose size is equal to or
greater than the given limit, matching the semantics used by the
list-objects-filter machinery.

When revs->filter.choice is LOFC_BLOB_LIMIT, the prepare_filters()
method stores the limit value in info->blob_limit and clears the filter
from revs. If the limit is zero, this degenerates to blob:none (all
blobs excluded), so info->blobs is set to 0 instead.

During walk_path(), blob batches are filtered before being delivered to
the callback: each blob's size is checked via odb_read_object_info(),
and only blobs strictly smaller than the limit are included. Blobs whose
size cannot be determined (e.g. missing in a partial clone) are
conservatively included, matching the existing filter behavior. Empty
batches after filtering are skipped entirely.

The check for inclusion in the path batch looks a little strange at
first glance. We use odb_read_object_info() to read the object's size.
Based on all of the assumptions to this point, this _should_ return
OBJ_BLOB. Since we are focused on the size filter, we use a
short-circuited OR (||) to skip the size check if that method returns a
different object type.

Notice that this inspection of object sizes requires the content to be
present in the repository. The odb_read_object_info() call will download
a missing blob on-demand. This means that the use of the path-walk API
within 'git backfill' would not operate nicely with this filter type.
The intention of that command is to download missing blobs in batches.
Downloading objects one-by-one would go against the point. Update the
validation in 'git backfill' to add its own compatibility check on top
of path_walk_filter_compatible().

Add tests for blob:limit=0 (equivalent to blob:none) and blob:limit=3
(which exercises partial filtering within a batch where some blobs are
kept and others are excluded).

Co-authored-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-pack-objects.adoc

@@ -404,7 +404,7 @@ will be automatically changed to version `1`.
 +
 Incompatible with `--delta-islands`. The `--use-bitmap-index` option is
 ignored in the presence of `--path-walk`. The `--path-walk` option
-supports the `--filter=<spec>` form `blob:none`.
+supports the `--filter=<spec>` forms `blob:none` and `blob:limit=<n>`.
 
 
 DELTA ISLANDS

builtin/backfill.c

@@ -98,6 +98,8 @@ static void reject_unsupported_rev_list_options(struct rev_info *revs)
 		    "--diff-merges");
 	if (!path_walk_filter_compatible(&revs->filter))
 		die(_("cannot backfill with these filter options"));
+	if (revs->filter.blob_limit_value)
+		die(_("cannot backfill with blob size limits"));
 }
 
 static int do_backfill(struct backfill_context *ctx)

path-walk.c

@@ -10,6 +10,7 @@
 #include "hex.h"
 #include "list-objects.h"
 #include "list-objects-filter-options.h"
+#include "odb.h"
 #include "object.h"
 #include "oid-array.h"
 #include "path.h"
@@ -327,13 +328,35 @@ static int walk_path(struct path_walk_context *ctx,
 	/*
 	 * Evaluate function pointer on this data, if requested.
 	 * Ignore object type filters for tagged objects (path starts
-	 * with `/`).
+	 * with `/`), first for blobs and then other types.
 	 */
-	if ((list->type == OBJ_TREE && (ctx->info->trees || path_is_for_direct_objects(path))) ||
-	    (list->type == OBJ_BLOB && (ctx->info->blobs || path_is_for_direct_objects(path))) ||
-	    (list->type == OBJ_TAG && ctx->info->tags))
+	if (list->type == OBJ_BLOB &&
+	    ctx->info->blob_limit &&
+	    !path_is_for_direct_objects(path)) {
+		struct oid_array filtered = OID_ARRAY_INIT;
+
+		for (size_t i = 0; i < list->oids.nr; i++) {
+			unsigned long size;
+
+			if (odb_read_object_info(ctx->repo->objects,
+						 &list->oids.oid[i],
+						 &size) != OBJ_BLOB ||
+				size < ctx->info->blob_limit)
+				oid_array_append(&filtered,
+						 &list->oids.oid[i]);
+		}
+
+		if (filtered.nr)
+			ret = ctx->info->path_fn(path, &filtered, list->type,
+						 ctx->info->path_fn_data);
+		oid_array_clear(&filtered);
+	} else if (path_is_for_direct_objects(path) ||
+		   (list->type == OBJ_TREE && ctx->info->trees) ||
+		   (list->type == OBJ_BLOB && ctx->info->blobs) ||
+		   (list->type == OBJ_TAG && ctx->info->tags)) {
 		ret = ctx->info->path_fn(path, &list->oids, list->type,
 					ctx->info->path_fn_data);
+	}
 
 	/* Expand data for children. */
 	if (list->type == OBJ_TREE) {
@@ -510,6 +533,16 @@ static int prepare_filters(struct path_walk_info *info,
 		}
 		return 1;
 
+	case LOFC_BLOB_LIMIT:
+		if (info) {
+			if (!options->blob_limit_value)
+				info->blobs = 0;
+			else
+				info->blob_limit = options->blob_limit_value;
+			list_objects_filter_release(options);
+		}
+		return 1;
+
 	default:
 		error(_("object filter '%s' not supported by the path-walk API"),
 		      list_objects_filter_spec(options));

path-walk.h

@@ -47,6 +47,13 @@ struct path_walk_info {
 	int blobs;
 	int tags;
 
+	/**
+	 * If non-zero, specifies a maximum blob size. Blobs with a
+	 * size equal to or greater than this limit will not be
+	 * emitted unless included in 'pending'.
+	 */
+	unsigned long blob_limit;
+
 	/**
 	 * When 'prune_all_uninteresting' is set and a path has all objects
 	 * marked as UNINTERESTING, then the path-walk will not visit those

t/t5620-backfill.sh

@@ -20,7 +20,7 @@ test_expect_success 'backfill rejects incompatible filter options' '
 	test_grep "cannot backfill with these filter options" err &&
 
 	test_must_fail git backfill --objects --filter=blob:limit=10m 2>err &&
-	test_grep "cannot backfill with these filter options" err
+	test_grep "cannot backfill with blob size limits" err
 '
 
 # We create objects in the 'src' repo.

t/t6601-path-walk.sh

@@ -477,4 +477,86 @@ test_expect_success 'topic only, blob:none filter' '
 	test_cmp_sorted expect out
 '
 
+test_expect_success 'all, blob:limit=0 filter' '
+	test-tool path-walk --filter=blob:limit=0 -- --all >out &&
+
+	cat >expect <<-EOF &&
+	0:commit::$(git rev-parse topic)
+	0:commit::$(git rev-parse base)
+	0:commit::$(git rev-parse base~1)
+	0:commit::$(git rev-parse base~2)
+	1:tag:/tags:$(git rev-parse refs/tags/first)
+	1:tag:/tags:$(git rev-parse refs/tags/second.1)
+	1:tag:/tags:$(git rev-parse refs/tags/second.2)
+	1:tag:/tags:$(git rev-parse refs/tags/third)
+	1:tag:/tags:$(git rev-parse refs/tags/fourth)
+	1:tag:/tags:$(git rev-parse refs/tags/tree-tag)
+	1:tag:/tags:$(git rev-parse refs/tags/blob-tag)
+	2:blob:/tagged-blobs:$(git rev-parse refs/tags/blob-tag^{})
+	2:blob:/tagged-blobs:$(git rev-parse refs/tags/blob-tag2^{})
+	3:tree::$(git rev-parse topic^{tree})
+	3:tree::$(git rev-parse base^{tree})
+	3:tree::$(git rev-parse base~1^{tree})
+	3:tree::$(git rev-parse base~2^{tree})
+	3:tree::$(git rev-parse refs/tags/tree-tag^{})
+	3:tree::$(git rev-parse refs/tags/tree-tag2^{})
+	4:tree:a/:$(git rev-parse base:a)
+	5:tree:child/:$(git rev-parse refs/tags/tree-tag:child)
+	6:tree:left/:$(git rev-parse base:left)
+	6:tree:left/:$(git rev-parse base~2:left)
+	7:tree:right/:$(git rev-parse topic:right)
+	7:tree:right/:$(git rev-parse base~1:right)
+	7:tree:right/:$(git rev-parse base~2:right)
+	blobs:2
+	commits:4
+	tags:7
+	trees:13
+	EOF
+
+	test_cmp_sorted expect out
+'
+
+test_expect_success 'all, blob:limit=3 filter' '
+	test-tool path-walk --filter=blob:limit=3 -- --all >out &&
+
+	cat >expect <<-EOF &&
+	0:commit::$(git rev-parse topic)
+	0:commit::$(git rev-parse base)
+	0:commit::$(git rev-parse base~1)
+	0:commit::$(git rev-parse base~2)
+	1:tag:/tags:$(git rev-parse refs/tags/first)
+	1:tag:/tags:$(git rev-parse refs/tags/second.1)
+	1:tag:/tags:$(git rev-parse refs/tags/second.2)
+	1:tag:/tags:$(git rev-parse refs/tags/third)
+	1:tag:/tags:$(git rev-parse refs/tags/fourth)
+	1:tag:/tags:$(git rev-parse refs/tags/tree-tag)
+	1:tag:/tags:$(git rev-parse refs/tags/blob-tag)
+	2:blob:/tagged-blobs:$(git rev-parse refs/tags/blob-tag^{})
+	2:blob:/tagged-blobs:$(git rev-parse refs/tags/blob-tag2^{})
+	3:tree::$(git rev-parse topic^{tree})
+	3:tree::$(git rev-parse base^{tree})
+	3:tree::$(git rev-parse base~1^{tree})
+	3:tree::$(git rev-parse base~2^{tree})
+	3:tree::$(git rev-parse refs/tags/tree-tag^{})
+	3:tree::$(git rev-parse refs/tags/tree-tag2^{})
+	4:blob:a:$(git rev-parse base~2:a)
+	5:tree:a/:$(git rev-parse base:a)
+	6:tree:child/:$(git rev-parse refs/tags/tree-tag:child)
+	7:tree:left/:$(git rev-parse base:left)
+	7:tree:left/:$(git rev-parse base~2:left)
+	8:blob:left/b:$(git rev-parse base~2:left/b)
+	9:tree:right/:$(git rev-parse topic:right)
+	9:tree:right/:$(git rev-parse base~1:right)
+	9:tree:right/:$(git rev-parse base~2:right)
+	10:blob:right/c:$(git rev-parse base~2:right/c)
+	11:blob:right/d:$(git rev-parse base~1:right/d)
+	blobs:6
+	commits:4
+	tags:7
+	trees:13
+	EOF
+
+	test_cmp_sorted expect out
+'
+
 test_done