path-walk: support wildcard pathspecs for blob filtering

Previously, walk_objects_by_path() silently ignored pathspecs containing
wildcards or magic by clearing them. This caused all blobs to be
downloaded regardless of the given pathspec. Wildcard pathspecs like
"d/file.*.txt" are useful for narrowing which blobs to process (e.g.,
during 'git backfill').

Support wildcard pathspecs by making three changes:

 1. Add an 'exact_pathspecs' flag to path_walk_context. When the
    pathspec has no wildcards or magic, set this flag and use the
    existing fast-path prefix matching in add_tree_entries(). When
    wildcards are present, skip that block since prefix matching
    cannot handle glob patterns.

 2. Disable revision-level commit pruning (revs->prune = 0) for
    wildcard pathspecs. The revision walk uses the pathspec to filter
    commits via TREESAME detection. For exact prefix pathspecs this
    works well, but wildcard pathspecs may fail to match through
    TREESAME because fnmatch with WM_PATHNAME does not cross directory
    boundaries. Disabling pruning ensures all commits are visited and
    their trees are available for the path-walk to filter.

 3. Add a match_pathspec() check in walk_path() to filter out blobs
    whose full path does not match the pathspec. This provides the
    actual blob-level filtering for wildcard pathspecs.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: derrickstolee

path-walk.c

@@ -62,6 +62,8 @@ struct path_walk_context {
 	 */
 	struct prio_queue path_stack;
 	struct strset path_stack_pushed;
+
+	unsigned exact_pathspecs:1;
 };
 
 static int compare_by_type(const void *one, const void *two, void *cb_data)
@@ -206,7 +208,7 @@ static int add_tree_entries(struct path_walk_context *ctx,
 				 match != MATCHED)
 				continue;
 		}
-		if (ctx->revs->prune_data.nr) {
+		if (ctx->revs->prune_data.nr && ctx->exact_pathspecs) {
 			struct pathspec *pd = &ctx->revs->prune_data;
 			bool found = false;
 
@@ -317,6 +319,13 @@ static int walk_path(struct path_walk_context *ctx,
 			return 0;
 	}
 
+	if (list->type == OBJ_BLOB &&
+	    ctx->revs->prune_data.nr &&
+	    !match_pathspec(ctx->repo->index, &ctx->revs->prune_data,
+			   path, strlen(path), 0,
+			   NULL, 0))
+		return 0;
+
 	/* Evaluate function pointer on this data, if requested. */
 	if ((list->type == OBJ_TREE && ctx->info->trees) ||
 	    (list->type == OBJ_BLOB && ctx->info->blobs) ||
@@ -525,15 +534,12 @@ int walk_objects_by_path(struct path_walk_info *info)
 		info->revs->tag_objects = 1;
 
 	if (ctx.revs->prune_data.nr) {
-		/*
-		 * Only exact prefix pathspecs are currently supported.
-		 * Clear any wildcard or magic pathspecs to avoid
-		 * incorrect prefix matching.
-		 */
 		struct pathspec *pd = &ctx.revs->prune_data;
 
-		if (pd->has_wildcard || pd->magic)
-			pd->nr = 0;
+		if (!pd->has_wildcard && !pd->magic)
+			ctx.exact_pathspecs = 1;
+		else
+			ctx.revs->prune = 0;
 	}
 
 	/* Insert a single list for the root tree into the paths. */

t/t5620-backfill.sh

@@ -307,12 +307,11 @@ test_expect_success 'backfill with wildcard pathspec' '
 	git -C backfill-path rev-list --quiet --objects --missing=print HEAD >missing &&
 	test_line_count = 48 missing &&
 
-	# TODO: The wildcard pathspec should limit downloaded blobs,
-	# but currently all blobs are downloaded.
-	git -C backfill-path backfill HEAD -- "d/file.*.txt" &&
+	git -C backfill-path backfill HEAD -- "d/file.*.txt" 2>err &&
+	test_must_be_empty err &&
 
 	git -C backfill-path rev-list --quiet --objects --missing=print HEAD >missing &&
-	test_line_count = 0 missing
+	test_line_count = 40 missing
 '
 
 test_expect_success 'backfill with --all' '