backfill: work with prefix pathspecs

The previous change allowed specifying revision arguments over the 'git
backfill' command-line. This created the opportunity for restricting the
initial commit set by filtering the revision walk through a pathspec. Other
than filtering the commit set (and thereby the root trees), this did not
restrict the path-walk implementation of 'git backfill' and did not restrict
the blobs that were downloaded to only those matching the pathspec.

Update the path-walk API to accept certain kinds of pathspecs and to
silently ignore anything too complex, for now. We will update this in the
next change to properly restrict to even complex pathspecs.

The current behavior focuses on pathspecs that match paths exactly. This
includes exact filenames, including directory names as prefixes. Pathspecs
containing wildcards or magic are cleared so the path walk downloads all
blobs, as before.

The reason for this restriction is to allow for a faster execution by
pruning the path walk to only trees that could contribute towards one of
those paths as a parent directory.

The test directory 'd/f/' (next to 'd/file*.txt') was prepared in a
previous commit to exercise the subtlety in prefix matching.

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

path-walk.c

@@ -11,6 +11,7 @@
 #include "list-objects.h"
 #include "object.h"
 #include "oid-array.h"
+#include "path.h"
 #include "prio-queue.h"
 #include "repository.h"
 #include "revision.h"
@@ -206,6 +207,33 @@ static int add_tree_entries(struct path_walk_context *ctx,
 				 match != MATCHED)
 				continue;
 		}
+		if (ctx->revs->prune_data.nr) {
+			struct pathspec *pd = &ctx->revs->prune_data;
+			bool found = false;
+			int did_strip_suffix = strbuf_strip_suffix(&path, "/");
+
+
+			for (int i = 0; i < pd->nr; i++) {
+				struct pathspec_item *item = &pd->items[i];
+
+				/*
+				 * Continue if either is a directory prefix
+				 * of the other.
+				 */
+				if (dir_prefix(path.buf, item->match) ||
+				    dir_prefix(item->match, path.buf)) {
+					found = true;
+					break;
+				}
+			}
+
+			if (did_strip_suffix)
+				strbuf_addch(&path, '/');
+
+			/* Skip paths that do not match the prefix. */
+			if (!found)
+				continue;
+		}
 
 		add_path_to_list(ctx, path.buf, type, &entry.oid,
 				 !(o->flags & UNINTERESTING));
@@ -481,6 +509,17 @@ int walk_objects_by_path(struct path_walk_info *info)
 	if (info->tags)
 		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.
+		 */
+		if (ctx.revs->prune_data.has_wildcard ||
+		    ctx.revs->prune_data.magic)
+			clear_pathspec(&ctx.revs->prune_data);
+	}
+
 	/* Insert a single list for the root tree into the paths. */
 	CALLOC_ARRAY(root_tree_list, 1);
 	root_tree_list->type = OBJ_TREE;

path.c

@@ -56,7 +56,7 @@ static void strbuf_cleanup_path(struct strbuf *sb)
 		strbuf_remove(sb, 0, path - sb->buf);
 }
 
-static int dir_prefix(const char *buf, const char *dir)
+int dir_prefix(const char *buf, const char *dir)
 {
 	size_t len = strlen(dir);
 	return !strncmp(buf, dir, len) &&

path.h

@@ -112,6 +112,12 @@ const char *repo_submodule_path_replace(struct repository *repo,
 					const char *fmt, ...)
 	__attribute__((format (printf, 4, 5)));
 
+/*
+ * Given a directory name 'dir' (not ending with a trailing '/'),
+ * determine if 'buf' is equal to 'dir' or has prefix 'dir'+'/'.
+ */
+int dir_prefix(const char *buf, const char *dir);
+
 void report_linked_checkout_garbage(struct repository *r);
 
 /*

t/t5620-backfill.sh

@@ -273,13 +273,11 @@ test_expect_success 'backfill with prefix pathspec' '
 	git -C backfill-path rev-list --quiet --objects --missing=print HEAD >missing &&
 	test_line_count = 48 missing &&
 
-	# TODO: The pathspec should limit the downloaded blobs to
-	# only those matching the prefix "d/f", but currently all
-	# blobs are downloaded.
-	git -C backfill-path backfill HEAD -- d/f &&
+	git -C backfill-path backfill HEAD -- d/f 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 multiple pathspecs' '
@@ -292,13 +290,11 @@ test_expect_success 'backfill with multiple pathspecs' '
 	git -C backfill-path rev-list --quiet --objects --missing=print HEAD >missing &&
 	test_line_count = 48 missing &&
 
-	# TODO: The pathspecs should limit the downloaded blobs to
-	# only those matching "d/f" or "a", but currently all blobs
-	# are downloaded.
-	git -C backfill-path backfill HEAD -- d/f a &&
+	git -C backfill-path backfill HEAD -- d/f a 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 = 16 missing
 '
 
 test_expect_success 'backfill with wildcard pathspec' '