object-store: provide helpers for loose_objects_cache

Our object_directory struct has a loose objects cache that all users of
the struct can see. But the only one that knows how to load the cache is
find_short_object_filename(). Let's extract that logic in to a reusable
function.

While we're at it, let's also reset the cache when we re-read the object
directories. This shouldn't have an impact on performance, as re-reads
are meant to be rare (and are already expensive, so we avoid them with
things like OBJECT_INFO_QUICK).

Since the cache is already meant to be an approximation, it's tempting
to skip even this bit of safety. But it's necessary to allow more code
to use it. For instance, fetch-pack explicitly re-reads the object
directory after performing its fetch, and would be confused if we didn't
clear the cache.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object-store.h

@@ -11,11 +11,12 @@ struct object_directory {
 	struct object_directory *next;
 
 	/*
-	 * Used to store the results of readdir(3) calls when searching
-	 * for unique abbreviated hashes.  This cache is never
-	 * invalidated, thus it's racy and not necessarily accurate.
-	 * That's fine for its purpose; don't use it for tasks requiring
-	 * greater accuracy!
+	 * Used to store the results of readdir(3) calls when we are OK
+	 * sacrificing accuracy due to races for speed. That includes
+	 * our search for unique abbreviated hashes. Don't use it for tasks
+	 * requiring greater accuracy!
+	 *
+	 * Be sure to call odb_load_loose_cache() before using.
 	 */
 	char loose_objects_subdir_seen[256];
 	struct oid_array loose_objects_cache;
@@ -45,6 +46,13 @@ void add_to_alternates_file(const char *dir);
  */
 void add_to_alternates_memory(const char *dir);
 
+/*
+ * Populate an odb's loose object cache for one particular subdirectory (i.e.,
+ * the one that corresponds to the first byte of objects you're interested in,
+ * from 0 to 255 inclusive).
+ */
+void odb_load_loose_cache(struct object_directory *odb, int subdir_nr);
+
 struct packed_git {
 	struct packed_git *next;
 	struct list_head mru;

packfile.c

@@ -987,6 +987,14 @@ static void prepare_packed_git(struct repository *r)
 
 void reprepare_packed_git(struct repository *r)
 {
+	struct object_directory *odb;
+
+	for (odb = r->objects->odb; odb; odb = odb->next) {
+		oid_array_clear(&odb->loose_objects_cache);
+		memset(&odb->loose_objects_subdir_seen, 0,
+		       sizeof(odb->loose_objects_subdir_seen));
+	}
+
 	r->objects->approximate_object_count_valid = 0;
 	r->objects->packed_git_initialized = 0;
 	prepare_packed_git(r);

sha1-file.c

@@ -2125,6 +2125,32 @@ int for_each_loose_object(each_loose_object_fn cb, void *data,
 	return 0;
 }
 
+static int append_loose_object(const struct object_id *oid, const char *path,
+			       void *data)
+{
+	oid_array_append(data, oid);
+	return 0;
+}
+
+void odb_load_loose_cache(struct object_directory *odb, int subdir_nr)
+{
+	struct strbuf buf = STRBUF_INIT;
+
+	if (subdir_nr < 0 ||
+	    subdir_nr >= ARRAY_SIZE(odb->loose_objects_subdir_seen))
+		BUG("subdir_nr out of range");
+
+	if (odb->loose_objects_subdir_seen[subdir_nr])
+		return;
+
+	strbuf_addstr(&buf, odb->path);
+	for_each_file_in_obj_subdir(subdir_nr, &buf,
+				    append_loose_object,
+				    NULL, NULL,
+				    &odb->loose_objects_cache);
+	odb->loose_objects_subdir_seen[subdir_nr] = 1;
+}
+
 static int check_stream_sha1(git_zstream *stream,
 			     const char *hdr,
 			     unsigned long size,

sha1-name.c

@@ -83,36 +83,19 @@ static void update_candidates(struct disambiguate_state *ds, const struct object
 	/* otherwise, current can be discarded and candidate is still good */
 }
 
-static int append_loose_object(const struct object_id *oid, const char *path,
-			       void *data)
-{
-	oid_array_append(data, oid);
-	return 0;
-}
-
 static int match_sha(unsigned, const unsigned char *, const unsigned char *);
 
 static void find_short_object_filename(struct disambiguate_state *ds)
 {
 	int subdir_nr = ds->bin_pfx.hash[0];
 	struct object_directory *odb;
-	struct strbuf buf = STRBUF_INIT;
 
 	for (odb = the_repository->objects->odb;
 	     odb && !ds->ambiguous;
 	     odb = odb->next) {
 		int pos;
 
-		if (!odb->loose_objects_subdir_seen[subdir_nr]) {
-			strbuf_reset(&buf);
-			strbuf_addstr(&buf, odb->path);
-			for_each_file_in_obj_subdir(subdir_nr, &buf,
-						    append_loose_object,
-						    NULL, NULL,
-						    &odb->loose_objects_cache);
-			odb->loose_objects_subdir_seen[subdir_nr] = 1;
-		}
-
+		odb_load_loose_cache(odb, subdir_nr);
 		pos = oid_array_lookup(&odb->loose_objects_cache, &ds->bin_pfx);
 		if (pos < 0)
 			pos = -1 - pos;
@@ -125,8 +108,6 @@ static void find_short_object_filename(struct disambiguate_state *ds)
 			pos++;
 		}
 	}
-
-	strbuf_release(&buf);
 }
 
 static int match_sha(unsigned len, const unsigned char *a, const unsigned char *b)