refdb: make ref deletion after pack safer

In order not to undo concurrent modifications to references, we must
make sure that we only delete a loose reference if it still has the same
value as when we packed it.

This means we need to lock it and then compare the value with the one we
put in the packed file.

src/refdb_fs.c

@@ -901,30 +901,68 @@ static int packed_write_ref(struct packref *ref, git_filebuf *file)
 static int packed_remove_loose(refdb_fs_backend *backend)
 {
 	size_t i;
-	git_buf full_path = GIT_BUF_INIT;
-	int failed = 0;
+	git_buf ref_content = GIT_BUF_INIT;
+	int failed = 0, error = 0;
 
 	/* backend->refcache is already locked when this is called */
 
 	for (i = 0; i < git_sortedcache_entrycount(backend->refcache); ++i) {
 		struct packref *ref = git_sortedcache_entry(backend->refcache, i);
+		git_filebuf lock = GIT_FILEBUF_INIT;
+		git_oid current_id;
 
 		if (!ref || !(ref->flags & PACKREF_WAS_LOOSE))
 			continue;
 
-		if (git_buf_joinpath(&full_path, backend->path, ref->name) < 0)
-			return -1; /* critical; do not try to recover on oom */
+		/* We need to stop anybody from updating the ref while we try to do a safe delete */
+		error = loose_lock(&lock, backend, ref->name);
+		/* If someone else is updating it, let them do it */
+		if (error == GIT_EEXISTS)
+			continue;
+
+		if (error < 0) {
+			failed = 1;
+			continue;
+		}
 
-		if (git_path_exists(full_path.ptr) && p_unlink(full_path.ptr) < 0) {
+		error = git_futils_readbuffer(&ref_content, lock.path_original);
+		/* Someone else beat us to cleaning up the ref, let's simply continue */
+		if (error == GIT_ENOTFOUND) {
+			git_filebuf_cleanup(&lock);
+			continue;
+		}
+
+		/* This became a symref between us packing and trying to delete it, so ignore it */
+		if (!git__prefixcmp(ref_content.ptr, GIT_SYMREF)) {
+			git_filebuf_cleanup(&lock);
+			continue;
+		}
+
+		/* Figure out the current id; if we fail record it but don't fail the whole operation */
+		if ((error = loose_parse_oid(&current_id, lock.path_original, &ref_content)) < 0) {
+			failed = 1;
+			git_filebuf_cleanup(&lock);
+			continue;
+		}
+
+		/* If the ref moved since we packed it, we must not delete it */
+		if (!git_oid_equal(&current_id, &ref->oid)) {
+			git_filebuf_cleanup(&lock);
+			continue;
+		}
+
+		if (p_unlink(lock.path_original) < 0) {
 			if (failed)
 				continue;
 
 			giterr_set(GITERR_REFERENCE,
 				"Failed to remove loose reference '%s' after packing: %s",
-				full_path.ptr, strerror(errno));
+				lock.path_original, strerror(errno));
 			failed = 1;
 		}
 
+		git_filebuf_cleanup(&lock);
+
 		/*
 		 * if we fail to remove a single file, this is *not* good,
 		 * but we should keep going and remove as many as possible.
@@ -933,7 +971,6 @@ static int packed_remove_loose(refdb_fs_backend *backend)
 		 */
 	}
 
-	git_buf_free(&full_path);
 	return failed ? -1 : 0;
 }