Add extra locking around packfile open

We were still seeing a few issues in threaded access to packs.
This adds extra locks around the opening of the mwindow to
avoid a different race.

src/pack.c

@@ -205,13 +205,18 @@ static int pack_index_check(const char *path, struct git_pack_file *p)
 	if (fd < 0)
 		return fd;
 
-	if (p_fstat(fd, &st) < 0 ||
-		!S_ISREG(st.st_mode) ||
+	if (p_fstat(fd, &st) < 0) {
+		p_close(fd);
+		giterr_set(GITERR_OS, "Unable to stat pack index '%s'", path);
+		return -1;
+	}
+
+	if (!S_ISREG(st.st_mode) ||
 		!git__is_sizet(st.st_size) ||
 		(idx_size = (size_t)st.st_size) < 4 * 256 + 20 + 20)
 	{
 		p_close(fd);
-		giterr_set(GITERR_OS, "Failed to check pack index.");
+		giterr_set(GITERR_ODB, "Invalid pack index '%s'", path);
 		return -1;
 	}
 
@@ -799,9 +804,6 @@ void git_packfile_free(struct git_pack_file *p)
 	if (!p)
 		return;
 
-	if (git_mutex_lock(&p->lock) < 0)
-		return;
-
 	cache_free(&p->bases);
 
 	git_mwindow_free_all(&p->mwf);
@@ -813,8 +815,6 @@ void git_packfile_free(struct git_pack_file *p)
 
 	git__free(p->bad_object_sha1);
 
-	git_mutex_unlock(&p->lock);
-
 	git_mutex_free(&p->lock);
 	git__free(p);
 }
@@ -829,12 +829,19 @@ static int packfile_open(struct git_pack_file *p)
 	if (!p->index_map.data && pack_index_open(p) < 0)
 		return git_odb__error_notfound("failed to open packfile", NULL);
 
+	/* if mwf opened by another thread, return now */
+	if (git_mutex_lock(&p->lock) < 0)
+		return packfile_error("failed to get lock for open");
+
+	if (p->mwf.fd >= 0) {
+		git_mutex_unlock(&p->lock);
+		return 0;
+	}
+
 	/* TODO: open with noatime */
 	p->mwf.fd = git_futils_open_ro(p->pack_name);
-	if (p->mwf.fd < 0) {
-		p->mwf.fd = -1;
-		return -1;
-	}
+	if (p->mwf.fd < 0)
+		goto cleanup;
 
 	if (p_fstat(p->mwf.fd, &st) < 0 ||
 		git_mwindow_file_register(&p->mwf) < 0)
@@ -875,13 +882,20 @@ static int packfile_open(struct git_pack_file *p)
 
 	idx_sha1 = ((unsigned char *)p->index_map.data) + p->index_map.len - 40;
 
-	if (git_oid__cmp(&sha1, (git_oid *)idx_sha1) == 0)
+	if (git_oid__cmp(&sha1, (git_oid *)idx_sha1) != 0)
+		goto cleanup;
+
+	git_mutex_unlock(&p->lock);
 	return 0;
 
 cleanup:
 	giterr_set(GITERR_OS, "Invalid packfile '%s'", p->pack_name);
+
 	p_close(p->mwf.fd);
 	p->mwf.fd = -1;
+
+	git_mutex_unlock(&p->lock);
+
 	return -1;
 }