gluster: Support auto-read-only option

If read-only=off, but auto-read-only=on is given, open the file
read-write if we have the permissions, but instead of erroring out for
read-only files, just degrade to read-only.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NNiels de Vos <ndevos@redhat.com>

block/gluster.c

@@ -849,8 +849,16 @@ static int qemu_gluster_open(BlockDriverState *bs,  QDict *options,
     qemu_gluster_parse_flags(bdrv_flags, &open_flags);
 
     s->fd = glfs_open(s->glfs, gconf->path, open_flags);
-    if (!s->fd) {
-        ret = -errno;
+    ret = s->fd ? 0 : -errno;
+
+    if (ret == -EACCES || ret == -EROFS) {
+        /* Try to degrade to read-only, but if it doesn't work, still use the
+         * normal error message. */
+        if (bdrv_apply_auto_read_only(bs, NULL, NULL) == 0) {
+            open_flags = (open_flags & ~O_RDWR) | O_RDONLY;
+            s->fd = glfs_open(s->glfs, gconf->path, open_flags);
+            ret = s->fd ? 0 : -errno;
+        }
     }
 
     s->supports_seek_data = qemu_gluster_test_seek(s->fd);