block: Do not copy exact_filename from format file

If a format BDS's file BDS is in turn a format BDS, we cannot simply use
the same filename, because when opening a BDS tree based on a filename
alone, qemu will create only one format node on top of one protocol node
(disregarding a potential backing file).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20190201192935.18394-26-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -5731,9 +5731,21 @@ void bdrv_refresh_filename(BlockDriverState *bs)
 
         bs->exact_filename[0] = '\0';
 
-        /* If no specific options have been given for this BDS, the filename of
-         * the underlying file should suffice for this one as well */
-        if (bs->file->bs->exact_filename[0] && !generate_json_filename) {
+        /*
+         * We can use the underlying file's filename if:
+         * - it has a filename,
+         * - the file is a protocol BDS, and
+         * - opening that file (as this BDS's format) will automatically create
+         *   the BDS tree we have right now, that is:
+         *   - the user did not significantly change this BDS's behavior with
+         *     some explicit (strong) options
+         *   - no non-file child of this BDS has been overridden by the user
+         *   Both of these conditions are represented by generate_json_filename.
+         */
+        if (bs->file->bs->exact_filename[0] &&
+            bs->file->bs->drv->bdrv_file_open &&
+            !generate_json_filename)
+        {
             strcpy(bs->exact_filename, bs->file->bs->exact_filename);
         }
     }