block/qapi: always report full_backing_filename

Always report full_backing_filename, even if it's the same as
backing_filename. In the next patch, full_backing_filename may be
omitted if it cannot be generated instead of allowing e.g. drive_query
to abort if it runs into this scenario.

The presence or absence of the "full" field becomes useful information.
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Message-id: 1450122916-4706-3-git-send-email-jsnow@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

block/qapi.c

@@ -251,9 +251,10 @@ void bdrv_query_image_info(BlockDriverState *bs,
             return;
         }
 
-        if (strcmp(backing_filename, backing_filename2) != 0) {
-            info->full_backing_filename =
-                        g_strdup(backing_filename2);
+        /* Always report the full_backing_filename if present, even if it's the
+         * same as backing_filename. That they are same is useful info. */
+        if (backing_filename2) {
+            info->full_backing_filename = g_strdup(backing_filename2);
             info->has_full_backing_filename = true;
         }
 

tests/qemu-iotests/043.out

@@ -44,6 +44,7 @@ cluster_size: 65536
         "filename": "TEST_DIR/t.IMGFMT",
         "cluster-size": 65536,
         "format": "IMGFMT",
+        "full-backing-filename": "TEST_DIR/t.IMGFMT.2.base",
         "backing-filename": "TEST_DIR/t.IMGFMT.2.base",
         "dirty-flag": false
     },
@@ -52,6 +53,7 @@ cluster_size: 65536
         "filename": "TEST_DIR/t.IMGFMT.2.base",
         "cluster-size": 65536,
         "format": "IMGFMT",
+        "full-backing-filename": "TEST_DIR/t.IMGFMT.1.base",
         "backing-filename": "TEST_DIR/t.IMGFMT.1.base",
         "dirty-flag": false
     },