mirror: Fail gracefully for source == target

blockdev-mirror with the same node for source and target segfaults
today: A node is in its own backing chain, so mirror_start_job() decides
that this is an active commit. When adding the intermediate nodes with
block_job_add_bdrv(), it starts the iteration through the subchain with
the backing file of source, though, so it never reaches target and
instead runs into NULL at the base.

While we could fix that by starting with source itself, there is no
point in allowing mirroring a node into itself and I wouldn't be
surprised if this caused more problems later.

So just check for this scenario and error out.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

block/mirror.c

@@ -1499,6 +1499,11 @@ static void mirror_start_job(const char *job_id, BlockDriverState *bs,
         buf_size = DEFAULT_MIRROR_BUF_SIZE;
     }
 
+    if (bs == target) {
+        error_setg(errp, "Can't mirror node into itself");
+        return;
+    }
+
     /* In the case of active commit, add dummy driver to provide consistent
      * reads on the top, while disabling it in the intermediate nodes, and make
      * the backing chain writable. */

tests/qemu-iotests/041

@@ -234,6 +234,12 @@ class TestSingleBlockdev(TestSingleDrive):
         result = self.vm.qmp("blockdev-add", **args)
         self.assert_qmp(result, 'return', {})
 
+    def test_mirror_to_self(self):
+        result = self.vm.qmp(self.qmp_cmd, job_id='job0',
+                             device=self.qmp_target, sync='full',
+                             target=self.qmp_target)
+        self.assert_qmp(result, 'error/class', 'GenericError')
+
     test_large_cluster = None
     test_image_not_found = None
     test_small_buffer2 = None

tests/qemu-iotests/041.out

@@ -1,5 +1,5 @@
-.....................................................................................
+........................................................................................
 ----------------------------------------------------------------------
-Ran 85 tests
+Ran 88 tests
 
 OK