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>

1 files changed, 6 insertions, 0 deletions

diff --git a/tests/qemu-iotests/041 b/tests/qemu-iotests/041
index c20ac7d..9336ab6 100755
--- a/tests/qemu-iotests/041
+++ b/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