iotests: Test mirror with temporarily disabled target backing file

The newly tested scenario is a common live storage migration scenario:
The target node is opened without a backing file so that the active
layer is mirrored while its backing chain can be copied in the
background.

The backing chain should be attached to the mirror target node when
finalising the job, just before switching the users of the source node
to the new copy (at which point the mirror job still has a reference to
the node). drive-mirror did this automatically, but with blockdev-mirror
this is the job of the QMP client.

This patch adds test cases for two ways to achieve the desired result,
using either x-blockdev-reopen or blockdev-snapshot.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200310113831.27293-5-kwolf@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/155

@@ -45,10 +45,15 @@ target_img = os.path.join(iotests.test_dir, 'target.' + iotests.imgfmt)
 #                      image during runtime, only makes sense if
 #                      target_blockdev_backing is not None
 #                      (None: same as target_backing)
+# target_open_with_backing: If True, the target image is added with its backing
+#                           chain opened right away. If False, blockdev-add
+#                           opens it without a backing file and job completion
+#                           is supposed to open the backing chain.
 
 class BaseClass(iotests.QMPTestCase):
     target_blockdev_backing = None
     target_real_backing = None
+    target_open_with_backing = True
 
     def setUp(self):
         qemu_img('create', '-f', iotests.imgfmt, back0_img, '1440K')
@@ -80,9 +85,13 @@ class BaseClass(iotests.QMPTestCase):
                 options = { 'node-name': 'target',
                             'driver': iotests.imgfmt,
                             'file': { 'driver': 'file',
+                                      'node-name': 'target-file',
                                       'filename': target_img } }
-                if self.target_blockdev_backing:
-                    options['backing'] = self.target_blockdev_backing
+
+                if not self.target_open_with_backing:
+                        options['backing'] = None
+                elif self.target_blockdev_backing:
+                        options['backing'] = self.target_blockdev_backing
 
                 result = self.vm.qmp('blockdev-add', **options)
                 self.assert_qmp(result, 'return', {})
@@ -147,10 +156,14 @@ class BaseClass(iotests.QMPTestCase):
 # cmd: Mirroring command to execute, either drive-mirror or blockdev-mirror
 
 class MirrorBaseClass(BaseClass):
+    def openBacking(self):
+        pass
+
     def runMirror(self, sync):
         if self.cmd == 'blockdev-mirror':
             result = self.vm.qmp(self.cmd, job_id='mirror-job', device='source',
-                                 sync=sync, target='target')
+                                 sync=sync, target='target',
+                                 auto_finalize=False)
         else:
             if self.existing:
                 mode = 'existing'
@@ -159,11 +172,12 @@ class MirrorBaseClass(BaseClass):
             result = self.vm.qmp(self.cmd, job_id='mirror-job', device='source',
                                  sync=sync, target=target_img,
                                  format=iotests.imgfmt, mode=mode,
-                                 node_name='target')
+                                 node_name='target', auto_finalize=False)
 
         self.assert_qmp(result, 'return', {})
 
-        self.complete_and_wait('mirror-job')
+        self.vm.run_job('mirror-job', use_log=False, auto_finalize=False,
+                        pre_finalize=self.openBacking, auto_dismiss=True)
 
     def testFull(self):
         self.runMirror('full')
@@ -221,6 +235,38 @@ class TestBlockdevMirrorForcedBacking(MirrorBaseClass):
     target_blockdev_backing = { 'driver': 'null-co' }
     target_real_backing = 'null-co://'
 
+# Attach the backing chain only during completion, with blockdev-reopen
+class TestBlockdevMirrorReopen(MirrorBaseClass):
+    cmd = 'blockdev-mirror'
+    existing = True
+    target_backing = 'null-co://'
+    target_open_with_backing = False
+
+    def openBacking(self):
+        if not self.target_open_with_backing:
+            result = self.vm.qmp('blockdev-add', node_name="backing",
+                                 driver="null-co")
+            self.assert_qmp(result, 'return', {})
+            result = self.vm.qmp('x-blockdev-reopen', node_name="target",
+                                 driver=iotests.imgfmt, file="target-file",
+                                 backing="backing")
+            self.assert_qmp(result, 'return', {})
+
+# Attach the backing chain only during completion, with blockdev-snapshot
+class TestBlockdevMirrorSnapshot(MirrorBaseClass):
+    cmd = 'blockdev-mirror'
+    existing = True
+    target_backing = 'null-co://'
+    target_open_with_backing = False
+
+    def openBacking(self):
+        if not self.target_open_with_backing:
+            result = self.vm.qmp('blockdev-add', node_name="backing",
+                                 driver="null-co")
+            self.assert_qmp(result, 'return', {})
+            result = self.vm.qmp('blockdev-snapshot', node="backing",
+                                 overlay="target")
+            self.assert_qmp(result, 'return', {})
 
 class TestCommit(BaseClass):
     existing = False

tests/qemu-iotests/155.out

@@ -1,5 +1,5 @@
-...................
+.........................
 ----------------------------------------------------------------------
-Ran 19 tests
+Ran 25 tests
 
 OK