fix(instances): name snapshot files libvirt could not remove

When libvirt cannot remove a snapshot file it merged or discarded, it
only logs a warning and the delete or revert still succeeds. On NFS it
removes them as the qemu user, so an image directory the qemu user may
not write to left every merged overlay on disk without a word.

Revert and delete now compare the files in use before and after and
show a warning naming the ones still on disk. Only a snapshot's own
files (overlays, memory) and overlays on a snapshot point count, and a
disk the revert detached is never named, so a kept disk is not offered
for removal.

vrtManager/tests/test_external_snapshots.py

@@ -78,6 +78,7 @@ def proxy(status=1, internal=(), external=(), xml=DOMAIN, jobs=None, managed_sav
     dom.hasManagedSaveImage.return_value = managed_save
     dom.isActive.return_value = status in (1, 3)
     dom.blockJobInfo.side_effect = lambda dev, flags: (jobs or {}).get(dev, {})
+    dom.XMLDesc.side_effect = lambda flags: xml
     p.instance = dom
     return p
 
@@ -365,6 +366,110 @@ class DeleteTestCase(unittest.TestCase):
         snap.delete.assert_not_called()
 
 
+CHAIN = """<domain><devices>
+  <disk type='file' device='disk'><source file='/pool/a.two'/><target dev='vda'/>
+    <backingStore type='file'><source file='/pool/a.one'/>
+      <backingStore type='file'><source file='/pool/a.qcow2'/><backingStore/></backingStore>
+    </backingStore>
+  </disk>
+</devices></domain>"""
+
+
+def chain_snap(name, overlay, memory=None):
+    snap = fake_snap(name)
+    mem = f"<memory snapshot='external' file='{memory}'/>" if memory else "<memory snapshot='no'/>"
+    snap.getXMLDesc.return_value = (
+        f"<domainsnapshot><name>{name}</name><creationTime>1700000000</creationTime>{mem}"
+        f"<disks><disk name='vda' snapshot='external'><source file='{overlay}'/></disk></disks>"
+        "<domain><devices/></domain></domainsnapshot>"
+    )
+    return snap
+
+
+class LeftBehindTestCase(unittest.TestCase):
+    """libvirt only logs a warning when it cannot remove a merged or
+    discarded file (on NFS it removes them as the qemu user), so the delete
+    and revert report the files nothing uses any more that still exist."""
+
+    def run_delete(self, still_there):
+        one = chain_snap("one", "/pool/a.one", memory="/pool/vm.one.mem")
+        two = chain_snap("two", "/pool/a.two")
+        p = proxy(external=[one, two])
+        after = CHAIN.replace("<source file='/pool/a.one'/>", "<source file='/pool/a.qcow2'/>").replace(
+            "<backingStore type='file'><source file='/pool/a.qcow2'/><backingStore/></backingStore>", "<backingStore/>"
+        )
+        p.instance.XMLDesc.side_effect = [CHAIN, after]
+        p.instance.listAllSnapshots.side_effect = [[one, two], [two]]
+
+        def volume(path):
+            if path not in still_there:
+                raise libvirtError("no volume")
+            return MagicMock()
+
+        p.get_volume_by_path = volume
+        return p.delete_external_snapshot("one")
+
+    def test_files_libvirt_could_not_remove_are_reported(self):
+        self.assertEqual(
+            self.run_delete({"/pool/a.one", "/pool/vm.one.mem", "/pool/a.two", "/pool/a.qcow2"}),
+            ["/pool/a.one", "/pool/vm.one.mem"],
+        )
+
+    def test_nothing_is_reported_when_libvirt_removed_them(self):
+        # files still in use (base, the newer overlay) are never reported
+        self.assertEqual(self.run_delete({"/pool/a.two", "/pool/a.qcow2"}), [])
+
+    def revert(self, before, after, backing):
+        """Revert to one; backing maps each file still on disk to its backing file."""
+        one = chain_snap("one", "/pool/a.one")
+        p = proxy(external=[one])
+        p.instance.XMLDesc.side_effect = [before, after]
+        p.instance.listAllSnapshots.side_effect = [[one], [one]]
+
+        def volume(path):
+            if path not in backing:
+                raise libvirtError("no volume")
+            parent = f"<backingStore><path>{backing[path]}</path></backingStore>" if backing[path] else ""
+            return MagicMock(**{"XMLDesc.return_value": f"<volume><target><path>{path}</path></target>{parent}</volume>"})
+
+        p.get_volume_by_path = volume
+        return p.revert_external_snapshot("one")
+
+    def test_revert_reports_a_discarded_overlay_left_on_disk(self):
+        before = CHAIN.replace("/pool/a.two", "/pool/a.after")
+        after = CHAIN.replace("/pool/a.two", "/pool/a.1700000001")
+        left = self.revert(before, after, {"/pool/a.after": "/pool/a.one", "/pool/a.1700000001": "/pool/a.one"})
+        self.assertEqual(left, ["/pool/a.after"])
+
+    def test_a_shut_off_vm_overlay_is_found_through_the_storage(self):
+        # a shut-off VM's definition does not show the backing chain
+        def plain(top):
+            return f"<domain><devices><disk type='file' device='disk'><source file='{top}'/><target dev='vda'/></disk></devices></domain>"
+
+        left = self.revert(plain("/pool/a.after"), plain("/pool/a.1700000001"), {"/pool/a.after": "/pool/a.one"})
+        self.assertEqual(left, ["/pool/a.after"])
+
+    def test_revert_never_reports_a_detached_disk(self):
+        # vdb and vdc were added after the snapshot: the revert detaches them
+        # and keeps their files on purpose, also when vdc is itself an overlay
+        later = (
+            "<disk type='file' device='disk'><source file='/pool/later.qcow2'/><target dev='vdb'/></disk>"
+            "<disk type='file' device='disk'><source file='/pool/clone.qcow2'/><target dev='vdc'/>"
+            "<backingStore type='file'><source file='/templates/base.qcow2'/><backingStore/></backingStore></disk>"
+        )
+        before = CHAIN.replace("</devices>", later + "</devices>")
+        left = self.revert(before, CHAIN, {"/pool/later.qcow2": None, "/pool/clone.qcow2": "/templates/base.qcow2"})
+        self.assertEqual(left, [])
+
+    def test_revert_never_reports_a_detached_overlay_of_a_snapshot_file(self):
+        linked = (
+            "<disk type='file' device='disk'><source file='/pool/linked.qcow2'/><target dev='vdb'/>"
+            "<backingStore type='file'><source file='/pool/a.one'/><backingStore/></backingStore></disk>"
+        )
+        before = CHAIN.replace("</devices>", linked + "</devices>")
+        left = self.revert(before, CHAIN, {"/pool/linked.qcow2": "/pool/a.one"})
+        self.assertEqual(left, [])
+
 class KindTestCase(unittest.TestCase):
     def test_internal_endpoints_refuse_external_names(self):
         snap = fake_snap("e")