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/instance.py

@@ -1598,7 +1598,8 @@ class wvmInstance(wvmConnect):
     def revert_external_snapshot(self, name):
         """libvirt returns the disks and the definition to the snapshot's. A
         disk-only snapshot keeps the VM running, paused or shut off as it is;
-        a snapshot with memory resumes the VM as it was saved."""
+        a snapshot with memory resumes the VM as it was saved. Returns the
+        files libvirt left behind (see _left_behind)."""
         snap = self._snapshot(name, external=True)
         self._check_snapshot_state()
         tree = etree.fromstring(snap.getXMLDesc(0))
@@ -1618,6 +1619,7 @@ class wvmInstance(wvmConnect):
             flags = {1: VIR_DOMAIN_SNAPSHOT_REVERT_RUNNING, 3: VIR_DOMAIN_SNAPSHOT_REVERT_PAUSED}.get(
                 self.get_status(), 0
             )
+        before = self._snapshot_files()
         # libvirt names the new overlays after the second, so a revert within
         # the second of the previous one finds them taken, after it has
         # already stopped a running VM; one retry a second later finishes it.
@@ -1634,14 +1636,55 @@ class wvmInstance(wvmConnect):
                 raise util.OperationError(
                     f"The revert failed twice; try again in a moment ({retry_err}). The VM is {state}."
                 ) from None
-        self.refresh_instance_pools()
+        return self._left_behind(before)
 
     def delete_external_snapshot(self, name):
-        """libvirt merges the snapshot's overlays and removes its files."""
+        """libvirt merges the snapshot's overlays and removes its files.
+        Returns the files it left behind (see _left_behind)."""
         snap = self._snapshot(name, external=True)
         self._check_snapshot_state()
+        before = self._snapshot_files()
         snap.delete(0)
+        return self._left_behind(before)
+
+    def _snapshot_files(self):
+        """(disks, snapshot files, snapshot points): the files each disk uses
+        by target, the external snapshots' own files (overlays, memory), and
+        the files snapshots were taken on, which libvirt's overlays sit on."""
+        disks = {
+            disk.xpath("string(target/@dev)"): set(disk.xpath(".//source/@file"))
+            for disk in etree.fromstring(self.instance.XMLDesc(0)).xpath("devices/disk[@device='disk']")
+        }
+        own, points = set(), set()
+        for snap in self.instance.listAllSnapshots(VIR_DOMAIN_SNAPSHOT_LIST_EXTERNAL):
+            tree = etree.fromstring(snap.getXMLDesc(0))
+            own.update(tree.xpath("disks/disk/source/@file|memory/@file"))
+            saved = self._saved_definition(tree)
+            if saved is not None:
+                points.update(self._disk_sources(saved).values())
+        return disks, own, points | own
+
+    def _left_behind(self, before):
+        """Files libvirt merged or discarded but could not remove: it only
+        logs a warning then (on NFS it removes them as the qemu user, who
+        needs write access to their directory). Only a snapshot's own file or
+        an overlay on a snapshot point counts; a disk a revert detached is
+        kept on purpose. A file outside every storage pool is not checked."""
+        disks, own, points = before
         self.refresh_instance_pools()
+        now_disks, now_own, _ = self._snapshot_files()
+        # only disks still attached: one a revert detached keeps its files,
+        # whatever they sit on
+        attached = set().union(*(files for dev, files in disks.items() if dev in now_disks))
+        left = []
+        for path in sorted((attached | own) - set().union(now_own, *now_disks.values())):
+            try:
+                vol = self.get_volume_by_path(path)
+            except libvirtError:
+                continue
+            if path in own or util.get_xml_path(vol.XMLDesc(0), "/volume/backingStore/path") in points:
+                left.append(path)
+        return left
 
     @staticmethod
     def _disk_sources(xml):