fix(instances): a snapshot file another VM uses is not called left behind

After a snapshot revert or delete, files libvirt could not remove are
named in a warning that says nothing uses them and asks to remove them
by hand. A file another VM on the same host uses (as a disk, a backing
file or a snapshot file) is no longer named: removing it would break
that VM. The warning now says no VM on this host uses the files.

instances/views.py

@@ -1383,7 +1383,7 @@ def _warn_left_behind(request, files):
         messages.warning(
             request,
             _(
-                "These snapshot files were not removed, although nothing uses them any more: %(files)s. "
+                "These snapshot files were not removed, although no VM on this host uses them any more: %(files)s. "
                 "On NFS libvirt removes them as the qemu user, who needs write access to their directory. "
                 "Remove them by hand."
             )

vrtManager/instance.py

@@ -1715,9 +1715,10 @@ class wvmInstance(wvmConnect):
         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,
-        nor is anything when the pools could not be refreshed: a stale pool
-        still lists the removed files."""
+        kept on purpose, and a file another VM on this host uses (or one
+        below it) is not reported. A file outside every storage pool is not
+        checked, nor is anything when the pools could not be refreshed: a
+        stale pool still lists the removed files."""
         disks, own, points = before
         if not self._refresh_pools_after_snapshot():
             return []
@@ -1734,6 +1735,14 @@ class wvmInstance(wvmConnect):
             except libvirtError:  # not in a pool, or removed meanwhile
                 continue
             left.append(path)
+        if left:
+            # another VM on this host may use one; their own backing files
+            # do not keep each other, but a file in use keeps the files below it
+            in_use = self.paths_in_use(own=left)
+            for path in [path for path in left if path in in_use]:
+                while (path := self._backing_path(path)) and path not in in_use:
+                    in_use.add(path)
+            left = [path for path in left if path not in in_use]
         return left
 
     @staticmethod

vrtManager/tests/test_external_snapshots.py

@@ -4,7 +4,7 @@ cases are in instances/tests/test_live_snapshots.py."""
 
 import unittest
 from datetime import datetime
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, patch
 
 from django.conf import settings
 
@@ -440,6 +440,24 @@ class LeftBehindTestCase(unittest.TestCase):
             ["/pool/a.one", "/pool/vm.one.mem"],
         )
 
+    def test_a_file_another_vm_uses_is_not_reported(self):
+        # on NFS libvirt could not remove it, but another VM uses it now
+        with patch.object(wvmInstance, "paths_in_use", return_value={"/pool/a.one"}) as in_use, \
+                patch.object(wvmInstance, "_backing_path", return_value=None):
+            left = self.run_delete({"/pool/a.one", "/pool/vm.one.mem", "/pool/a.two", "/pool/a.qcow2"})
+        self.assertEqual(left, ["/pool/vm.one.mem"])
+        # their own backing files do not keep each other
+        in_use.assert_called_once_with(own=["/pool/a.one", "/pool/vm.one.mem"])
+
+    def test_a_file_below_one_another_vm_uses_is_not_reported(self):
+        # the other VM is shut off: its definition does not list the backing
+        # file, and the in-use top's own backing reference does not count
+        backing = {"/pool/a.one": "/pool/vm.one.mem"}
+        with patch.object(wvmInstance, "paths_in_use", return_value={"/pool/a.one"}), \
+                patch.object(wvmInstance, "_backing_path", side_effect=backing.get):
+            left = self.run_delete({"/pool/a.one", "/pool/vm.one.mem", "/pool/a.two", "/pool/a.qcow2"})
+        self.assertEqual(left, [])
+
     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"}), [])