fix #4012: migrate: mention snapshots referencing a local ISO image

When a VM has no local ISO image attached anymore, but one of its
snapshots still references it, migration fails with:

  can't migrate local disk 'local:iso/debian.iso': local cdrom image

which does not tell where the image is coming from, since it is not
visible in the current configuration of the VM.

Add the names of the snapshots referencing the image to the error in
that case, like it is already done for a physical CD/DVD drive. The
check for the physical drive now reuses the same hint.

Signed-off-by: Michal Fox <me@dualfroz.com>

src/PVE/QemuMigrate.pm

@@ -412,15 +412,15 @@ sub scan_local_volumes {
             }
 
             my $snaprefs = $attr->{referenced_in_snapshot};
+            my $snapshot_hint = '';
+            if (defined($snaprefs) && !$attr->{is_attached}) {
+                my $snapnames = join(', ', sort keys %$snaprefs);
+                $snapshot_hint = " (referenced in snapshot - $snapnames)";
+            }
 
             if ($attr->{cdrom}) {
                 if ($volid eq 'cdrom') {
-                    my $msg = "can't migrate local cdrom drive";
-                    if (defined($snaprefs) && !$attr->{is_attached}) {
-                        my $snapnames = join(', ', sort keys %$snaprefs);
-                        $msg .= " (referenced in snapshot - $snapnames)";
-                    }
-                    &$log_error("$msg\n");
+                    &$log_error("can't migrate local cdrom drive$snapshot_hint\n");
                     return;
                 }
                 return if $volid eq 'none';
@@ -475,7 +475,7 @@ sub scan_local_volumes {
                     $local_volumes->{$volid}->{ref} = 'generated';
                     return;
                 }
-                die "local cdrom image\n";
+                die "local cdrom image$snapshot_hint\n";
             }
 
             my ($path, $owner) = PVE::Storage::path($storecfg, $volid);

src/test/run_qemu_migrate_tests.pl

@@ -37,6 +37,7 @@ my $storage_config = {
         local => {
             content => {
                 images => 1,
+                iso => 1,
             },
             path => "/var/lib/vz",
             type => "dir",
@@ -346,6 +347,17 @@ my $source_vdisks = {
             'volid' => 'local-dir:4567/vm-4567-state-snap2.raw',
         },
     ],
+    'local' => [
+        {
+            'ctime' => 1589439681,
+            'format' => 'iso',
+            'parent' => undef,
+            'size' => 663748608,
+            'used' => 663748608,
+            'vmid' => '0',
+            'volid' => 'local:iso/debian.iso',
+        },
+    ],
     'local-lvm' => [
         {
             'ctime' => '1589277334',
@@ -1236,6 +1248,41 @@ my $tests = [
             },
         },
     },
+    {
+        name => '105_local_iso_in_snapshot',
+        target => 'pve1',
+        vmid => 105,
+        vm_status => {
+            running => 0,
+        },
+        config_patch => {
+            snapshots => {
+                ohsnap => {
+                    ide2 => 'local:iso/debian.iso,media=cdrom',
+                },
+            },
+        },
+        expected_calls => {},
+        expect_die =>
+            "can't migrate local disk 'local:iso/debian.iso': local cdrom image (referenced in snapshot - ohsnap)",
+        expected => {
+            source_volids => local_volids_for_vm(105),
+            target_volids => {},
+            vm_config => get_patched_config(
+                105,
+                {
+                    snapshots => {
+                        ohsnap => {
+                            ide2 => 'local:iso/debian.iso,media=cdrom',
+                        },
+                    },
+                },
+            ),
+            vm_status => {
+                running => 0,
+            },
+        },
+    },
     {
         name => '105_cdrom',
         target => 'pve1',