plugin: qemu block device: add support for snapshot option

This is mostly in preparation for external qcow2 snapshot support.

For internal qcow2 snapshots, which currently are the only supported
variant, it is not possible to attach the snapshot only. If access to
that is required it will need to be handled differently, e.g. via a
FUSE/NBD export.

Such accesses are currently not done for running VMs via '-drive'
either, so there still is feature parity.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>

src/PVE/Storage/ISCSIDirectPlugin.pm

@@ -113,6 +113,9 @@ sub path {
 sub qemu_blockdev_options {
     my ($class, $scfg, $storeid, $volname, $options) = @_;
 
+    die "volume snapshot is not possible on iscsi device\n"
+        if $options->{'snapshot-name'};
+
     my $lun = ($class->parse_volname($volname))[1];
 
     return {

src/PVE/Storage/Plugin.pm

@@ -2009,6 +2009,9 @@ disk of a VM, containing its OMVF variables.
 
 =back
 
+=item C<< $options->{'snapshot-name'} >>: (optional) The snapshot name. Set when the associated snapshot should be opened rather than the
+volume itself.
+
 =back
 
 =back
@@ -2020,9 +2023,15 @@ sub qemu_blockdev_options {
 
     my $blockdev = {};
 
-    my ($path) = $class->filesystem_path($scfg, $volname);
+    my ($path) = $class->filesystem_path($scfg, $volname, $options->{'snapshot-name'});
 
     if ($path =~ m|^/|) {
+        # For qcow2 and qed the path of a snapshot will be the same, but it's not possible to attach
+        # the snapshot alone.
+        my $format = ($class->parse_volname($volname))[6];
+        die "cannot attach only the snapshot of a '$format' image\n"
+            if $options->{'snapshot-name'} && ($format eq 'qcow2' || $format eq 'qed');
+
         # The 'file' driver only works for regular files. The check below is taken from
         # block/file-posix.c:hdev_probe_device() in QEMU. Do not bother with detecting 'host_cdrom'
         # devices here, those are not managed by the storage layer.

src/PVE/Storage/RBDPlugin.pm

@@ -530,6 +530,7 @@ sub qemu_blockdev_options {
     my ($name) = ($class->parse_volname($volname))[1];
 
     if ($scfg->{krbd}) {
+        $name .= '@' . $options->{'snapshot-name'} if $options->{'snapshot-name'};
         my $rbd_dev_path = get_rbd_dev_path($scfg, $storeid, $name);
         return { driver => 'host_device', filename => $rbd_dev_path };
     }
@@ -540,6 +541,7 @@ sub qemu_blockdev_options {
         image => "$name",
     };
     $blockdev->{namespace} = "$scfg->{namespace}" if defined($scfg->{namespace});
+    $blockdev->{snapshot} = $options->{'snapshot-name'} if $options->{'snapshot-name'};
 
     $blockdev->{conf} = $cmd_option->{ceph_conf} if $cmd_option->{ceph_conf};
 

src/PVE/Storage/ZFSPlugin.pm

@@ -250,6 +250,9 @@ sub path {
 sub qemu_blockdev_options {
     my ($class, $scfg, $storeid, $volname, $options) = @_;
 
+    die "direct access to snapshots not implemented\n"
+        if $options->{'snapshot-name'};
+
     my $name = ($class->parse_volname($volname))[1];
     my $guid = $class->zfs_get_lu_name($scfg, $name);
     my $lun = $class->zfs_get_lun_number($scfg, $guid);

src/PVE/Storage/ZFSPoolPlugin.pm

@@ -169,6 +169,8 @@ sub qemu_blockdev_options {
 
     die "volume '$volname' not usable as VM image\n" if $format ne 'raw';
 
+    die "cannot attach only the snapshot of a zvol\n" if $options->{'snapshot-name'};
+
     my ($path) = $class->path($scfg, $volname, $storeid);
 
     my $blockdev = { driver => 'host_device', filename => $path };