[pve-devel] [PATCH pve-guest-common 1/1] partially fix #2530: snapshots: add pre/post/failed-snapshot hooks

[Stefan Hanreich]

This commit adds hooks to the snapshotting process, which can be used
to run additional setup scripts to prepare the VM for snapshotting.

Examples for use cases include:
* forcing processes to flush their writes
* blocking processes from writing
* altering the configuration of the VM to make snapshotting possible

The prepare step has been split into two parts, so the configuration
can be locked a bit earlier during the snapshotting process. Doing it
this way ensures that the configuration is already locked during the
pre-snapshot hook. Because of this split, the VM config gets written
in two stages now, rather than one.

In case of failure during the preparation step - after the lock is
written - error handling has been added so the lock gets released
properly. The failed-snapshot hook runs when the snapshot fails, if
the pre-snapshot hook ran already. This enables users to revert any
changes done during the pre-snapshot hookscript.

The preparation step assumes that the hook does not convert the
current VM into a template, which is why the basic checks are not
re-run after the pre-snapshot hook. The storage check runs after the
pre-snapshot hook, because the hook might get used to setup the
storage for snapshotting. If the hook would run after the storage
checks, this becomes impossible.

cfs_update() gets called after every invocation of a hookscript, since
it is impossible to know which changes get made by the hookscript.
Doing this ensures that we see the updated state of the CFS after the
hookscript got invoked.

Signed-off-by: Stefan Hanreich <s.hanreich at proxmox.com>
---
 src/PVE/AbstractConfig.pm | 49 ++++++++++++++++++++++++++++++++++-----
 1 file changed, 43 insertions(+), 6 deletions(-)

diff --git a/src/PVE/AbstractConfig.pm b/src/PVE/AbstractConfig.pm
index a0c0bc6..3bff600 100644
--- a/src/PVE/AbstractConfig.pm
+++ b/src/PVE/AbstractConfig.pm
@@ -710,8 +710,7 @@ sub __snapshot_prepare {
 
     my $snap;
 
-    my $updatefn =  sub {
-
+    my $run_checks = sub {
 	my $conf = $class->load_config($vmid);
 
 	die "you can't take a snapshot if it's a template\n"
@@ -721,15 +720,21 @@ sub __snapshot_prepare {
 
 	$conf->{lock} = 'snapshot';
 
-	my $snapshots = $conf->{snapshots};
-
 	die "snapshot name '$snapname' already used\n"
-	    if defined($snapshots->{$snapname});
+	    if defined($conf->{snapshots}->{$snapname});
+
+	$class->write_config($vmid, $conf);
+    };
 
+    my $updatefn = sub {
+	my $conf = $class->load_config($vmid);
 	my $storecfg = PVE::Storage::config();
+
 	die "snapshot feature is not available\n"
 	    if !$class->has_feature('snapshot', $conf, $storecfg, undef, undef, $snapname eq 'vzdump');
 
+	my $snapshots = $conf->{snapshots};
+
 	for my $snap (sort keys %$snapshots) {
 	    my $parent_name = $snapshots->{$snap}->{parent} // '';
 	    if ($snapname eq $parent_name) {
@@ -753,7 +758,32 @@ sub __snapshot_prepare {
 	$class->write_config($vmid, $conf);
     };
 
-    $class->lock_config($vmid, $updatefn);
+    $class->lock_config($vmid, $run_checks);
+
+    eval {
+	my $conf = $class->load_config($vmid);
+	PVE::GuestHelpers::exec_hookscript($conf, $vmid, "pre-snapshot", 1);
+    };
+    my $err = $@;
+
+    PVE::Cluster::cfs_update();
+
+    if ($err) {
+	$class->remove_lock($vmid, 'snapshot');
+	die $err;
+    }
+
+    eval {
+	$class->lock_config($vmid, $updatefn);
+    };
+    if (my $err = $@) {
+	my $conf = $class->load_config($vmid);
+	PVE::GuestHelpers::exec_hookscript($conf, $vmid, "failed-snapshot");
+	PVE::Cluster::cfs_update();
+
+	$class->remove_lock($vmid, 'snapshot');
+	die $err;
+    }
 
     return $snap;
 }
@@ -837,11 +867,18 @@ sub snapshot_create {
 
     if ($err) {
 	warn "snapshot create failed: starting cleanup\n";
+
+	PVE::GuestHelpers::exec_hookscript($conf, $vmid, "failed-snapshot");
+	PVE::Cluster::cfs_update();
+
 	eval { $class->snapshot_delete($vmid, $snapname, 1, $drivehash); };
 	warn "$@" if $@;
 	die "$err\n";
     }
 
+    PVE::GuestHelpers::exec_hookscript($conf, $vmid, "post-snapshot");
+    PVE::Cluster::cfs_update();
+
     $class->__snapshot_commit($vmid, $snapname);
 }
 
-- 
2.30.2