g_regex_unref reports an error if called with a NULL argument.

We have two cases in the code where we (possibly) call it on a NULL
argument. The interesting one is in virDomainQemuMonitorEventCleanup.

Based on VIR_CONNECT_DOMAIN_QEMU_MONITOR_EVENT_REGISTER_REGEX, we unref
data->regex, which has two problems:

* On the client side, flags is -1 so the comparison is true even if no
  regex was used, reproducible by:
  $ virsh qemu-monitor-event --timeout 1
  which results in an ugly error:
(process:1289846): GLib-CRITICAL **: 14:58:42.631: g_regex_unref: assertion 'regex != NULL' failed
* On the server side, we only create the regex if both the flag and the
  string are present, so it's possible to trigger this message by:
  $ virsh qemu-monitor-event --regex --timeout 1

Use a non-NULL comparison instead of the flag to decide whether we need
to unref the regex.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Fixes: 71efb59a4de7c51b1bc889a316f1796ebf55738f
https://bugzilla.redhat.com/show_bug.cgi?id=1861176
---
 src/conf/domain_event.c     | 2 +-
 tests/vboxsnapshotxmltest.c | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/conf/domain_event.c b/src/conf/domain_event.c
index 33fbf10406..d3acde0236 100644
--- a/src/conf/domain_event.c
+++ b/src/conf/domain_event.c
@@ -2194,7 +2194,7 @@ virDomainQemuMonitorEventCleanup(void *opaque)
     virDomainQemuMonitorEventData *data = opaque;
 
     VIR_FREE(data->event);
-    if (data->flags & VIR_CONNECT_DOMAIN_QEMU_MONITOR_EVENT_REGISTER_REGEX)
+    if (data->regex)
         g_regex_unref(data->regex);
     if (data->freecb)
         (data->freecb)(data->opaque);
diff --git a/tests/vboxsnapshotxmltest.c b/tests/vboxsnapshotxmltest.c
index 7e3f85cc58..d2beb7858d 100644
--- a/tests/vboxsnapshotxmltest.c
+++ b/tests/vboxsnapshotxmltest.c
@@ -135,7 +135,8 @@ mymain(void)
     DO_TEST("2disks-3snap-brother");
 
  cleanup:
-    g_regex_unref(testSnapshotXMLVariableLineRegex);
+    if (testSnapshotXMLVariableLineRegex)
+        g_regex_unref(testSnapshotXMLVariableLineRegex);
     return ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
 }
 
-- 
2.26.2

On Tue, Sep 08, 2020 at 03:20:52PM +0200, Ján Tomko wrote:
>g_regex_unref reports an error if called with a NULL argument.
>
>We have two cases in the code where we (possibly) call it on a NULL
>argument. The interesting one is in virDomainQemuMonitorEventCleanup.
>
>Based on VIR_CONNECT_DOMAIN_QEMU_MONITOR_EVENT_REGISTER_REGEX, we unref
>data->regex, which has two problems:
>
>* On the client side, flags is -1 so the comparison is true even if no
>  regex was used, reproducible by:
>  $ virsh qemu-monitor-event --timeout 1
>  which results in an ugly error:
>(process:1289846): GLib-CRITICAL **: 14:58:42.631: g_regex_unref: assertion 'regex != NULL' failed
>* On the server side, we only create the regex if both the flag and the
>  string are present, so it's possible to trigger this message by:
>  $ virsh qemu-monitor-event --regex --timeout 1
>
>Use a non-NULL comparison instead of the flag to decide whether we need
>to unref the regex.
>
>Signed-off-by: Ján Tomko <jtomko@redhat.com>
>Fixes: 71efb59a4de7c51b1bc889a316f1796ebf55738f
>https://bugzilla.redhat.com/show_bug.cgi?id=1861176

Reviewed-by: Martin Kletzander <mkletzan@redhat.com>

Any reason why regex_unref does not accept NULLs?  It caught a misuse in the
first case, but the second one could just make the code cleaner.  Maybe we could
create our own auto type over it? =D

On Tue, Sep 08, 2020 at 15:20:52 +0200, Ján Tomko wrote:
> g_regex_unref reports an error if called with a NULL argument.
> 
> We have two cases in the code where we (possibly) call it on a NULL
> argument. The interesting one is in virDomainQemuMonitorEventCleanup.
> 
> Based on VIR_CONNECT_DOMAIN_QEMU_MONITOR_EVENT_REGISTER_REGEX, we unref
> data->regex, which has two problems:
> 
> * On the client side, flags is -1 so the comparison is true even if no
>   regex was used, reproducible by:
>   $ virsh qemu-monitor-event --timeout 1
>   which results in an ugly error:
> (process:1289846): GLib-CRITICAL **: 14:58:42.631: g_regex_unref: assertion 'regex != NULL' failed
> * On the server side, we only create the regex if both the flag and the
>   string are present, so it's possible to trigger this message by:
>   $ virsh qemu-monitor-event --regex --timeout 1
> 
> Use a non-NULL comparison instead of the flag to decide whether we need
> to unref the regex.

Please modify this last sentence to describe both fixes. I've flushed
out that there's another case by the time I've read the end here.

> Signed-off-by: Ján Tomko <jtomko@redhat.com>
> Fixes: 71efb59a4de7c51b1bc889a316f1796ebf55738f
> https://bugzilla.redhat.com/show_bug.cgi?id=1861176
> ---
>  src/conf/domain_event.c     | 2 +-
>  tests/vboxsnapshotxmltest.c | 3 ++-
>  2 files changed, 3 insertions(+), 2 deletions(-)

Reviewed-by: Peter Krempa <pkrempa@redhat.com>