[libvirt] [PATCH] qemu: Don't send any monitor commands afer SHUTDOWN event

[Ján Tomko]

On Fri, Jan 19, 2018 at 10:02:45AM +0100, Michal Privoznik wrote:
>After aeda1b8c56dc5 we tried to stop reporting I/O errors on
>expected monitor HUP. We've achieved that by not setting
>mon->lastError once we've received SHUTDOWN event. However, this
>makes us to deadlock in case there's thread that enters the
>monitor after the event is received. The problem is, we're no
>longer setting mon->lastError and therefore qemuMonitorSend()
>does not return early and continues execution until virCondWait()
>(which will never wake up).
>
>Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
>---
> src/qemu/qemu_monitor.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
>diff --git a/src/qemu/qemu_monitor.c b/src/qemu/qemu_monitor.c
>index 85c7d68a1..9f3e3eb14 100644
>--- a/src/qemu/qemu_monitor.c
>+++ b/src/qemu/qemu_monitor.c
>@@ -1063,7 +1063,7 @@ qemuMonitorSend(qemuMonitorPtr mon,
> {
>     int ret = -1;
>
>-    /* Check whether qemu quit unexpectedly */
>+    /* Check whether qemu quit unexpectedly, */
>     if (mon->lastError.code != VIR_ERR_OK) {
>         VIR_DEBUG("Attempt to send command while error is set %s",
>                   NULLSTR(mon->lastError.message));
>@@ -1071,6 +1071,12 @@ qemuMonitorSend(qemuMonitorPtr mon,
>         return -1;
>     }
>
>+    /* or expectedly. */
>+    if (mon->willhangup) {
>+        VIR_DEBUG("Attempt to send command while domain is shutting down");
>+        return -1;
>+    }
>+

Not reporting an error here is not desirable in all the cases.

Quietly failing to send a monitor command might be fine for the
GetAllDomainStats use case, but exiting an API called on a specific
domain without setting an error is not okay.

IMO the right fix of the deadlock is to revert the patch.

Jan

>     mon->msg = msg;
>     qemuMonitorUpdateWatch(mon);
>
>-- 
>2.13.6
>
>--
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