qemu: Fix race between async and query jobs

If an async job run on a domain will stop the domain at the end of the
job, a concurrently run query job can hang in qemu monitor and nothing
can be done with that domain from this point on. An attempt to start
such domain results in "Timed out during operation: cannot acquire state
change lock" error.

However, quite a few things have to happen at the right time... There
must be an async job running which stops a domain at the end. This race
was reported with dump --crash but other similar jobs, such as
(managed)save and migration, should be able to trigger this bug as well.
While this async job is processing its last monitor command, that is a
query-migrate to which qemu replies with status "completed", a new
libvirt API that results in a query job must arrive and stay waiting
until the query-migrate command finishes. Once query-migrate is done but
before the async job closes qemu monitor while stopping the domain, the
other thread needs to wake up and call qemuMonitorSend to send its
command to qemu. Before qemu gets a chance to respond to this command,
the async job needs to close the monitor. At this point, the query job
thread is waiting for a condition that no-one will ever signal so it
never finishes the job.

src/qemu/qemu_monitor.c

@@ -750,6 +750,27 @@ void qemuMonitorClose(qemuMonitorPtr mon)
         VIR_FORCE_CLOSE(mon->fd);
     }
 
+    /* In case another thread is waiting for its monitor command to be
+     * processed, we need to wake it up with appropriate error set.
+     */
+    if (mon->msg) {
+        if (mon->lastError.code == VIR_ERR_OK) {
+            virErrorPtr err = virSaveLastError();
+
+            qemuReportError(VIR_ERR_OPERATION_FAILED,
+                            _("Qemu monitor was closed"));
+            virCopyLastError(&mon->lastError);
+            if (err) {
+                virSetError(err);
+                virFreeError(err);
+            } else {
+                virResetLastError();
+            }
+        }
+        mon->msg->finished = 1;
+        virCondSignal(&mon->notify);
+    }
+
     if (qemuMonitorUnref(mon) > 0)
         qemuMonitorUnlock(mon);
 }