qemu: snapshot: Avoid libvirtd crash when qemu crashes while snapshotting

We shouldn't access the domain definition while we are in the monitor
section as the domain is unlocked. Additionally after we exit from the
monitor we need to check if the VM is still alive. Not doing so resulted
in a crash if qemu exits while attempting to do an external VM snapshot.

src/qemu/qemu_driver.c

@@ -12564,7 +12564,8 @@ qemuDomainSnapshotCreateSingleDiskActive(virQEMUDriverPtr driver,
                                          virDomainDiskDefPtr disk,
                                          virDomainDiskDefPtr persistDisk,
                                          virJSONValuePtr actions,
-                                         bool reuse)
+                                         bool reuse,
+                                         enum qemuDomainAsyncJob asyncJob)
 {
     qemuDomainObjPrivatePtr priv = vm->privateData;
     char *device = NULL;
@@ -12616,8 +12617,25 @@ qemuDomainSnapshotCreateSingleDiskActive(virQEMUDriverPtr driver,
     /* create the actual snapshot */
     if (snap->format)
         formatStr = virStorageFileFormatTypeToString(snap->format);
+
+    /* The monitor is only accessed if qemu doesn't support transactions.
+     * Otherwise the following monitor command only constructs the command.
+     */
+    if (!actions &&
+        qemuDomainObjEnterMonitorAsync(driver, vm, asyncJob) < 0)
+        goto cleanup;
+
     ret = qemuMonitorDiskSnapshot(priv->mon, actions, device, source,
                                   formatStr, reuse);
+    if (!actions) {
+        qemuDomainObjExitMonitor(driver, vm);
+        if (!virDomainObjIsActive(vm)) {
+            virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                           _("domain crashed while taking the snapshot"));
+            ret = -1;
+        }
+    }
+
     virDomainAuditDisk(vm, disk->src, source, "snapshot", ret >= 0);
     if (ret < 0)
         goto cleanup;
@@ -12723,9 +12741,6 @@ qemuDomainSnapshotCreateDiskActive(virQEMUDriverPtr driver,
      * Based on earlier qemuDomainSnapshotPrepare, all
      * disks in this list are now either SNAPSHOT_NO, or
      * SNAPSHOT_EXTERNAL with a valid file name and qcow2 format.  */
-    if (qemuDomainObjEnterMonitorAsync(driver, vm, asyncJob) < 0)
-        goto cleanup;
-
     for (i = 0; i < snap->def->ndisks; i++) {
         virDomainDiskDefPtr persistDisk = NULL;
 
@@ -12735,24 +12750,36 @@ qemuDomainSnapshotCreateDiskActive(virQEMUDriverPtr driver,
             int indx = virDomainDiskIndexByName(vm->newDef,
                                                 vm->def->disks[i]->dst,
                                                 false);
-            if (indx >= 0) {
+            if (indx >= 0)
                 persistDisk = vm->newDef->disks[indx];
-                persist = true;
-            }
         }
 
         ret = qemuDomainSnapshotCreateSingleDiskActive(driver, vm,
                                                        &snap->def->disks[i],
                                                        vm->def->disks[i],
                                                        persistDisk, actions,
-                                                       reuse);
+                                                       reuse, asyncJob);
         if (ret < 0)
             break;
     }
     if (actions) {
-        if (ret == 0)
-            ret = qemuMonitorTransaction(priv->mon, actions);
+        if (ret == 0) {
+            if (qemuDomainObjEnterMonitorAsync(driver, vm, asyncJob) == 0) {
+                ret = qemuMonitorTransaction(priv->mon, actions);
+                qemuDomainObjExitMonitor(driver, vm);
+                if (!virDomainObjIsActive(vm)) {
+                    virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                                   _("domain crashed while taking the snapshot"));
+                    ret = -1;
+                }
+            } else {
+                /* failed to enter monitor, clean stuff up and quit */
+                ret = -1;
+            }
+        }
+
         virJSONValueFree(actions);
+
         if (ret < 0) {
             /* Transaction failed; undo the changes to vm.  */
             bool need_unlink = !(flags & VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT);
@@ -12766,8 +12793,11 @@ qemuDomainSnapshotCreateDiskActive(virQEMUDriverPtr driver,
                     int indx = virDomainDiskIndexByName(vm->newDef,
                                                         vm->def->disks[i]->dst,
                                                         false);
-                    if (indx >= 0)
+                    if (indx >= 0) {
                         persistDisk = vm->newDef->disks[indx];
+                        persist = true;
+                    }
+
                 }
 
                 qemuDomainSnapshotUndoSingleDiskActive(driver, vm,
@@ -12778,7 +12808,6 @@ qemuDomainSnapshotCreateDiskActive(virQEMUDriverPtr driver,
             }
         }
     }
-    qemuDomainObjExitMonitor(driver, vm);
 
 cleanup: