snapshot: rudimentary qemu support for atomic disk snapshot

Taking an external snapshot of just one disk is atomic, without having
to pause and resume the VM.  This also paves the way for later patches
to interact with the new qemu 'transaction' monitor command.

The various scenarios when requesting atomic are:
online, 1 disk, old qemu - safe, allowed by this patch
online, more than 1 disk, old qemu - failure, this patch
offline snapshot - safe, once a future patch implements offline disk snapshot
online, 1 or more disks, new qemu - safe, once future patch uses transaction

Taking an online system checkpoint snapshot is atomic, since it is
done via a single 'savevm' monitor command.  Taking an offline system
checkpoint snapshot is atomic, thanks to the previous patch.

* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateXML): Support
new flag for single-disk setups.
(qemuDomainSnapshotDiskPrepare): Check for atomic here.
(qemuDomainSnapshotCreateDiskActive): Skip pausing the VM when
atomic supported.
(qemuDomainSnapshotIsAllowed): Use bool instead of int.

src/qemu/qemu_driver.c

@@ -9536,7 +9536,8 @@ cleanup:
     return ret;
 }
 
-static int qemuDomainSnapshotIsAllowed(virDomainObjPtr vm)
+static bool
+qemuDomainSnapshotIsAllowed(virDomainObjPtr vm)
 {
     int i;
 
@@ -9551,11 +9552,11 @@ static int qemuDomainSnapshotIsAllowed(virDomainObjPtr vm)
             qemuReportError(VIR_ERR_OPERATION_INVALID,
                             _("Disk '%s' does not support snapshotting"),
                             vm->def->disks[i]->src);
-            return 0;
+            return false;
         }
     }
 
-    return 1;
+    return true;
 }
 
 static int
@@ -9713,13 +9714,17 @@ endjob:
 
 static int
 qemuDomainSnapshotDiskPrepare(virDomainObjPtr vm, virDomainSnapshotDefPtr def,
-                              bool allow_reuse)
+                              unsigned int *flags)
 {
     int ret = -1;
     int i;
     bool found = false;
     bool active = virDomainObjIsActive(vm);
     struct stat st;
+    bool allow_reuse = (*flags & VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT) != 0;
+    bool atomic = (*flags & VIR_DOMAIN_SNAPSHOT_CREATE_ATOMIC) != 0;
+    int external = 0;
+    qemuDomainObjPrivatePtr priv = vm->privateData;
 
     for (i = 0; i < def->ndisks; i++) {
         virDomainSnapshotDiskDefPtr disk = &def->disks[i];
@@ -9774,6 +9779,7 @@ qemuDomainSnapshotDiskPrepare(virDomainObjPtr vm, virDomainSnapshotDefPtr def,
                 goto cleanup;
             }
             found = true;
+            external++;
             break;
 
         case VIR_DOMAIN_DISK_SNAPSHOT_NO:
@@ -9793,6 +9799,17 @@ qemuDomainSnapshotDiskPrepare(virDomainObjPtr vm, virDomainSnapshotDefPtr def,
                           "selected for snapshot"));
         goto cleanup;
     }
+    if (active) {
+        if (external == 1 ||
+            qemuCapsGet(priv->qemuCaps, QEMU_CAPS_TRANSACTION)) {
+            *flags |= VIR_DOMAIN_SNAPSHOT_CREATE_ATOMIC;
+        } else if (atomic && external > 1) {
+            qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                            _("atomic live snapshot of multiple disks "
+                              "is unsupported"));
+            goto cleanup;
+        }
+    }
 
     ret = 0;
 
@@ -9921,6 +9938,7 @@ qemuDomainSnapshotCreateDiskActive(virConnectPtr conn,
     int i;
     bool persist = false;
     int thaw = 0; /* 1 if freeze succeeded, -1 if freeze failed */
+    bool atomic = (flags & VIR_DOMAIN_SNAPSHOT_CREATE_ATOMIC) != 0;
 
     if (qemuDomainObjBeginJobWithDriver(driver, vm, QEMU_JOB_MODIFY) < 0)
         return -1;
@@ -9945,14 +9963,14 @@ qemuDomainSnapshotCreateDiskActive(virConnectPtr conn,
         }
     }
 
-    if (virDomainObjGetState(vm, NULL) == VIR_DOMAIN_RUNNING) {
-        /* In qemu, snapshot_blkdev on a single disk will pause cpus,
-         * but this confuses libvirt since notifications are not given
-         * when qemu resumes.  And for multiple disks, libvirt must
-         * pause externally to get all snapshots to be at the same
-         * point in time.  For simplicitly, we always pause ourselves
-         * rather than relying on qemu doing pause.
-         */
+    /* For multiple disks, libvirt must pause externally to get all
+     * snapshots to be at the same point in time, unless qemu supports
+     * transactions.  For a single disk, snapshot is atomic without
+     * requiring a pause.  Thanks to qemuDomainSnapshotDiskPrepare, if
+     * we got to this point, the atomic flag now says whether we need
+     * to pause, and a capability bit says whether to use transaction.
+     */
+    if (!atomic && virDomainObjGetState(vm, NULL) == VIR_DOMAIN_RUNNING) {
         if (qemuProcessStopCPUs(driver, vm, VIR_DOMAIN_PAUSED_SAVE,
                                 QEMU_ASYNC_JOB_NONE) < 0)
             goto cleanup;
@@ -10040,10 +10058,10 @@ endjob:
             ret = -1;
     }
     if (vm && (qemuDomainObjEndJob(driver, vm) == 0)) {
-            /* Only possible if a transient vm quit while our locks were down,
-             * in which case we don't want to save snapshot metadata.  */
-            *vmptr = NULL;
-            ret = -1;
+        /* Only possible if a transient vm quit while our locks were down,
+         * in which case we don't want to save snapshot metadata.  */
+        *vmptr = NULL;
+        ret = -1;
     }
 
     return ret;
@@ -10071,7 +10089,8 @@ qemuDomainSnapshotCreateXML(virDomainPtr domain,
                   VIR_DOMAIN_SNAPSHOT_CREATE_HALT |
                   VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY |
                   VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT |
-                  VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE, NULL);
+                  VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE |
+                  VIR_DOMAIN_SNAPSHOT_CREATE_ATOMIC, NULL);
 
     if ((flags & VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE) &&
         !(flags & VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY)) {
@@ -10213,14 +10232,11 @@ qemuDomainSnapshotCreateXML(virDomainPtr domain,
             goto cleanup;
 
         if (flags & VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY) {
-            bool allow_reuse;
-
-            allow_reuse = (flags & VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT) != 0;
             if (virDomainSnapshotAlignDisks(def,
                                             VIR_DOMAIN_DISK_SNAPSHOT_EXTERNAL,
                                             false) < 0)
                 goto cleanup;
-            if (qemuDomainSnapshotDiskPrepare(vm, def, allow_reuse) < 0)
+            if (qemuDomainSnapshotDiskPrepare(vm, def, &flags) < 0)
                 goto cleanup;
             def->state = VIR_DOMAIN_DISK_SNAPSHOT;
         } else {