qemu: snapshot: Introduce helpers for creating overlays on <transient/> disks

To implement <transient/> disks we'll need to install an overlay on top
of the original disk image which will be discarded after the VM is
turned off. This was initially implemented by qemu but libvirt never
picked up this option as the overlays were created by qemu without
libvirt involvment which didn't work with SELinux.

With blockdev the qemu feature became unsupported so we need to do this
via the snapshot code anyways.

The helpers introduced in this patch prepare a fake snapshot disk
definition for a disk which is configured as <transient/> and use it to
create a snapshot (without actually modifying metadata or persistent
def).

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Tested-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Tested-by: Ján Tomko <jtomko@redhat.com>

src/qemu/qemu_snapshot.c

@@ -986,6 +986,7 @@ qemuSnapshotDiskPrepareOne(virDomainObjPtr vm,
                            qemuSnapshotDiskDataPtr dd,
                            virHashTablePtr blockNamedNodeData,
                            bool reuse,
+                           bool updateConfig,
                            qemuDomainAsyncJob asyncJob,
                            virJSONValuePtr actions)
 {
@@ -1008,7 +1009,8 @@ qemuSnapshotDiskPrepareOne(virDomainObjPtr vm,
         return -1;
 
     /* modify disk in persistent definition only when the source is the same */
-    if (vm->newDef &&
+    if (updateConfig &&
+        vm->newDef &&
         (persistdisk = virDomainDiskByTarget(vm->newDef, dd->disk->dst)) &&
         virStorageSourceIsSameLocation(dd->disk->src, persistdisk->src)) {
 
@@ -1116,6 +1118,55 @@ qemuSnapshotDiskPrepareActiveExternal(virDomainObjPtr vm,
                                        snapctxt->dd + snapctxt->ndd++,
                                        blockNamedNodeData,
                                        reuse,
+                                       true,
+                                       asyncJob,
+                                       snapctxt->actions) < 0)
+            return NULL;
+    }
+
+    return g_steal_pointer(&snapctxt);
+}
+
+
+static qemuSnapshotDiskContextPtr
+qemuSnapshotDiskPrepareDisksTransient(virDomainObjPtr vm,
+                                      virQEMUDriverConfigPtr cfg,
+                                      virHashTablePtr blockNamedNodeData,
+                                      qemuDomainAsyncJob asyncJob)
+{
+    g_autoptr(qemuSnapshotDiskContext) snapctxt = NULL;
+    size_t i;
+
+    snapctxt = qemuSnapshotDiskContextNew(vm->def->ndisks, vm, asyncJob);
+
+    for (i = 0; i < vm->def->ndisks; i++) {
+        virDomainDiskDefPtr domdisk = vm->def->disks[i];
+        g_autoptr(virDomainSnapshotDiskDef) snapdisk = g_new0(virDomainSnapshotDiskDef, 1);
+
+        if (!domdisk->transient)
+            continue;
+
+        /* validation code makes sure that we do this only for local disks
+         * with a file source */
+        snapdisk->name = g_strdup(domdisk->dst);
+        snapdisk->snapshot = VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL;
+        snapdisk->src = virStorageSourceNew();
+        snapdisk->src->type = VIR_STORAGE_TYPE_FILE;
+        snapdisk->src->format = VIR_STORAGE_FILE_QCOW2;
+        snapdisk->src->path = g_strdup_printf("%s.TRANSIENT", domdisk->src->path);
+
+        if (virFileExists(snapdisk->src->path)) {
+            virReportError(VIR_ERR_OPERATION_UNSUPPORTED,
+                           _("Overlay file '%s' for transient disk '%s' already exists"),
+                           snapdisk->src->path, domdisk->dst);
+            return NULL;
+        }
+
+        if (qemuSnapshotDiskPrepareOne(vm, cfg, domdisk, snapdisk,
+                                       snapctxt->dd + snapctxt->ndd++,
+                                       blockNamedNodeData,
+                                       false,
+                                       false,
                                        asyncJob,
                                        snapctxt->actions) < 0)
             return NULL;
@@ -1253,6 +1304,45 @@ qemuSnapshotCreateActiveExternalDisks(virDomainObjPtr vm,
 }
 
 
+/**
+ * qemuSnapshotCreateDisksTransient:
+ * @vm: domain object
+ * @asyncJob: qemu async job type
+ *
+ * Creates overlays on top of disks which are configured as <transient/>. Note
+ * that the validation code ensures that <transient> disks have appropriate
+ * configuration.
+ */
+int
+qemuSnapshotCreateDisksTransient(virDomainObjPtr vm,
+                                 qemuDomainAsyncJob asyncJob)
+{
+    qemuDomainObjPrivatePtr priv = vm->privateData;
+    virQEMUDriverPtr driver = priv->driver;
+    g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(driver);
+    g_autoptr(qemuSnapshotDiskContext) snapctxt = NULL;
+    g_autoptr(virHashTable) blockNamedNodeData = NULL;
+
+    if (virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_BLOCKDEV)) {
+        if (!(blockNamedNodeData = qemuBlockGetNamedNodeData(vm, asyncJob)))
+            return -1;
+
+        if (!(snapctxt = qemuSnapshotDiskPrepareDisksTransient(vm, cfg,
+                                                               blockNamedNodeData,
+                                                               asyncJob)))
+            return -1;
+
+        if (qemuSnapshotDiskCreate(snapctxt, cfg) < 0)
+            return -1;
+    }
+
+    /* the overlays are established, so they can be deleted on shutdown */
+    priv->inhibitDiskTransientDelete = false;
+
+    return 0;
+}
+
+
 static int
 qemuSnapshotCreateActiveExternal(virQEMUDriverPtr driver,
                                  virDomainObjPtr vm,

src/qemu/qemu_snapshot.h

@@ -21,6 +21,7 @@
 #include "virconftypes.h"
 #include "datatypes.h"
 #include "qemu_conf.h"
+#include "qemu_domainjob.h"
 
 virDomainMomentObjPtr
 qemuSnapObjFromName(virDomainObjPtr vm,
@@ -53,3 +54,7 @@ int
 qemuSnapshotDelete(virDomainObjPtr vm,
                    virDomainSnapshotPtr snapshot,
                    unsigned int flags);
+
+int
+qemuSnapshotCreateDisksTransient(virDomainObjPtr vm,
+                                 qemuDomainAsyncJob asyncJob);