snapshot: allow deletion of just snapshot metadata

A future patch will make it impossible to remove a domain if it
would leave behind any libvirt-tracked metadata about snapshots,
since stale metadata interferes with a new domain by the same name.
But requiring snaphot contents to be deleted before removing a
domain is harsh; with qemu, qemu-img can still make use of the
contents after the libvirt domain is gone.  Therefore, we need
an option to get rid of libvirt tracking information, but not
the actual contents.  For hypervisors that do not track any
metadata in libvirt, the implementation is trivial; all remaining
hypervisors (really, just qemu) will be dealt with separately.

* include/libvirt/libvirt.h.in
(VIR_DOMAIN_SNAPSHOT_DELETE_METADATA_ONLY): New flag.
* src/libvirt.c (virDomainSnapshotDelete): Document it.
* src/esx/esx_driver.c (esxDomainSnapshotDelete): Trivially
supported when there is no libvirt metadata.
* src/vbox/vbox_tmpl.c (vboxDomainSnapshotDelete): Likewise.

include/libvirt/libvirt.h.in

@@ -2596,7 +2596,8 @@ int virDomainRevertToSnapshot(virDomainSnapshotPtr snapshot,
 
 /* Delete a snapshot */
 typedef enum {
-    VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN = (1 << 0),
+    VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN      = (1 << 0), /* Also delete children */
+    VIR_DOMAIN_SNAPSHOT_DELETE_METADATA_ONLY = (1 << 1), /* Delete just metadata */
 } virDomainSnapshotDeleteFlags;
 
 int virDomainSnapshotDelete(virDomainSnapshotPtr snapshot,

src/esx/esx_driver.c

@@ -4543,7 +4543,8 @@ esxDomainSnapshotDelete(virDomainSnapshotPtr snapshot, unsigned int flags)
     esxVI_TaskInfoState taskInfoState;
     char *taskInfoErrorMessage = NULL;
 
-    virCheckFlags(VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN, -1);
+    virCheckFlags(VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN |
+                  VIR_DOMAIN_SNAPSHOT_DELETE_METADATA_ONLY, -1);
 
     if (esxVI_EnsureSession(priv->primary) < 0) {
         return -1;
@@ -4561,6 +4562,13 @@ esxDomainSnapshotDelete(virDomainSnapshotPtr snapshot, unsigned int flags)
         goto cleanup;
     }
 
+    /* ESX snapshots do not require any libvirt metadata, making this
+     * flag trivial once we know we have a valid snapshot.  */
+    if (flags & VIR_DOMAIN_SNAPSHOT_DELETE_METADATA_ONLY) {
+        result = 0;
+        goto cleanup;
+    }
+
     if (esxVI_RemoveSnapshot_Task(priv->primary, snapshotTree->snapshot,
                                   removeChildren, &task) < 0 ||
         esxVI_WaitForTaskCompletion(priv->primary, task, snapshot->domain->uuid,

src/libvirt.c

@@ -15962,14 +15962,18 @@ error:
 /**
  * virDomainSnapshotDelete:
  * @snapshot: a domain snapshot object
- * @flags: flag parameters
+ * @flags: bitwise-or of supported virDomainSnapshotDeleteFlags
  *
  * Delete the snapshot.
  *
  * If @flags is 0, then just this snapshot is deleted, and changes from
  * this snapshot are automatically merged into children snapshots.  If
- * flags is VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN, then this snapshot
- * and any children snapshots are deleted.
+ * @flags includes VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN, then this snapshot
+ * and any children snapshots are deleted.  If @flags includes
+ * VIR_DOMAIN_SNAPSHOT_DELETE_METADATA_ONLY, then any snapshot metadata
+ * tracked by libvirt is removed while keeping the snapshot contents
+ * intact; if a hypervisor does not require any libvirt metadata to
+ * track snapshots, then this flag is silently ignored.
  *
  * Returns 0 if the snapshot was successfully deleted, -1 on error.
  */

src/vbox/vbox_tmpl.c

@@ -6361,7 +6361,8 @@ vboxDomainSnapshotDelete(virDomainSnapshotPtr snapshot,
     PRUint32 state;
     nsresult rc;
 
-    virCheckFlags(VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN, -1);
+    virCheckFlags(VIR_DOMAIN_SNAPSHOT_DELETE_CHILDREN |
+                  VIR_DOMAIN_SNAPSHOT_DELETE_METADATA_ONLY, -1);
 
     vboxIIDFromUUID(&domiid, dom->uuid);
     rc = VBOX_OBJECT_GET_MACHINE(domiid.value, &machine);
@@ -6382,6 +6383,13 @@ vboxDomainSnapshotDelete(virDomainSnapshotPtr snapshot,
         goto cleanup;
     }
 
+    /* VBOX snapshots do not require any libvirt metadata, making this
+     * flag trivial once we know we have a valid snapshot.  */
+    if (flags & VIR_DOMAIN_SNAPSHOT_DELETE_METADATA_ONLY) {
+        ret = 0;
+        goto cleanup;
+    }
+
     if (state >= MachineState_FirstOnline
         && state <= MachineState_LastOnline) {
         vboxError(VIR_ERR_OPERATION_INVALID, "%s",