[libvirt PATCH v2 21/24] qemu_snapshot: update backing store after deleting external snapshot

[Pavel Hrdina <phrdina@xxxxxxxxxx>]

With introduction of external snapshot revert we will have to update
backing store of qcow images not actively used be QEMU manually.
The need for this patch comes from the fact that we stop and start QEMU
process therefore after revert not all existing snapshots will be known
to that QEMU process due to reverting to non-leaf snapshot or having
multiple branches.

We need to loop over all existing snapshots and check all disks to see
if they happen to have the image we are deleting as backing store and
update them to point to the new image except for images currently used
by the running QEMU process doing the merge operation.

Signed-off-by: Pavel Hrdina <phrdina@xxxxxxxxxx>
---
 src/qemu/qemu_snapshot.c | 95 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 95 insertions(+)

diff --git a/src/qemu/qemu_snapshot.c b/src/qemu/qemu_snapshot.c
index 2950ad7d77..337c83f151 100644
--- a/src/qemu/qemu_snapshot.c
+++ b/src/qemu/qemu_snapshot.c
@@ -2530,6 +2530,8 @@ typedef struct _qemuSnapshotDeleteExternalData {
     virStorageSource *parentDiskSrc; /* backing disk source of the @diskSrc */
     virStorageSource *prevDiskSrc; /* source of disk for which @diskSrc is
                                       backing disk */
+    GSList *disksWithBacking; /* list of storage source for which the
+                                 deleted storage source is backing store */
     qemuBlockJobData *job;
     bool merge;
 } qemuSnapshotDeleteExternalData;
@@ -2542,6 +2544,7 @@ qemuSnapshotDeleteExternalDataFree(qemuSnapshotDeleteExternalData *data)
         return;
 
     virObjectUnref(data->job);
+    g_slist_free(data->disksWithBacking);
 
     g_free(data);
 }
@@ -2591,6 +2594,60 @@ qemuSnapshotFindParentSnapForDisk(virDomainMomentObj *snap,
 }
 
 
+struct _qemuSnapshotDisksWithBackingStoreData {
+    virDomainMomentObj *current;
+    virStorageSource *diskSrc;
+    GSList **disksWithBacking;
+};
+
+
+static int
+qemuSnapshotDiskHasBackingDisk(void *payload,
+                               const char *name G_GNUC_UNUSED,
+                               void *opaque)
+{
+    virDomainMomentObj *snap = payload;
+    virDomainSnapshotDef *snapdef = virDomainSnapshotObjGetDef(snap);
+    struct _qemuSnapshotDisksWithBackingStoreData *data = opaque;
+    ssize_t i;
+
+    /* skip snapshots that are within the active snapshot tree as it will be handled
+     * by qemu */
+    if (virDomainMomentIsAncestor(data->current, snap) || data->current == snap)
+        return 0;
+
+    for (i = 0; i < snapdef->parent.dom->ndisks; i++) {
+        virDomainDiskDef *disk = snapdef->parent.dom->disks[i];
+
+        if (!virStorageSourceIsLocalStorage(disk->src))
+            continue;
+
+        if (!disk->src->backingStore)
+            ignore_value(virStorageSourceGetMetadata(disk->src, -1, -1, 1, false));
+
+        if (virStorageSourceIsSameLocation(disk->src->backingStore, data->diskSrc))
+            *data->disksWithBacking = g_slist_prepend(*data->disksWithBacking, disk->src);
+    }
+
+    return 0;
+}
+
+
+static void
+qemuSnapshotGetDisksWithBackingStore(virDomainObj *vm,
+                                     virDomainMomentObj *snap,
+                                     qemuSnapshotDeleteExternalData *data)
+{
+    struct _qemuSnapshotDisksWithBackingStoreData iterData;
+
+    iterData.current = virDomainSnapshotGetCurrent(vm->snapshots);
+    iterData.diskSrc = data->diskSrc;
+    iterData.disksWithBacking = &data->disksWithBacking;
+
+    virDomainMomentForEachDescendant(snap, qemuSnapshotDiskHasBackingDisk, &iterData);
+}
+
+
 /**
  * qemuSnapshotDeleteExternalPrepareData:
  * @vm: domain object
@@ -2682,6 +2739,8 @@ qemuSnapshotDeleteExternalPrepareData(virDomainObj *vm,
                                    _("snapshot VM disk source and parent disk source are not the same"));
                     return -1;
                 }
+
+                qemuSnapshotGetDisksWithBackingStore(vm, snap, data);
             }
 
             data->parentSnap = qemuSnapshotFindParentSnapForDisk(snap, data->snapDisk);
@@ -3063,6 +3122,40 @@ qemuSnapshotSetInvalid(virDomainObj *vm,
 }
 
 
+static void
+qemuSnapshotUpdateBackingStore(virDomainObj *vm,
+                               qemuSnapshotDeleteExternalData *data)
+{
+    GSList *cur = NULL;
+    const char *qemuImgPath;
+    virQEMUDriver *driver = QEMU_DOMAIN_PRIVATE(vm)->driver;
+
+    if (!(qemuImgPath = qemuFindQemuImgBinary(driver)))
+        return;
+
+    for (cur = data->disksWithBacking; cur; cur = g_slist_next(cur)) {
+        virStorageSource *diskSrc = cur->data;
+        g_autoptr(virCommand) cmd = NULL;
+
+        /* creates cmd line args: qemu-img create -f qcow2 -o */
+        if (!(cmd = virCommandNewArgList(qemuImgPath,
+                                         "rebase",
+                                         "-u",
+                                         "-F",
+                                         virStorageFileFormatTypeToString(data->parentDiskSrc->format),
+                                         "-f",
+                                         virStorageFileFormatTypeToString(diskSrc->format),
+                                         "-b",
+                                         data->parentDiskSrc->path,
+                                         diskSrc->path,
+                                         NULL)))
+            continue;
+
+        ignore_value(virCommandRun(cmd, NULL));
+    }
+}
+
+
 static int
 qemuSnapshotDiscardExternal(virDomainObj *vm,
                             virDomainMomentObj *snap,
@@ -3149,6 +3242,8 @@ qemuSnapshotDiscardExternal(virDomainObj *vm,
 
         qemuBlockJobSyncEnd(vm, data->job, VIR_ASYNC_JOB_SNAPSHOT);
 
+        qemuSnapshotUpdateBackingStore(vm, data);
+
         if (qemuSnapshotSetInvalid(vm, data->parentSnap, data->snapDisk, false) < 0)
             goto error;
     }
-- 
2.41.0