block: Add BlockDriver.bdrv_check_ext_snapshot.

This field is used by blkverify to disable external snapshots creation.
It will also be used by block filters like quorum to disable external
snapshot creation.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -4647,3 +4647,22 @@ int bdrv_amend_options(BlockDriverState *bs, QEMUOptionParameter *options)
     }
     return bs->drv->bdrv_amend_options(bs, options);
 }
+
+ExtSnapshotPerm bdrv_check_ext_snapshot(BlockDriverState *bs)
+{
+    if (bs->drv->bdrv_check_ext_snapshot) {
+        return bs->drv->bdrv_check_ext_snapshot(bs);
+    }
+
+    if (bs->file && bs->file->drv && bs->file->drv->bdrv_check_ext_snapshot) {
+        return bs->file->drv->bdrv_check_ext_snapshot(bs);
+    }
+
+    /* external snapshots are allowed by default */
+    return EXT_SNAPSHOT_ALLOWED;
+}
+
+ExtSnapshotPerm bdrv_check_ext_snapshot_forbidden(BlockDriverState *bs)
+{
+    return EXT_SNAPSHOT_FORBIDDEN;
+}

block/blkverify.c

@@ -417,6 +417,8 @@ static BlockDriver bdrv_blkverify = {
     .bdrv_aio_readv         = blkverify_aio_readv,
     .bdrv_aio_writev        = blkverify_aio_writev,
     .bdrv_aio_flush         = blkverify_aio_flush,
+
+    .bdrv_check_ext_snapshot = bdrv_check_ext_snapshot_forbidden,
 };
 
 static void bdrv_blkverify_init(void)

blockdev.c

@@ -1131,6 +1131,11 @@ static void external_snapshot_prepare(BlkTransactionState *common,
         }
     }
 
+    if (bdrv_check_ext_snapshot(state->old_bs) != EXT_SNAPSHOT_ALLOWED) {
+        error_set(errp, QERR_FEATURE_DISABLED, "snapshot");
+        return;
+    }
+
     flags = state->old_bs->open_flags;
 
     /* create new image w/backing file */

include/block/block.h

@@ -248,6 +248,20 @@ int bdrv_check(BlockDriverState *bs, BdrvCheckResult *res, BdrvCheckMode fix);
 
 int bdrv_amend_options(BlockDriverState *bs_new, QEMUOptionParameter *options);
 
+/* external snapshots */
+
+typedef enum {
+    EXT_SNAPSHOT_ALLOWED,
+    EXT_SNAPSHOT_FORBIDDEN,
+} ExtSnapshotPerm;
+
+/* return EXT_SNAPSHOT_ALLOWED if external snapshot is allowed
+ * return EXT_SNAPSHOT_FORBIDDEN if external snapshot is forbidden
+ */
+ExtSnapshotPerm bdrv_check_ext_snapshot(BlockDriverState *bs);
+/* helper used to forbid external snapshots like in blkverify */
+ExtSnapshotPerm bdrv_check_ext_snapshot_forbidden(BlockDriverState *bs);
+
 /* async block I/O */
 typedef void BlockDriverDirtyHandler(BlockDriverState *bs, int64_t sector,
                                      int sector_num);

include/block/block_int.h

@@ -67,6 +67,12 @@ typedef struct BdrvTrackedRequest {
 struct BlockDriver {
     const char *format_name;
     int instance_size;
+
+    /* if not defined external snapshots are allowed
+     * future block filters will query their children to build the response
+     */
+    ExtSnapshotPerm (*bdrv_check_ext_snapshot)(BlockDriverState *bs);
+
     int (*bdrv_probe)(const uint8_t *buf, int buf_size, const char *filename);
     int (*bdrv_probe_device)(const char *filename);