Btrfs: use received_uuid of parent during send

Neil Horman pointed out a problem where if he did something like this

receive A
snap A B
change B
send -p A B

and then on another box do

recieve A
receive B

the receive B would fail because we use the UUID of A for the clone sources for
B. This makes sense most of the time because normally you are sending from the
original sources, not a received source. However when you use a recieved subvol
its UUID is going to be something completely different, so if you then try to
receive the diff on a different volume it won't find the UUID because the new A
will be something else. The only constant is the received uuid. So instead
check to see if we have received_uuid set on the root, and if so use that as the
clone source, as btrfs receive looks for matches either in received_uuid or
uuid. Thanks,

Reported-by: Neil Horman <[e-mail]>
Signed-off-by: Josef Bacik <[e-mail]>
Reviewed-by: Hugo Mills <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/send.c

···
 	TLV_PUT_U64(sctx, BTRFS_SEND_A_CTRANSID,
 		    le64_to_cpu(sctx->send_root->root_item.ctransid));
 	if (parent_root) {
-		TLV_PUT_UUID(sctx, BTRFS_SEND_A_CLONE_UUID,
-				sctx->parent_root->root_item.uuid);
+		if (!btrfs_is_empty_uuid(parent_root->root_item.received_uuid))
+			TLV_PUT_UUID(sctx, BTRFS_SEND_A_CLONE_UUID,
+				     parent_root->root_item.received_uuid);
+		else
+			TLV_PUT_UUID(sctx, BTRFS_SEND_A_CLONE_UUID,
+				     parent_root->root_item.uuid);
 		TLV_PUT_U64(sctx, BTRFS_SEND_A_CLONE_CTRANSID,
 			    le64_to_cpu(sctx->parent_root->root_item.ctransid));
 	}
···
 	if (ret < 0)
 		goto out;
 
-	TLV_PUT_UUID(sctx, BTRFS_SEND_A_CLONE_UUID,
-			clone_root->root->root_item.uuid);
+	/*
+	 * If the parent we're using has a received_uuid set then use that as
+	 * our clone source as that is what we will look for when doing a
+	 * receive.
+	 *
+	 * This covers the case that we create a snapshot off of a received
+	 * subvolume and then use that as the parent and try to receive on a
+	 * different host.
+	 */
+	if (!btrfs_is_empty_uuid(clone_root->root->root_item.received_uuid))
+		TLV_PUT_UUID(sctx, BTRFS_SEND_A_CLONE_UUID,
+			     clone_root->root->root_item.received_uuid);
+	else
+		TLV_PUT_UUID(sctx, BTRFS_SEND_A_CLONE_UUID,
+			     clone_root->root->root_item.uuid);
 	TLV_PUT_U64(sctx, BTRFS_SEND_A_CLONE_CTRANSID,
 		    le64_to_cpu(clone_root->root->root_item.ctransid));
 	TLV_PUT_PATH(sctx, BTRFS_SEND_A_CLONE_PATH, p);