ObserverNamenode handles addBlock rpc and throws a FileNotFoundException

ObserverNameNode currently can handle the addBlockLocation RPC, but it may throw a FileNotFoundException when it contains stale txid.

• AddBlock is not a coordinated method, so Observer will not check the statId.
• AddBlock does the validation with checkOperation(OperationCategory.READ)

So the observer can handle the addBlock rpc. If this observer cannot replay the edit of create file, it will throw a FileNotFoundException during doing validation.

The related code as follows:

checkOperation(OperationCategory.READ);
final FSPermissionChecker pc = getPermissionChecker();
FSPermissionChecker.setOperationType(operationName);
readLock();
try {
  checkOperation(OperationCategory.READ);
  r = FSDirWriteFileOp.validateAddBlock(this, pc, src, fileId, clientName,
                                        previous, onRetryBlock);
} finally {
  readUnlock(operationName);
}