// sendBatch unrolls a batched command and sends each constituent
// command in parallel.
func (tc *TxnCoordSender) sendBatch(batchArgs *proto.InternalBatchRequest, batchReply *proto.InternalBatchResponse) {
	// Prepare the calls by unrolling the batch. If the batchReply is
	// pre-initialized with replies, use those; otherwise create replies
	// as needed.
	// TODO(spencer): send calls in parallel.
	batchReply.Txn = batchArgs.Txn
	for i := range batchArgs.Requests {
		args := batchArgs.Requests[i].GetValue().(proto.Request)
		call := proto.Call{Args: args}
		// Disallow transaction, user and priority on individual calls, unless
		// equal.
		if args.Header().User != "" && args.Header().User != batchArgs.User {
			batchReply.Header().SetGoError(util.Error("cannot have individual user on call in batch"))
			return
		}
		args.Header().User = batchArgs.User
		if args.Header().UserPriority != nil && args.Header().GetUserPriority() != batchArgs.GetUserPriority() {
			batchReply.Header().SetGoError(util.Error("cannot have individual user priority on call in batch"))
			return
		}
		args.Header().UserPriority = batchArgs.UserPriority
		if txn := args.Header().Txn; txn != nil && !txn.Equal(batchArgs.Txn) {
			batchReply.Header().SetGoError(util.Error("cannot have individual transactional call in batch"))
			return
		}
		// Propagate batch Txn to each call.
		args.Header().Txn = batchArgs.Txn

		// Create a reply from the method type and add to batch response.
		if i >= len(batchReply.Responses) {
			call.Reply = args.CreateReply()
			batchReply.Add(call.Reply)
		} else {
			call.Reply = batchReply.Responses[i].GetValue().(proto.Response)
		}
		tc.sendOne(call)
		// Amalgamate transaction updates and propagate first error, if applicable.
		if batchReply.Txn != nil {
			batchReply.Txn.Update(call.Reply.Header().Txn)
		}
		if call.Reply.Header().Error != nil {
			batchReply.Error = call.Reply.Header().Error
			return
		}
	}
}

// sendBatch unrolls a batched command and sends each constituent
// command in parallel.
// TODO(tschottdorf): modify sendBatch so that it sends truly parallel requests
// when outside of a Transaction. This can then be used to address the TODO in
// (*TxnCoordSender).resolve().
func (tc *TxnCoordSender) sendBatch(ctx context.Context, batchArgs *proto.InternalBatchRequest, batchReply *proto.InternalBatchResponse) {
	// Prepare the calls by unrolling the batch. If the batchReply is
	// pre-initialized with replies, use those; otherwise create replies
	// as needed.
	// TODO(spencer): send calls in parallel.
	batchReply.Txn = batchArgs.Txn
	for i := range batchArgs.Requests {
		args := batchArgs.Requests[i].GetValue().(proto.Request)
		if err := updateForBatch(args.Header(), batchArgs.RequestHeader); err != nil {
			batchReply.Header().SetGoError(err)
			return
		}
		call := proto.Call{Args: args}
		// Create a reply from the method type and add to batch response.
		if i >= len(batchReply.Responses) {
			call.Reply = args.CreateReply()
			batchReply.Add(call.Reply)
		} else {
			call.Reply = batchReply.Responses[i].GetValue().(proto.Response)
		}
		tc.sendOne(ctx, call)
		// Amalgamate transaction updates and propagate first error, if applicable.
		if batchReply.Txn != nil {
			batchReply.Txn.Update(call.Reply.Header().Txn)
		}
		if call.Reply.Header().Error != nil {
			batchReply.Error = call.Reply.Header().Error
			return
		}
	}
}