// maybeBeginTxn begins a new transaction if a txn has been specified
// in the request but has a nil ID. The new transaction is initialized
// using the name and isolation in the otherwise uninitialized txn.
// The Priority, if non-zero is used as a minimum.
func (tc *TxnCoordSender) maybeBeginTxn(header *proto.RequestHeader) {
	if header.Txn != nil {
		if len(header.Txn.ID) == 0 {
			newTxn := proto.NewTransaction(header.Txn.Name, keys.KeyAddress(header.Key), header.GetUserPriority(),
				header.Txn.Isolation, tc.clock.Now(), tc.clock.MaxOffset().Nanoseconds())
			// Use existing priority as a minimum. This is used on transaction
			// aborts to ratchet priority when creating successor transaction.
			if newTxn.Priority < header.Txn.Priority {
				newTxn.Priority = header.Txn.Priority
			}
			header.Txn = newTxn
		}
	}
}

// updateForBatch updates the first argument (the header of a request contained
// in a batch) from the second one (the batch header), returning an error when
// inconsistencies are found.
// It is checked that the individual call does not have a User, UserPriority
// or Txn set that differs from the batch's.
func updateForBatch(aHeader *proto.RequestHeader, bHeader proto.RequestHeader) error {
	// Disallow transaction, user and priority on individual calls, unless
	// equal.
	if aHeader.User != "" && aHeader.User != bHeader.User {
		return util.Error("conflicting user on call in batch")
	}
	if aPrio := aHeader.GetUserPriority(); aPrio != proto.Default_RequestHeader_UserPriority && aPrio != bHeader.GetUserPriority() {
		return util.Error("conflicting user priority on call in batch")
	}
	if aHeader.Txn != nil && !aHeader.Txn.Equal(bHeader.Txn) {
		return util.Error("conflicting transaction in transactional batch")
	}

	aHeader.User = bHeader.User
	aHeader.UserPriority = bHeader.UserPriority
	aHeader.Txn = bHeader.Txn
	return nil
}

// beginCmd waits for any overlapping, already-executing commands via
// the command queue and adds itself to the queue to gate follow-on
// commands which overlap its key range. This method will block if
// there are any overlapping commands already in the queue. Returns
// the command queue insertion key, to be supplied to subsequent
// invocation of endCmd().
func (r *Range) beginCmd(header *proto.RequestHeader, readOnly bool) interface{} {
	r.Lock()
	var wg sync.WaitGroup
	r.cmdQ.GetWait(header.Key, header.EndKey, readOnly, &wg)
	cmdKey := r.cmdQ.Add(header.Key, header.EndKey, readOnly)
	r.Unlock()
	wg.Wait()
	// Update the incoming timestamp if unset. Wait until after any
	// preceding command(s) for key range are complete so that the node
	// clock has been updated to the high water mark of any commands
	// which might overlap this one in effect.
	if header.Timestamp.Equal(proto.ZeroTimestamp) {
		header.Timestamp = r.rm.Clock().Now()
	}
	return cmdKey
}

// updateResponseTxn updates the response txn based on the response
// timestamp and error. The timestamp may have changed upon
// encountering a newer write or read. Both the timestamp and the
// priority may change depending on error conditions.
func (tc *TxnCoordSender) updateResponseTxn(argsHeader *proto.RequestHeader, replyHeader *proto.ResponseHeader) {
	// Move txn timestamp forward to response timestamp if applicable.
	if replyHeader.Txn.Timestamp.Less(replyHeader.Timestamp) {
		replyHeader.Txn.Timestamp = replyHeader.Timestamp
	}

	// Take action on various errors.
	switch t := replyHeader.GoError().(type) {
	case *proto.ReadWithinUncertaintyIntervalError:
		// Mark the host as certain. See the protobuf comment for
		// Transaction.CertainNodes for details.
		replyHeader.Txn.CertainNodes.Add(argsHeader.Replica.NodeID)

		// If the reader encountered a newer write within the uncertainty
		// interval, move the timestamp forward, just past that write or
		// up to MaxTimestamp, whichever comes first.
		var candidateTS proto.Timestamp
		if t.ExistingTimestamp.Less(replyHeader.Txn.MaxTimestamp) {
			candidateTS = t.ExistingTimestamp
			candidateTS.Logical++
		} else {
			candidateTS = replyHeader.Txn.MaxTimestamp
		}
		// Only change the timestamp if we're moving it forward.
		if replyHeader.Txn.Timestamp.Less(candidateTS) {
			replyHeader.Txn.Timestamp = candidateTS
		}
		replyHeader.Txn.Restart(argsHeader.GetUserPriority(), replyHeader.Txn.Priority, replyHeader.Txn.Timestamp)
	case *proto.TransactionAbortedError:
		// Increase timestamp if applicable.
		if replyHeader.Txn.Timestamp.Less(t.Txn.Timestamp) {
			replyHeader.Txn.Timestamp = t.Txn.Timestamp
		}
		replyHeader.Txn.Priority = t.Txn.Priority
	case *proto.TransactionPushError:
		// Increase timestamp if applicable.
		if replyHeader.Txn.Timestamp.Less(t.PusheeTxn.Timestamp) {
			replyHeader.Txn.Timestamp = t.PusheeTxn.Timestamp
			replyHeader.Txn.Timestamp.Logical++ // ensure this txn's timestamp > other txn
		}
		replyHeader.Txn.Restart(argsHeader.GetUserPriority(), t.PusheeTxn.Priority-1, replyHeader.Txn.Timestamp)
	case *proto.TransactionRetryError:
		// Increase timestamp if applicable.
		if replyHeader.Txn.Timestamp.Less(t.Txn.Timestamp) {
			replyHeader.Txn.Timestamp = t.Txn.Timestamp
		}
		replyHeader.Txn.Restart(argsHeader.GetUserPriority(), t.Txn.Priority, replyHeader.Txn.Timestamp)
	}
}

// updateForBatch updates the first argument (the header of a request contained
// in a batch) from the second one (the batch header), returning an error when
// inconsistencies are found.
// It is checked that the individual call does not have a User, UserPriority
// or Txn set that differs from the batch's.
func updateForBatch(args proto.Request, bHeader proto.RequestHeader) error {
	// Disallow transaction, user and priority on individual calls, unless
	// equal.
	aHeader := args.Header()
	if aHeader.User != "" && aHeader.User != bHeader.User {
		return util.Error("conflicting user on call in batch")
	}
	if aPrio := aHeader.GetUserPriority(); aPrio != proto.Default_RequestHeader_UserPriority && aPrio != bHeader.GetUserPriority() {
		return util.Error("conflicting user priority on call in batch")
	}
	aHeader.User = bHeader.User
	aHeader.UserPriority = bHeader.UserPriority
	// Only allow individual transactions on the requests of a batch if
	// - the batch is non-transactional,
	// - the individual transaction does not write intents, and
	// - the individual transaction is initialized.
	// The main usage of this is to allow mass-resolution of intents, which
	// entails sending a non-txn batch of transactional InternalResolveIntent.
	if aHeader.Txn != nil && !aHeader.Txn.Equal(bHeader.Txn) {
		if len(aHeader.Txn.ID) == 0 || proto.IsTransactionWrite(args) || bHeader.Txn != nil {
			return util.Error("conflicting transaction in transactional batch")
		}
	} else {
		aHeader.Txn = bHeader.Txn
	}
	return nil
}

//.........这里部分代码省略.........
		epoch := -1
		// Start a txn that does read-after-write.
		// The txn will be restarted twice, and the out-of-order put
		// will happen in the second epoch.
		if err := store.DB().Txn(func(txn *client.Txn) error {
			epoch++

			if epoch == 1 {
				// Wait until the second get operation is issued.
				close(waitTxnRestart)
				<-waitSecondGet
			}

			updatedVal := []byte("updatedVal")
			if err := txn.Put(key, updatedVal); err != nil {
				return err
			}

			// Make sure a get will return the value that was just written.
			actual, err := txn.Get(key)
			if err != nil {
				return err
			}
			if !bytes.Equal(actual.ValueBytes(), updatedVal) {
				t.Fatalf("unexpected get result: %s", actual)
			}

			if epoch == 0 {
				// Wait until the first get operation will push the txn timestamp.
				close(waitPut)
				<-waitFirstGet
			}

			b := &client.Batch{}
			err = txn.Commit(b)
			return err
		}); err != nil {
			t.Fatal(err)
		}

		if epoch != 2 {
			t.Fatalf("unexpected number of txn retries: %d", epoch)
		}

		close(waitTxnComplete)
	}()

	<-waitPut

	// Start the Reader.

	// Advance the clock and send a get operation with higher
	// priority to trigger the txn restart.
	manualClock.Increment(100)

	priority := int32(math.MaxInt32)
	requestHeader := proto.RequestHeader{
		Key:          proto.Key(key),
		RaftID:       1,
		Replica:      proto.Replica{StoreID: store.StoreID()},
		UserPriority: &priority,
		Timestamp:    clock.Now(),
	}
	getCall := proto.Call{
		Args: &proto.GetRequest{
			RequestHeader: requestHeader,
		},
		Reply: &proto.GetResponse{},
	}
	err = store.ExecuteCmd(context.Background(), getCall)
	if err != nil {
		t.Fatalf("failed to get: %s", err)
	}

	// Wait until the writer restarts the txn.
	close(waitFirstGet)
	<-waitTxnRestart

	// Advance the clock and send a get operation again. This time
	// we use TestingCommandFilter so that a get operation is not
	// processed after the write intent is resolved (to prevent the
	// timestamp cache from being updated).
	manualClock.Increment(100)

	requestHeader.Timestamp = clock.Now()
	getCall = proto.Call{
		Args: &proto.GetRequest{
			RequestHeader: requestHeader,
		},
		Reply: &proto.GetResponse{},
	}

	err = store.ExecuteCmd(context.Background(), getCall)
	if err == nil {
		t.Fatal("unexpected success of get")
	}

	close(waitSecondGet)
	<-waitTxnComplete
}

// UpdateForBatch updates the first argument (the header of a request contained
// in a batch) from the second one (the batch header), returning an error when
// inconsistencies are found.
// It is checked that the individual call does not have a UserPriority
// or Txn set that differs from the batch's.
// TODO(tschottdorf): will go with #2143.
func updateForBatch(args proto.Request, bHeader proto.RequestHeader) error {
	// Disallow transaction, user and priority on individual calls, unless
	// equal.
	aHeader := args.Header()
	if aPrio := aHeader.GetUserPriority(); aPrio != proto.Default_RequestHeader_UserPriority && aPrio != bHeader.GetUserPriority() {
		return util.Errorf("conflicting user priority on call in batch")
	}
	aHeader.UserPriority = bHeader.UserPriority
	aHeader.Txn = bHeader.Txn // reqs always take Txn from batch
	return nil
}