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distributed with this work for additional information
regarding copyright ownership. The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License. You may obtain a copy of the License at
Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
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under the License.
package electron
import (
// Receiver is a Link that receives messages.
type Receiver interface {
// Receive blocks until a message is available or until the Receiver is closed
// and has no more buffered messages.
Receive() (ReceivedMessage, error)
// ReceiveTimeout is like Receive but gives up after timeout, see Timeout.
// Note that that if Prefetch is false, after a Timeout the credit issued by
// Receive remains on the link. It will be used by the next call to Receive.
ReceiveTimeout(timeout time.Duration) (ReceivedMessage, error)
// Prefetch==true means the Receiver will automatically issue credit to the
// remote sender to keep its buffer as full as possible, i.e. it will
// "pre-fetch" messages independently of the application calling
// Receive(). This gives good throughput for applications that handle a
// continuous stream of messages. Larger capacity may improve throughput, the
// optimal value depends on the characteristics of your application.
// Prefetch==false means the Receiver will issue only issue credit when you
// call Receive(), and will only issue enough credit to satisfy the calls
// actually made. This gives lower throughput but will not fetch any messages
// in advance. It is good for synchronous applications that need to evaluate
// each message before deciding whether to receive another. The
// request-response pattern is a typical example. If you make concurrent
// calls to Receive with pre-fetch disabled, you can improve performance by
// setting the capacity close to the expected number of concurrent calls.
Prefetch() bool
// Capacity is the size (number of messages) of the local message buffer
// These are messages received but not yet returned to the application by a call to Receive()
Capacity() int
// Receiver implementation
type receiver struct {
buffer chan ReceivedMessage
callers int
func (r *receiver) Capacity() int { return cap(r.buffer) }
func (r *receiver) Prefetch() bool { return r.prefetch }
// Call in proton goroutine
func newReceiver(ls linkSettings) *receiver {
r := &receiver{link: link{linkSettings: ls}}
if r.capacity < 1 {
r.capacity = 1
r.buffer = make(chan ReceivedMessage, r.capacity)
r.handler().addLink(r.pLink, r)
if r.prefetch {
return r
// Call in proton goroutine. Max additional credit we can request.
func (r *receiver) maxFlow() int { return cap(r.buffer) - len(r.buffer) - r.pLink.Credit() }
func (r *receiver) flow(credit int) {
if credit > 0 {
// Inject flow check per-caller call when prefetch is off.
// Called with inc=1 at start of call, inc = -1 at end
func (r *receiver) caller(inc int) {
_ = r.engine().Inject(func() {
r.callers += inc
need := r.callers - (len(r.buffer) + r.pLink.Credit())
max := r.maxFlow()
if need > max {
need = max
// Inject flow top-up if prefetch is enabled
func (r *receiver) flowTopUp() {
if r.prefetch {
_ = r.engine().Inject(func() { r.flow(r.maxFlow()) })
func (r *receiver) Receive() (rm ReceivedMessage, err error) {
return r.ReceiveTimeout(Forever)
func (r *receiver) ReceiveTimeout(timeout time.Duration) (rm ReceivedMessage, err error) {
if r.buffer == nil {
panic(fmt.Errorf("Receiver is not open: %s", r))
if !r.prefetch { // Per-caller flow control
select { // Check for immediate availability, avoid caller() inject
case rm2, ok := <-r.buffer:
if ok {
rm = rm2
} else {
err = r.Error()
default: // Not immediately available, inject caller() counts
defer r.caller(-1)
rmi, err := timedReceive(r.buffer, timeout)
switch err {
case nil:
rm = rmi.(ReceivedMessage)
case Closed:
err = r.Error()
// Called in proton goroutine on MMessage event.
func (r *receiver) message(delivery proton.Delivery) {
if r.pLink.State().RemoteClosed() {
localClose(r.pLink, r.pLink.RemoteCondition().Error())
if delivery.HasMessage() {
bytes, err := delivery.MessageBytes()
var m amqp.Message
if err == nil {
m = amqp.NewMessage()
err =, bytes)
if err != nil {
localClose(r.pLink, err)
if r.pLink.Credit() < 0 {
localClose(r.pLink, fmt.Errorf("received message in excess of credit limit"))
} else {
// We never issue more credit than cap(buffer) so this will not block.
r.buffer <- ReceivedMessage{m, delivery, r}
func (r *receiver) closed(err error) error {
e :=
if r.buffer != nil {
return e
// ReceivedMessage contains an amqp.Message and allows the message to be acknowledged.
type ReceivedMessage struct {
// Message is the received message.
Message amqp.Message
pDelivery proton.Delivery
receiver Receiver
// Acknowledge a ReceivedMessage with the given delivery status.
func (rm *ReceivedMessage) acknowledge(status uint64) error {
return rm.receiver.(*receiver).engine().Inject(func() {
// Deliveries are valid as long as the connection is, unless settled.
// Accept tells the sender that we take responsibility for processing the message.
func (rm *ReceivedMessage) Accept() error { return rm.acknowledge(proton.Accepted) }
// Reject tells the sender we consider the message invalid and unusable.
func (rm *ReceivedMessage) Reject() error { return rm.acknowledge(proton.Rejected) }
// Release tells the sender we will not process the message but some other
// receiver might.
func (rm *ReceivedMessage) Release() error { return rm.acknowledge(proton.Released) }
// IncomingReceiver is sent on the Connection.Incoming() channel when there is
// an incoming request to open a receiver link.
type IncomingReceiver struct {
func newIncomingReceiver(sn *session, pLink proton.Link) *IncomingReceiver {
return &IncomingReceiver{
incoming: makeIncoming(pLink),
linkSettings: makeIncomingLinkSettings(pLink, sn),
// SetCapacity sets the capacity of the incoming receiver, call before Accept()
func (in *IncomingReceiver) SetCapacity(capacity int) { in.capacity = capacity }
// SetPrefetch sets the pre-fetch mode of the incoming receiver, call before Accept()
func (in *IncomingReceiver) SetPrefetch(prefetch bool) { in.prefetch = prefetch }
// Accept accepts an incoming receiver endpoint
func (in *IncomingReceiver) Accept() Endpoint {
return in.accept(func() Endpoint { return newReceiver(in.linkSettings) })