feat(producer): add sync pool for channel reuse (#3109)

Use a sync.Pool for the sync producer expectation channels to allow for re-use. 

Signed-off-by: k.torgaev <k.torgaev@tinkoff.ru>

sync_producer.go

@@ -2,6 +2,12 @@ package sarama
 
 import "sync"
 
+var expectationsPool = sync.Pool{
+	New: func() interface{} {
+		return make(chan *ProducerError, 1)
+	},
+}
+
 // SyncProducer publishes Kafka messages, blocking until they have been acknowledged. It routes messages to the correct
 // broker, refreshing metadata as appropriate, and parses responses for errors. You must call Close() on a producer
 // to avoid leaks, it may not be garbage-collected automatically when it passes out of scope.
@@ -110,32 +116,38 @@ func verifyProducerConfig(config *Config) error {
 }
 
 func (sp *syncProducer) SendMessage(msg *ProducerMessage) (partition int32, offset int64, err error) {
-	expectation := make(chan *ProducerError, 1)
+	expectation := expectationsPool.Get().(chan *ProducerError)
 	msg.expectation = expectation
 	sp.producer.Input() <- msg
-
-	if pErr := <-expectation; pErr != nil {
+	pErr := <-expectation
+	msg.expectation = nil
+	expectationsPool.Put(expectation)
+	if pErr != nil {
 		return -1, -1, pErr.Err
 	}
 
 	return msg.Partition, msg.Offset, nil
 }
 
 func (sp *syncProducer) SendMessages(msgs []*ProducerMessage) error {
-	expectations := make(chan chan *ProducerError, len(msgs))
+	indices := make(chan int, len(msgs))
 	go func() {
-		for _, msg := range msgs {
-			expectation := make(chan *ProducerError, 1)
+		for i, msg := range msgs {
+			expectation := expectationsPool.Get().(chan *ProducerError)
 			msg.expectation = expectation
 			sp.producer.Input() <- msg
-			expectations <- expectation
+			indices <- i
 		}
-		close(expectations)
+		close(indices)
 	}()
 
 	var errors ProducerErrors
-	for expectation := range expectations {
-		if pErr := <-expectation; pErr != nil {
+	for i := range indices {
+		expectation := msgs[i].expectation
+		pErr := <-expectation
+		msgs[i].expectation = nil
+		expectationsPool.Put(expectation)
+		if pErr != nil {
 			errors = append(errors, pErr)
 		}
 	}