fix: only update preferredReadReplica if valid

FetchResponse from a follower will _not_ contain a PreferredReadReplica.
It seems like the partitionConsumer would overwrite the previously
assigned value from the leader with -1 which would then trigger the
"reconnect to the current leader" changes from IBM#1936 causing a flip-flop
effect.

Contributes-to: IBM#2071

Signed-off-by: Dominic Evans <[email protected]>

consumer.go

@@ -132,16 +132,17 @@ func (c *consumer) Partitions(topic string) ([]int32, error) {
 
 func (c *consumer) ConsumePartition(topic string, partition int32, offset int64) (PartitionConsumer, error) {
 	child := &partitionConsumer{
-		consumer:  c,
-		conf:      c.conf,
-		topic:     topic,
-		partition: partition,
-		messages:  make(chan *ConsumerMessage, c.conf.ChannelBufferSize),
-		errors:    make(chan *ConsumerError, c.conf.ChannelBufferSize),
-		feeder:    make(chan *FetchResponse, 1),
-		trigger:   make(chan none, 1),
-		dying:     make(chan none),
-		fetchSize: c.conf.Consumer.Fetch.Default,
+		consumer:             c,
+		conf:                 c.conf,
+		topic:                topic,
+		partition:            partition,
+		messages:             make(chan *ConsumerMessage, c.conf.ChannelBufferSize),
+		errors:               make(chan *ConsumerError, c.conf.ChannelBufferSize),
+		feeder:               make(chan *FetchResponse, 1),
+		preferredReadReplica: invalidPreferredReplicaID,
+		trigger:              make(chan none, 1),
+		dying:                make(chan none),
+		fetchSize:            c.conf.Consumer.Fetch.Default,
 	}
 
 	if err := child.chooseStartingOffset(offset); err != nil {
@@ -605,7 +606,9 @@ func (child *partitionConsumer) parseResponse(response *FetchResponse) ([]*Consu
 
 	consumerBatchSizeMetric.Update(int64(nRecs))
 
-	child.preferredReadReplica = block.PreferredReadReplica
+	if block.PreferredReadReplica != invalidPreferredReplicaID {
+		child.preferredReadReplica = block.PreferredReadReplica
+	}
 
 	if nRecs == 0 {
 		partialTrailingMessage, err := block.isPartial()

fetch_response.go

@@ -5,6 +5,8 @@ import (
 	"time"
 )
 
+const invalidPreferredReplicaID = -1
+
 type AbortedTransaction struct {
 	ProducerID  int64
 	FirstOffset int64