Revert client consumer group (#1027)

* Revert "Fixes for consumer group (#1022)"

This reverts commit a5f270d.

* Revert "Refactor Consumer Group to use Client (#947)"

This reverts commit ee37c7f.

conn.go

@@ -133,15 +133,16 @@ const (
 	ReadCommitted   IsolationLevel = 1
 )
 
-// DefaultClientID is the default value used as ClientID of kafka
-// connections.
-var DefaultClientID string
+var (
+	// DefaultClientID is the default value used as ClientID of kafka
+	// connections.
+	DefaultClientID string
+)
 
 func init() {
 	progname := filepath.Base(os.Args[0])
 	hostname, _ := os.Hostname()
 	DefaultClientID = fmt.Sprintf("%s@%s (github.com/segmentio/kafka-go)", progname, hostname)
-	DefaultTransport.(*Transport).ClientID = DefaultClientID
 }
 
 // NewConn returns a new kafka connection for the given topic and partition.
@@ -262,12 +263,10 @@ func (c *Conn) Controller() (broker Broker, err error) {
 			}
 			for _, brokerMeta := range res.Brokers {
 				if brokerMeta.NodeID == res.ControllerID {
-					broker = Broker{
-						ID:   int(brokerMeta.NodeID),
+					broker = Broker{ID: int(brokerMeta.NodeID),
 						Port: int(brokerMeta.Port),
 						Host: brokerMeta.Host,
-						Rack: brokerMeta.Rack,
-					}
+						Rack: brokerMeta.Rack}
 					break
 				}
 			}
@@ -323,6 +322,7 @@ func (c *Conn) findCoordinator(request findCoordinatorRequestV0) (findCoordinato
 	err := c.readOperation(
 		func(deadline time.Time, id int32) error {
 			return c.writeRequest(findCoordinator, v0, id, request)
+
 		},
 		func(deadline time.Time, size int) error {
 			return expectZeroSize(func() (remain int, err error) {
@@ -340,6 +340,32 @@ func (c *Conn) findCoordinator(request findCoordinatorRequestV0) (findCoordinato
 	return response, nil
 }
 
+// heartbeat sends a heartbeat message required by consumer groups
+//
+// See http://kafka.apache.org/protocol.html#The_Messages_Heartbeat
+func (c *Conn) heartbeat(request heartbeatRequestV0) (heartbeatResponseV0, error) {
+	var response heartbeatResponseV0
+
+	err := c.writeOperation(
+		func(deadline time.Time, id int32) error {
+			return c.writeRequest(heartbeat, v0, id, request)
+		},
+		func(deadline time.Time, size int) error {
+			return expectZeroSize(func() (remain int, err error) {
+				return (&response).readFrom(&c.rbuf, size)
+			}())
+		},
+	)
+	if err != nil {
+		return heartbeatResponseV0{}, err
+	}
+	if response.ErrorCode != 0 {
+		return heartbeatResponseV0{}, Error(response.ErrorCode)
+	}
+
+	return response, nil
+}
+
 // joinGroup attempts to join a consumer group
 //
 // See http://kafka.apache.org/protocol.html#The_Messages_JoinGroup
@@ -726,8 +752,9 @@ func (c *Conn) ReadBatch(minBytes, maxBytes int) *Batch {
 // ReadBatchWith in every way is similar to ReadBatch. ReadBatch is configured
 // with the default values in ReadBatchConfig except for minBytes and maxBytes.
 func (c *Conn) ReadBatchWith(cfg ReadBatchConfig) *Batch {
+
 	var adjustedDeadline time.Time
-	maxFetch := int(c.fetchMaxBytes)
+	var maxFetch = int(c.fetchMaxBytes)
 
 	if cfg.MinBytes < 0 || cfg.MinBytes > maxFetch {
 		return &Batch{err: fmt.Errorf("kafka.(*Conn).ReadBatch: minBytes of %d out of [1,%d] bounds", cfg.MinBytes, maxFetch)}
@@ -933,6 +960,7 @@ func (c *Conn) readOffset(t int64) (offset int64, err error) {
 // connection. If there are none, the method fetches all partitions of the kafka
 // cluster.
 func (c *Conn) ReadPartitions(topics ...string) (partitions []Partition, err error) {
+
 	if len(topics) == 0 {
 		if len(c.topic) != 0 {
 			defaultTopics := [...]string{c.topic}
@@ -1132,10 +1160,11 @@ func (c *Conn) writeCompressedMessages(codec CompressionCodec, msgs ...Message)
 			deadline = adjustDeadlineForRTT(deadline, now, defaultRTT)
 			switch produceVersion {
 			case v7:
-				recordBatch, err := newRecordBatch(
-					codec,
-					msgs...,
-				)
+				recordBatch, err :=
+					newRecordBatch(
+						codec,
+						msgs...,
+					)
 				if err != nil {
 					return err
 				}
@@ -1150,10 +1179,11 @@ func (c *Conn) writeCompressedMessages(codec CompressionCodec, msgs ...Message)
 					recordBatch,
 				)
 			case v3:
-				recordBatch, err := newRecordBatch(
-					codec,
-					msgs...,
-				)
+				recordBatch, err :=
+					newRecordBatch(
+						codec,
+						msgs...,
+					)
 				if err != nil {
 					return err
 				}
@@ -1218,6 +1248,7 @@ func (c *Conn) writeCompressedMessages(codec CompressionCodec, msgs ...Message)
 						}
 						return size, err
 					}
+
 				})
 				if err != nil {
 					return size, err
@@ -1577,7 +1608,7 @@ func (c *Conn) saslAuthenticate(data []byte) ([]byte, error) {
 		return nil, err
 	}
 	if version == v1 {
-		request := saslAuthenticateRequestV0{Data: data}
+		var request = saslAuthenticateRequestV0{Data: data}
 		var response saslAuthenticateResponseV0
 
 		err := c.writeOperation(