Introduced top-level ConsumerGroup construct (segmentio#277)

This is a lower-level API that unlocks use cases such as consuming
each assigned partition using a separate Reader or to overwrite
offsets for a group.

Refactored logic out of Reader and into ConsumerGroup so that
Reader uses the ConsumerGroup when GroupID is set.

Author: Steve van Loben Sels

conn.go

@@ -496,10 +496,10 @@ func (c *Conn) offsetFetch(request offsetFetchRequestV1) (offsetFetchResponseV1,
 	return response, nil
 }
 
-// syncGroups completes the handshake to join a consumer group
+// syncGroup completes the handshake to join a consumer group
 //
 // See http://kafka.apache.org/protocol.html#The_Messages_SyncGroup
-func (c *Conn) syncGroups(request syncGroupRequestV0) (syncGroupResponseV0, error) {
+func (c *Conn) syncGroup(request syncGroupRequestV0) (syncGroupResponseV0, error) {
 	var response syncGroupResponseV0
 
 	err := c.readOperation(
@@ -767,7 +767,6 @@ func (c *Conn) ReadBatchWith(cfg ReadBatchConfig) *Batch {
 	id, err := c.doRequest(&c.rdeadline, func(deadline time.Time, id int32) error {
 		now := time.Now()
 		deadline = adjustDeadlineForRTT(deadline, now, defaultRTT)
-		adjustedDeadline = deadline
 		switch c.fetchVersion {
 		case v10:
 			return c.wb.writeFetchRequestV10(

conn_test.go

@@ -597,7 +597,7 @@ func createGroup(t *testing.T, conn *Conn, groupID string) (generationID int32,
 	joinGroup := join()
 
 	// sync the group
-	_, err := conn.syncGroups(syncGroupRequestV0{
+	_, err := conn.syncGroup(syncGroupRequestV0{
 		GroupID:      groupID,
 		GenerationID: joinGroup.GenerationID,
 		MemberID:     joinGroup.MemberID,
@@ -609,7 +609,7 @@ func createGroup(t *testing.T, conn *Conn, groupID string) (generationID int32,
 		},
 	})
 	if err != nil {
-		t.Fatalf("bad syncGroups: %s", err)
+		t.Fatalf("bad syncGroup: %s", err)
 	}
 
 	generationID = joinGroup.GenerationID
@@ -710,7 +710,7 @@ func testConnHeartbeatErr(t *testing.T, conn *Conn) {
 	groupID := makeGroupID()
 	createGroup(t, conn, groupID)
 
-	_, err := conn.syncGroups(syncGroupRequestV0{
+	_, err := conn.syncGroup(syncGroupRequestV0{
 		GroupID: groupID,
 	})
 	if err != UnknownMemberId && err != NotCoordinatorForGroup {
@@ -734,7 +734,7 @@ func testConnSyncGroupErr(t *testing.T, conn *Conn) {
 	groupID := makeGroupID()
 	waitForCoordinator(t, conn, groupID)
 
-	_, err := conn.syncGroups(syncGroupRequestV0{
+	_, err := conn.syncGroup(syncGroupRequestV0{
 		GroupID: groupID,
 	})
 	if err != UnknownMemberId && err != NotCoordinatorForGroup {
@@ -844,6 +844,7 @@ func testConnFetchAndCommitOffsets(t *testing.T, conn *Conn) {
 }
 
 func testConnWriteReadConcurrently(t *testing.T, conn *Conn) {
+
 	const N = 1000
 	var msgs = make([]string, N)
 	var done = make(chan struct{})