kgo: no-op mark functions when not using AutoCommitMarks

Closes #598.

pkg/kgo/consumer_group.go

@@ -2201,14 +2201,6 @@ func (g *groupConsumer) getSetAssigns(setOffsets map[string]map[int32]EpochOffse
 // offsets are always updated on calls to PollFetches.
 //
 // If there are no uncommitted offsets, this returns nil.
-//
-// Note that, if manually committing, you should be careful with committing
-// during group rebalances. You must ensure you commit before the group's
-// session timeout is reached, otherwise this client will be kicked from the
-// group and the commit will fail.
-//
-// If using a cooperative balancer, commits while consuming during rebalancing
-// may fail with REBALANCE_IN_PROGRESS.
 func (cl *Client) UncommittedOffsets() map[string]map[int32]EpochOffset {
 	if g := cl.consumer.g; g != nil {
 		return g.getUncommitted(true)
@@ -2218,27 +2210,14 @@ func (cl *Client) UncommittedOffsets() map[string]map[int32]EpochOffset {
 
 // MarkedOffsets returns the latest marked offsets. When autocommitting, a
 // marked offset is an offset that can be committed, in comparison to a dirty
-// offset that cannot yet be committed. You usually see marked offsets with
-// AutoCommitMarks and MarkCommitRecords, but you can also use this function to
-// grab the current offsets that are candidates for committing from normal
-// autocommitting.
-//
-// If you set a custom OnPartitionsRevoked, marked offsets are not committed
-// when partitions are revoked. You can use this function to mark records and
-// issue a commit inside your OnPartitionsRevoked.
-//
-// Note that, if manually committing, you should be careful with committing
-// during group rebalances. You must ensure you commit before the group's
-// session timeout is reached, otherwise this client will be kicked from the
-// group and the commit will fail.
-//
-// If using a cooperative balancer, commits while consuming during rebalancing
-// may fail with REBALANCE_IN_PROGRESS.
+// offset that cannot yet be committed. MarkedOffsets returns nil if you are
+// not using AutoCommitMarks.
 func (cl *Client) MarkedOffsets() map[string]map[int32]EpochOffset {
-	if g := cl.consumer.g; g != nil {
-		return g.getUncommitted(false)
+	g := cl.consumer.g
+	if g == nil || !cl.cfg.autocommitMarks {
+		return nil
 	}
-	return nil
+	return g.getUncommitted(false)
 }
 
 // CommittedOffsets returns the latest committed offsets. Committed offsets are
@@ -2464,13 +2443,13 @@ func (cl *Client) CommitUncommittedOffsets(ctx context.Context) error {
 	return cl.commitOffsets(ctx, cl.UncommittedOffsets())
 }
 
-// CommitMarkedOffsets issues a synchronous offset commit for any
-// partition that has been consumed from that has marked offsets.
-// Retryable errors are retried up to the configured retry limit, and any
-// unretryable error is returned.
+// CommitMarkedOffsets issues a synchronous offset commit for any partition
+// that has been consumed from that has marked offsets.  Retryable errors are
+// retried up to the configured retry limit, and any unretryable error is
+// returned.
 //
-// This function is useful if you have marked offsets with MarkCommitRecords
-// when using AutoCommitMarks.
+// This function is only useful if you have marked offsets with
+// MarkCommitRecords when using AutoCommitMarks, otherwise this is a no-op.
 //
 // The recommended pattern for using this function is to have a poll / process
 // / commit loop. First PollFetches, then process every record,
@@ -2480,7 +2459,11 @@ func (cl *Client) CommitUncommittedOffsets(ctx context.Context) error {
 // As an alternative if you want to commit specific records, see CommitRecords.
 func (cl *Client) CommitMarkedOffsets(ctx context.Context) error {
 	// This function is just the tail end of CommitRecords just above.
-	return cl.commitOffsets(ctx, cl.MarkedOffsets())
+	marked := cl.MarkedOffsets()
+	if len(marked) == 0 {
+		return nil
+	}
+	return cl.commitOffsets(ctx, marked)
 }
 
 func (cl *Client) commitOffsets(ctx context.Context, offsets map[string]map[int32]EpochOffset) error {