kgo: allow PreTxnCommitFnContext to modify empty offsets

This builds the TxnOffsetCommitRequest early so that the hook can modify
it. If the modified request has no topics to commit, then we abort as
though uncommitted was empty.

pkg/kgo/txn.go

@@ -8,8 +8,9 @@ import (
 	"sync"
 	"time"
 
-	"github.com/twmb/franz-go/pkg/kerr"
 	"github.com/twmb/franz-go/pkg/kmsg"
+
+	"github.com/twmb/franz-go/pkg/kerr"
 )
 
 // TransactionEndTry is simply a named bool.
@@ -1060,7 +1061,13 @@ func (cl *Client) commitTransactionOffsets(
 		onDone(kmsg.NewPtrTxnOffsetCommitRequest(), kmsg.NewPtrTxnOffsetCommitResponse(), errNotGroup)
 		return nil
 	}
-	if len(uncommitted) == 0 {
+
+	req, err := g.prepareTxnOffsetCommit(ctx, uncommitted)
+	if err != nil {
+		onDone(req, kmsg.NewPtrTxnOffsetCommitResponse(), err)
+		return g
+	}
+	if len(req.Topics) == 0 {
 		onDone(kmsg.NewPtrTxnOffsetCommitRequest(), kmsg.NewPtrTxnOffsetCommitResponse(), nil)
 		return g
 	}
@@ -1088,7 +1095,7 @@ func (cl *Client) commitTransactionOffsets(
 	g.mu.Lock()
 	defer g.mu.Unlock()
 
-	g.commitTxn(ctx, uncommitted, unblockJoinSync)
+	g.commitTxn(ctx, req, unblockJoinSync)
 	return g
 }
 
@@ -1139,18 +1146,10 @@ func (cl *Client) addOffsetsToTxn(ctx context.Context, group string) error {
 // commitTxn is ALMOST EXACTLY THE SAME as commit, but changed for txn types
 // and we avoid updateCommitted. We avoid updating because we manually
 // SetOffsets when ending the transaction.
-func (g *groupConsumer) commitTxn(
-	ctx context.Context,
-	uncommitted map[string]map[int32]EpochOffset,
-	onDone func(*kmsg.TxnOffsetCommitRequest, *kmsg.TxnOffsetCommitResponse, error),
-) {
+func (g *groupConsumer) commitTxn(ctx context.Context, req *kmsg.TxnOffsetCommitRequest, onDone func(*kmsg.TxnOffsetCommitRequest, *kmsg.TxnOffsetCommitResponse, error)) {
 	if onDone == nil { // note we must always call onDone
 		onDone = func(_ *kmsg.TxnOffsetCommitRequest, _ *kmsg.TxnOffsetCommitResponse, _ error) {}
 	}
-	if len(uncommitted) == 0 { // only empty if called thru autocommit / default revoke
-		onDone(kmsg.NewPtrTxnOffsetCommitRequest(), kmsg.NewPtrTxnOffsetCommitResponse(), nil)
-		return
-	}
 
 	if g.commitCancel != nil {
 		g.commitCancel() // cancel any prior commit
@@ -1169,22 +1168,6 @@ func (g *groupConsumer) commitTxn(
 	g.commitCancel = commitCancel
 	g.commitDone = commitDone
 
-	// We issue this request even if the producer ID is failed; the request
-	// will fail if it is.
-	//
-	// The id must have been set at least once by this point because of
-	// addOffsetsToTxn.
-	id, epoch, _ := g.cl.producerID()
-	req := kmsg.NewPtrTxnOffsetCommitRequest()
-	req.TransactionalID = *g.cl.cfg.txnID
-	req.Group = g.cfg.group
-	req.ProducerID = id
-	req.ProducerEpoch = epoch
-	memberID, generation := g.memberGen.load()
-	req.Generation = generation
-	req.MemberID = memberID
-	req.InstanceID = g.cfg.instanceID
-
 	if ctx.Done() != nil {
 		go func() {
 			select {
@@ -1207,28 +1190,7 @@ func (g *groupConsumer) commitTxn(
 				<-priorDone // wait for any prior request to finish
 			}
 		}
-		g.cl.cfg.logger.Log(LogLevelDebug, "issuing txn offset commit", "uncommitted", uncommitted)
-
-		for topic, partitions := range uncommitted {
-			reqTopic := kmsg.NewTxnOffsetCommitRequestTopic()
-			reqTopic.Topic = topic
-			for partition, eo := range partitions {
-				reqPartition := kmsg.NewTxnOffsetCommitRequestTopicPartition()
-				reqPartition.Partition = partition
-				reqPartition.Offset = eo.Offset
-				reqPartition.LeaderEpoch = eo.Epoch
-				reqPartition.Metadata = &req.MemberID
-				reqTopic.Partitions = append(reqTopic.Partitions, reqPartition)
-			}
-			req.Topics = append(req.Topics, reqTopic)
-		}
-
-		if fn, ok := ctx.Value(txnCommitContextFn).(func(*kmsg.TxnOffsetCommitRequest) error); ok {
-			if err := fn(req); err != nil {
-				onDone(req, nil, err)
-				return
-			}
-		}
+		g.cl.cfg.logger.Log(LogLevelDebug, "issuing txn offset commit", "uncommitted", req)
 
 		var resp *kmsg.TxnOffsetCommitResponse
 		var err error
@@ -1242,3 +1204,44 @@ func (g *groupConsumer) commitTxn(
 		onDone(req, resp, nil)
 	}()
 }
+
+func (g *groupConsumer) prepareTxnOffsetCommit(ctx context.Context, uncommitted map[string]map[int32]EpochOffset) (*kmsg.TxnOffsetCommitRequest, error) {
+	req := kmsg.NewPtrTxnOffsetCommitRequest()
+
+	// We're now generating the producerID before addOffsetsToTxn.
+	// We will not make this request until after addOffsetsToTxn, but it's possible to fail here due to a failed producerID.
+	id, epoch, err := g.cl.producerID()
+	if err != nil {
+		return req, err
+	}
+
+	req.TransactionalID = *g.cl.cfg.txnID
+	req.Group = g.cfg.group
+	req.ProducerID = id
+	req.ProducerEpoch = epoch
+	memberID, generation := g.memberGen.load()
+	req.Generation = generation
+	req.MemberID = memberID
+	req.InstanceID = g.cfg.instanceID
+
+	for topic, partitions := range uncommitted {
+		reqTopic := kmsg.NewTxnOffsetCommitRequestTopic()
+		reqTopic.Topic = topic
+		for partition, eo := range partitions {
+			reqPartition := kmsg.NewTxnOffsetCommitRequestTopicPartition()
+			reqPartition.Partition = partition
+			reqPartition.Offset = eo.Offset
+			reqPartition.LeaderEpoch = eo.Epoch
+			reqPartition.Metadata = &req.MemberID
+			reqTopic.Partitions = append(reqTopic.Partitions, reqPartition)
+		}
+		req.Topics = append(req.Topics, reqTopic)
+	}
+
+	if fn, ok := ctx.Value(txnCommitContextFn).(func(*kmsg.TxnOffsetCommitRequest) error); ok {
+		if err := fn(req); err != nil {
+			return req, err
+		}
+	}
+	return req, nil
+}