kgo: add PreCommitTxnFnContext

Allows modifying TxnOffsetCommitRequest before it is issued --
particularly, to modify the metadata field.

Closes #559.

pkg/kgo/consumer_group.go

@@ -2238,6 +2238,20 @@ func PreCommitFnContext(ctx context.Context, fn func(*kmsg.OffsetCommitRequest)
 	return context.WithValue(ctx, commitContextFn, fn)
 }
 
+type commitTxnContextFnT struct{}
+
+var commitTxnContextFn commitTxnContextFnT
+
+// PreCommitTxnFnContext attaches fn to the context through WithValue. Using
+// the context while committing a transaction allows fn to be called just
+// before the commit is issued. This can be used to modify the actual commit,
+// such as by associating metadata with partitions (for transactions, the
+// default internal metadata is the client's current member ID). If fn returns
+// an error, the commit is not attempted.
+func PreCommitTxnFnContext(ctx context.Context, fn func(*kmsg.TxnOffsetCommitRequest) error) context.Context {
+	return context.WithValue(ctx, commitTxnContextFn, fn)
+}
+
 // CommitRecords issues a synchronous offset commit for the offsets contained
 // within rs. Retryable errors are retried up to the configured retry limit,
 // and any unretryable error is returned.

pkg/kgo/txn.go

@@ -1222,6 +1222,13 @@ func (g *groupConsumer) commitTxn(
 			req.Topics = append(req.Topics, reqTopic)
 		}
 
+		if fn, ok := ctx.Value(commitTxnContextFn).(func(*kmsg.TxnOffsetCommitRequest) error); ok {
+			if err := fn(req); err != nil {
+				onDone(req, nil, err)
+				return
+			}
+		}
+
 		var resp *kmsg.TxnOffsetCommitResponse
 		var err error
 		if len(req.Topics) > 0 {