kgo: further fix for cd65d77

The prior commit was insufficient -- we left a dangling lock, and we
had that same dangling lock in the cancelable commit offsets.

Author: twmb

pkg/kgo/consumer_group.go

@@ -2580,13 +2580,13 @@ func (g *groupConsumer) commitOffsetsSync(
 		onDone = func(*Client, *kmsg.OffsetCommitRequest, *kmsg.OffsetCommitResponse, error) {}
 	}
 
-	g.syncCommitMu.Lock() // block all other concurrent commits until our OnDone is done.
-
 	if err := g.waitJoinSyncMu(ctx); err != nil {
 		onDone(g.cl, kmsg.NewPtrOffsetCommitRequest(), kmsg.NewPtrOffsetCommitResponse(), err)
 		close(done)
 		return
 	}
+
+	g.syncCommitMu.Lock() // block all other concurrent commits until our OnDone is done.
 	unblockCommits := func(cl *Client, req *kmsg.OffsetCommitRequest, resp *kmsg.OffsetCommitResponse, err error) {
 		g.noCommitDuringJoinAndSync.RUnlock()
 		defer close(done)
@@ -2663,19 +2663,16 @@ func (cl *Client) CommitOffsets(
 		return
 	}
 
-	g.syncCommitMu.RLock() // block sync commit, but allow other concurrent Commit to cancel us
-	unblockSyncCommit := func(cl *Client, req *kmsg.OffsetCommitRequest, resp *kmsg.OffsetCommitResponse, err error) {
-		defer g.syncCommitMu.RUnlock()
-		onDone(cl, req, resp, err)
-	}
-
 	if err := g.waitJoinSyncMu(ctx); err != nil {
 		onDone(g.cl, kmsg.NewPtrOffsetCommitRequest(), kmsg.NewPtrOffsetCommitResponse(), err)
 		return
 	}
+
+	g.syncCommitMu.RLock() // block sync commit, but allow other concurrent Commit to cancel us
 	unblockJoinSync := func(cl *Client, req *kmsg.OffsetCommitRequest, resp *kmsg.OffsetCommitResponse, err error) {
 		g.noCommitDuringJoinAndSync.RUnlock()
-		unblockSyncCommit(cl, req, resp, err)
+		defer g.syncCommitMu.RUnlock()
+		onDone(cl, req, resp, err)
 	}
 
 	g.mu.Lock()