zmq4: fix race condition in Rep cancellation

* There is a race condition in repWriter write(ctx context.Context, msg Msg):

if the run() loop of repWriter has exited because r.ctx was cancelled then <- repSendPayload will block, because nobody is reading the channel anymore

I'm adding a test that reproduces the issue with a fix.

* We should also abort when the writer context is cancelled.

rep.go

@@ -257,8 +257,14 @@ func (r *repWriter) rmConn(conn *Conn) {
 
 func (r *repWriter) write(ctx context.Context, msg Msg) error {
 	conn, preamble := r.state.Get()
-	r.sendCh <- repSendPayload{conn, preamble, msg}
-	return nil
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	case <-r.ctx.Done(): // repWriter.run() terminates on this, sendCh <- will not complete
+		return r.ctx.Err()
+	case r.sendCh <- repSendPayload{conn, preamble, msg}:
+		return nil
+	}
 }
 
 func (r *repWriter) run() {

rep_test.go

@@ -6,6 +6,7 @@ package zmq4_test
 
 import (
 	"context"
+	"errors"
 	"sync"
 	"testing"
 
@@ -92,3 +93,77 @@ func TestIssue99(t *testing.T) {
 		t.Fatalf("message length mismatch: got=%d, want=%d", got, want)
 	}
 }
+
+func TestCancellation(t *testing.T) {
+	// if the context is cancelled during a rep.Send both the requester and the responder should get an error
+	var wg sync.WaitGroup
+
+	ep, err := EndPoint("tcp")
+	if err != nil {
+		t.Fatalf("could not find endpoint: %+v", err)
+	}
+
+	responderStarted := make(chan bool)
+
+	requester := func() {
+		defer wg.Done()
+		<-responderStarted
+
+		req := zmq4.NewReq(context.Background())
+		defer req.Close()
+
+		err := req.Dial(ep)
+		if err != nil {
+			t.Errorf("could not dial: %+v", err)
+			return
+		}
+
+		err = req.Send(zmq4.NewMsgString("ping"))
+		if err != nil {
+			t.Errorf("could not send: %+v", err)
+			return
+		}
+
+		msg, err := req.Recv()
+		if err == nil {
+			t.Errorf("requester should have gotten an error, but got: %+v", msg)
+		}
+	}
+
+	responder := func() {
+
+		defer wg.Done()
+		repCtx, cancel := context.WithCancel(context.Background())
+		rep := zmq4.NewRep(repCtx)
+		defer rep.Close()
+
+		err := rep.Listen(ep)
+		if err != nil {
+			t.Errorf("could not dial: %+v", err)
+			return
+		}
+
+		responderStarted <- true
+
+		_, err = rep.Recv()
+		if err != nil {
+			t.Errorf("could not recv: %+v", err)
+			return
+		}
+
+		// cancel the context right before sending the response
+		cancel()
+		err = rep.Send(zmq4.NewMsgString("pong"))
+
+		if !errors.Is(err, context.Canceled) {
+			t.Errorf("context should be cancelled: %+v", err)
+		}
+	}
+
+	wg.Add(2)
+
+	go requester()
+	go responder()
+
+	wg.Wait()
+}