Add CallOpt SetID

SetID allows a caller to control the ID of the request. Previously it was
impossible to set the ID of a call.

call_opt.go

@@ -18,3 +18,13 @@ func Meta(meta interface{}) CallOption {
 		return r.SetMeta(meta)
 	})
 }
+
+// PickID returns a call option which sets the ID on a request. Care must be
+// taken to ensure there are no conflicts with any previously picked ID, nor
+// with the default sequence ID.
+func PickID(id ID) CallOption {
+	return callOptionFunc(func(r *Request) error {
+		r.ID = id
+		return nil
+	})
+}

jsonrpc2.go

@@ -332,6 +332,10 @@ func (c *Conn) send(ctx context.Context, m *anyMessage, wait bool) (cc *call, er
 	c.sending.Lock()
 	defer c.sending.Unlock()
 
+	// m.request.ID could be changed, so we store a copy to correctly
+	// clean up pending
+	var id ID
+
 	c.mu.Lock()
 	if c.shutdown || c.closing {
 		c.mu.Unlock()
@@ -342,8 +346,12 @@ func (c *Conn) send(ctx context.Context, m *anyMessage, wait bool) (cc *call, er
 	// responses.
 	if m.request != nil && wait {
 		cc = &call{request: m.request, seq: c.seq, done: make(chan error, 1)}
-		c.pending[ID{Num: c.seq}] = cc // use next seq as call ID
-		m.request.ID.Num = c.seq
+		if !m.request.ID.IsString && m.request.ID.Num == 0 {
+			// unset, use next seq as call ID
+			m.request.ID.Num = c.seq
+		}
+		id = m.request.ID
+		c.pending[id] = cc
 		c.seq++
 	}
 	c.mu.Unlock()
@@ -364,7 +372,7 @@ func (c *Conn) send(ctx context.Context, m *anyMessage, wait bool) (cc *call, er
 		if err != nil {
 			if cc != nil {
 				c.mu.Lock()
-				delete(c.pending, ID{Num: cc.seq})
+				delete(c.pending, id)
 				c.mu.Unlock()
 			}
 		}

jsonrpc2_test.go

@@ -208,6 +208,76 @@ func testClientServer(ctx context.Context, t *testing.T, stream jsonrpc2.ObjectS
 	hb.mu.Unlock()
 }
 
+func inMemoryPeerConns() (io.ReadWriteCloser, io.ReadWriteCloser) {
+	sr, cw := io.Pipe()
+	cr, sw := io.Pipe()
+	return &pipeReadWriteCloser{sr, sw}, &pipeReadWriteCloser{cr, cw}
+}
+
+type pipeReadWriteCloser struct {
+	*io.PipeReader
+	*io.PipeWriter
+}
+
+func (c *pipeReadWriteCloser) Close() error {
+	err1 := c.PipeReader.Close()
+	err2 := c.PipeWriter.Close()
+	if err1 != nil {
+		return err1
+	}
+	return err2
+}
+
+type handlerFunc func(context.Context, *jsonrpc2.Conn, *jsonrpc2.Request)
+
+func (h handlerFunc) Handle(ctx context.Context, conn *jsonrpc2.Conn, req *jsonrpc2.Request) {
+	h(ctx, conn, req)
+}
+
+func TestPickID(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	a, b := inMemoryPeerConns()
+	defer a.Close()
+	defer b.Close()
+
+	handler := handlerFunc(func(ctx context.Context, conn *jsonrpc2.Conn, req *jsonrpc2.Request) {
+		if err := conn.Reply(ctx, req.ID, fmt.Sprintf("hello, #%s: %s", req.ID, *req.Params)); err != nil {
+			t.Error(err)
+		}
+	})
+	connA := jsonrpc2.NewConn(ctx, jsonrpc2.NewBufferedStream(a, jsonrpc2.VSCodeObjectCodec{}), handler)
+	connB := jsonrpc2.NewConn(ctx, jsonrpc2.NewBufferedStream(b, jsonrpc2.VSCodeObjectCodec{}), noopHandler{})
+	defer connA.Close()
+	defer connB.Close()
+
+	const n = 100
+	for i := 0; i < n; i++ {
+		var opts []jsonrpc2.CallOption
+		id := jsonrpc2.ID{Num: uint64(i)}
+
+		// This is the actual test, every 3rd request we specify the
+		// ID and ensure we get a response with the correct ID echoed
+		// back
+		if i%3 == 0 {
+			id = jsonrpc2.ID{
+				Str:      fmt.Sprintf("helloworld-%d", i/3),
+				IsString: true,
+			}
+			opts = append(opts, jsonrpc2.PickID(id))
+		}
+
+		var got string
+		if err := connB.Call(ctx, "f", []int32{1, 2, 3}, &got, opts...); err != nil {
+			t.Fatal(err)
+		}
+		if want := fmt.Sprintf("hello, #%s: [1,2,3]", id); got != want {
+			t.Errorf("got result %q, want %q", got, want)
+		}
+	}
+}
+
 type noopHandler struct{}
 
 func (noopHandler) Handle(ctx context.Context, conn *jsonrpc2.Conn, req *jsonrpc2.Request) {}