Add SyncWrite method to doppel.Conn

mtglib/internal/doppel/conn.go

@@ -16,18 +16,48 @@ type Conn struct {
 }
 
 type connPayload struct {
-	ctx         context.Context
-	ctxCancel   context.CancelCauseFunc
-	clock       Clock
-	wg          sync.WaitGroup
-	writeLock   sync.Mutex
-	writeStream bytes.Buffer
+	ctx           context.Context
+	ctxCancel     context.CancelCauseFunc
+	clock         Clock
+	wg            sync.WaitGroup
+	syncWriteLock sync.RWMutex
+	writeStream   bytes.Buffer
+	writeCond     *sync.Cond
 }
 
 func (c Conn) Write(p []byte) (int, error) {
-	c.p.writeLock.Lock()
+	c.p.syncWriteLock.RLock()
+	defer c.p.syncWriteLock.RUnlock()
+
+	c.p.writeCond.L.Lock()
 	c.p.writeStream.Write(p)
-	c.p.writeLock.Unlock()
+	c.p.writeCond.L.Unlock()
+
+	return len(p), context.Cause(c.p.ctx)
+}
+
+func (c Conn) SyncWrite(p []byte) (int, error) {
+	c.p.syncWriteLock.Lock()
+	defer c.p.syncWriteLock.Unlock()
+
+	c.p.writeCond.L.Lock()
+	// wait until buffer is exhausted
+	for c.p.writeStream.Len() != 0 && context.Cause(c.p.ctx) == nil {
+		c.p.writeCond.Wait()
+	}
+	c.p.writeStream.Write(p)
+	c.p.writeCond.L.Unlock()
+
+	if err := context.Cause(c.p.ctx); err != nil {
+		return len(p), err
+	}
+
+	c.p.writeCond.L.Lock()
+	// wait until data will be sent
+	for c.p.writeStream.Len() != 0 && context.Cause(c.p.ctx) == nil {
+		c.p.writeCond.Wait()
+	}
+	c.p.writeCond.L.Unlock()
 
 	return len(p), context.Cause(c.p.ctx)
 }
@@ -39,6 +69,8 @@ func (c Conn) Start() {
 }
 
 func (c Conn) start() {
+	defer c.p.writeCond.Broadcast()
+
 	buf := [tls.MaxRecordSize]byte{}
 
 	for {
@@ -48,9 +80,9 @@ func (c Conn) start() {
 		case <-c.p.clock.tick:
 		}
 
-		c.p.writeLock.Lock()
+		c.p.writeCond.L.Lock()
 		n, err := c.p.writeStream.Read(buf[:c.p.clock.stats.Size()])
-		c.p.writeLock.Unlock()
+		c.p.writeCond.L.Unlock()
 
 		if n == 0 || err != nil {
 			continue
@@ -60,6 +92,8 @@ func (c Conn) start() {
 			c.p.ctxCancel(err)
 			return
 		}
+
+		c.p.writeCond.Signal()
 	}
 }
 
@@ -75,6 +109,7 @@ func NewConn(ctx context.Context, conn essentials.Conn, stats *Stats) Conn {
 		p: &connPayload{
 			ctx:       ctx,
 			ctxCancel: cancel,
+			writeCond: sync.NewCond(&sync.Mutex{}),
 			clock: Clock{
 				stats: stats,
 				tick:  make(chan struct{}),

mtglib/internal/doppel/conn_test.go

@@ -157,6 +157,136 @@ func (suite *ConnTestSuite) TestStopOnUnderlyingWriteError() {
 	}, 2*time.Second, time.Millisecond)
 }
 
+func (suite *ConnTestSuite) TestSyncWriteDataSent() {
+	suite.connMock.
+		On("Write", mock.AnythingOfType("[]uint8")).
+		Return(0, nil).
+		Maybe()
+
+	c := suite.makeConn()
+	defer c.Stop()
+
+	payload := []byte("sync hello")
+	n, err := c.SyncWrite(payload)
+	suite.NoError(err)
+	suite.Equal(len(payload), n)
+
+	// SyncWrite returns only after data is flushed to the wire.
+	assembled := &bytes.Buffer{}
+	reader := bytes.NewReader(suite.connMock.Written())
+
+	for {
+		header := make([]byte, tls.SizeHeader)
+		if _, err := io.ReadFull(reader, header); err != nil {
+			break
+		}
+
+		suite.Equal(byte(tls.TypeApplicationData), header[0])
+
+		length := binary.BigEndian.Uint16(header[tls.SizeRecordType+tls.SizeVersion:])
+		rec := make([]byte, length)
+		_, err := io.ReadFull(reader, rec)
+		suite.NoError(err)
+
+		assembled.Write(rec)
+	}
+
+	suite.Equal(payload, assembled.Bytes())
+}
+
+func (suite *ConnTestSuite) TestSyncWriteDrainsBufferFirst() {
+	suite.connMock.
+		On("Write", mock.AnythingOfType("[]uint8")).
+		Return(0, nil).
+		Maybe()
+
+	c := suite.makeConn()
+	defer c.Stop()
+
+	// Buffer some data via async Write.
+	_, err := c.Write([]byte("first"))
+	suite.NoError(err)
+
+	// SyncWrite must drain "first" before sending "second".
+	n, err := c.SyncWrite([]byte("second"))
+	suite.NoError(err)
+	suite.Equal(6, n)
+
+	// All data should be on the wire now.
+	assembled := &bytes.Buffer{}
+	reader := bytes.NewReader(suite.connMock.Written())
+
+	for {
+		header := make([]byte, tls.SizeHeader)
+		if _, err := io.ReadFull(reader, header); err != nil {
+			break
+		}
+
+		length := binary.BigEndian.Uint16(header[tls.SizeRecordType+tls.SizeVersion:])
+		rec := make([]byte, length)
+		_, err := io.ReadFull(reader, rec)
+		suite.NoError(err)
+
+		assembled.Write(rec)
+	}
+
+	suite.Equal([]byte("firstsecond"), assembled.Bytes())
+}
+
+func (suite *ConnTestSuite) TestSyncWriteBlocksAsyncWrite() {
+	suite.connMock.
+		On("Write", mock.AnythingOfType("[]uint8")).
+		Return(0, nil).
+		Maybe()
+
+	c := suite.makeConn()
+	defer c.Stop()
+
+	// Start SyncWrite — it holds exclusive lock.
+	syncDone := make(chan struct{})
+
+	go func() {
+		defer close(syncDone)
+		c.SyncWrite([]byte("exclusive"))
+	}()
+
+	// Give SyncWrite time to acquire the lock.
+	time.Sleep(10 * time.Millisecond)
+
+	// Async Write should block until SyncWrite completes.
+	writeDone := make(chan struct{})
+
+	go func() {
+		defer close(writeDone)
+		c.Write([]byte("blocked"))
+	}()
+
+	// SyncWrite should finish first.
+	<-syncDone
+
+	select {
+	case <-writeDone:
+		// Write completed after SyncWrite — correct.
+	case <-time.After(2 * time.Second):
+		suite.Fail("async Write did not unblock after SyncWrite completed")
+	}
+}
+
+func (suite *ConnTestSuite) TestSyncWriteReturnsErrorAfterStop() {
+	suite.connMock.
+		On("Write", mock.AnythingOfType("[]uint8")).
+		Return(0, nil).
+		Maybe()
+
+	c := suite.makeConn()
+	c.Stop()
+
+	time.Sleep(10 * time.Millisecond)
+
+	_, err := c.SyncWrite([]byte("too late"))
+	suite.Error(err)
+}
+
 func TestConn(t *testing.T) {
 	t.Parallel()
 	suite.Run(t, &ConnTestSuite{})