Address common gotchas with package

Update documentation to explicitly state that applications must break out of a
read loop on error.

Detect application read loops spinning on a failed connection and panic.

Detect concurrent writes and panic. The detection is best-effort.

Update documentation to state that connections respond to close frames.

conn.go

@@ -214,6 +214,7 @@ type Conn struct {
 	writeFrameType int    // type of the current frame.
 	writeSeq       int    // incremented to invalidate message writers.
 	writeDeadline  time.Time
+	isWriting      bool // for best-effort concurrent write detection
 
 	// Read fields
 	readErr       error
@@ -227,6 +228,7 @@ type Conn struct {
 	readMaskKey   [4]byte
 	handlePong    func(string) error
 	handlePing    func(string) error
+	readErrCount  int
 }
 
 func newConn(conn net.Conn, isServer bool, readBufferSize, writeBufferSize int) *Conn {
@@ -440,9 +442,22 @@ func (c *Conn) flushFrame(final bool, extra []byte) error {
 		}
 	}
 
-	// Write the buffers to the connection.
+	// Write the buffers to the connection with best-effort detection of
+	// concurrent writes. See the concurrency section in the package
+	// documentation for more info.
+
+	if c.isWriting {
+		panic("concurrent write to websocket connection")
+	}
+	c.isWriting = true
+
 	c.writeErr = c.write(c.writeFrameType, c.writeDeadline, c.writeBuf[framePos:c.writePos], extra)
 
+	if !c.isWriting {
+		panic("concurrent write to websocket connection")
+	}
+	c.isWriting = false
+
 	// Setup for next frame.
 	c.writePos = maxFrameHeaderSize
 	c.writeFrameType = continuationFrame
@@ -752,9 +767,10 @@ func (c *Conn) handleProtocolError(message string) error {
 // There can be at most one open reader on a connection. NextReader discards
 // the previous message if the application has not already consumed it.
 //
-// Errors returned from NextReader are permanent. If NextReader returns a
+// Applications must break out of the application's read loop when this method
-// non-nil error, then all subsequent calls to NextReader return the same
+// returns a non-nil error value. Errors returned from this method are
-// error.
+// permanent. Once this method returns a non-nil error, all subsequent calls to
+// this method return the same error.
 func (c *Conn) NextReader() (messageType int, r io.Reader, err error) {
 
 	c.readSeq++
@@ -770,6 +786,15 @@ func (c *Conn) NextReader() (messageType int, r io.Reader, err error) {
 			return frameType, messageReader{c, c.readSeq}, nil
 		}
 	}
+
+	// Applications that do handle the error returned from this method spin in
+	// tight loop on connection failure. To help application developers detect
+	// this error, panic on repeated reads to the failed connection.
+	c.readErrCount++
+	if c.readErrCount >= 1000 {
+		panic("repeated read on failed websocket connection")
+	}
+
 	return noFrame, nil, c.readErr
 }
 

conn_test.go

@@ -311,3 +311,57 @@ func TestUnexpectedCloseErrors(t *testing.T) {
 		}
 	}
 }
+
+type blockingWriter struct {
+	c1, c2 chan struct{}
+}
+
+func (w blockingWriter) Write(p []byte) (int, error) {
+	// Allow main to continue
+	close(w.c1)
+	// Wait for panic in main
+	<-w.c2
+	return len(p), nil
+}
+
+func TestConcurrentWritePanic(t *testing.T) {
+	w := blockingWriter{make(chan struct{}), make(chan struct{})}
+	c := newConn(fakeNetConn{Reader: nil, Writer: w}, false, 1024, 1024)
+	go func() {
+		c.WriteMessage(TextMessage, []byte{})
+	}()
+
+	// wait for goroutine to block in write.
+	<-w.c1
+
+	defer func() {
+		close(w.c2)
+		if v := recover(); v != nil {
+			return
+		}
+	}()
+
+	c.WriteMessage(TextMessage, []byte{})
+	t.Fatal("should not get here")
+}
+
+type failingReader struct{}
+
+func (r failingReader) Read(p []byte) (int, error) {
+	return 0, io.EOF
+}
+
+func TestFailedConnectionReadPanic(t *testing.T) {
+	c := newConn(fakeNetConn{Reader: failingReader{}, Writer: nil}, false, 1024, 1024)
+
+	defer func() {
+		if v := recover(); v != nil {
+			return
+		}
+	}()
+
+	for i := 0; i < 20000; i++ {
+		c.ReadMessage()
+	}
+	t.Fatal("should not get here")
+}

doc.go

@@ -85,14 +85,28 @@
 // and pong. Call the connection WriteControl, WriteMessage or NextWriter
 // methods to send a control message to the peer.
 //
-// Connections handle received ping and pong messages by invoking a callback
+// Connections handle received ping and pong messages by invoking callback
-// function set with SetPingHandler and SetPongHandler methods. These callback
+// functions set with SetPingHandler and SetPongHandler methods. The default
-// functions can be invoked from the ReadMessage method, the NextReader method
+// ping handler sends a pong to the client. The callback functions can be
-// or from a call to the data message reader returned from NextReader.
+// invoked from the NextReader, ReadMessage or the message Read method.
 //
-// Connections handle received close messages by returning an error from the
+// Connections handle received close messages by sending a close message to the
-// ReadMessage method, the NextReader method or from a call to the data message
+// peer and returning a *CloseError from the the NextReader, ReadMessage or the
-// reader returned from NextReader.
+// message Read method.
+//
+// The application must read the connection to process ping and close messages
+// sent from the peer. If the application is not otherwise interested in
+// messages from the peer, then the application should start a goroutine to
+// read and discard messages from the peer. A simple example is:
+//
+//  func readLoop(c *websocket.Conn) {
+//      for {
+//          if _, _, err := c.NextReader(); err != nil {
+//              c.Close()
+//              break
+//          }
+//      }
+//  }
 //
 // Concurrency
 //
@@ -107,22 +121,6 @@
 // The Close and WriteControl methods can be called concurrently with all other
 // methods.
 //
-// Read is Required
-//
-// The application must read the connection to process ping and close messages
-// sent from the peer. If the application is not otherwise interested in
-// messages from the peer, then the application should start a goroutine to read
-// and discard messages from the peer. A simple example is:
-//
-//  func readLoop(c *websocket.Conn) {
-//      for {
-//          if _, _, err := c.NextReader(); err != nil {
-//              c.Close()
-//              break
-//          }
-//      }
-//  }
-//
 // Origin Considerations
 //
 // Web browsers allow Javascript applications to open a WebSocket connection to