一个websocket连接为什么不能有多个goroutine访问

曾经终败给现在 2023-07-05 11:42 5阅读 0赞

# 林深时见鹿 海蓝时见鲸 梦醒时见你 #

今天在看源码时看到的，分享出来。

所用的websocket库：**github.com/gorilla/websocket**

**代码位置：vendor/github.com/gorilla/websocket/conn.go，**WriteMessage方法

// WriteMessage is a helper method for getting a writer using NextWriter,
    // writing the message and closing the writer.
    func (c *Conn) WriteMessage(messageType int, data []byte) error {
    	wr, err := c.NextWriter(messageType)
    	if err != nil {
    		return err
    	}
    	w := wr.(messageWriter)
    	if _, err := w.write(true, data); err != nil {
    		return err
    	}
    	if c.writeSeq == w.seq {
    		if err := c.flushFrame(true, nil); err != nil {
    			return err
    		}
    	}
    	return nil
    }

看下510行的**NextWriter**方法：

// NextWriter returns a writer for the next message to send.  The writer's
    // Close method flushes the complete message to the network.
    //
    // There can be at most one open writer on a connection. NextWriter closes the
    // previous writer if the application has not already done so.
    //
    // The NextWriter method and the writers returned from the method cannot be
    // accessed by more than one goroutine at a time.
    func (c *Conn) NextWriter(messageType int) (io.WriteCloser, error) {
    	if c.writeErr != nil {
    		return nil, c.writeErr
    	}
    
    	if c.writeFrameType != noFrame {
    		if err := c.flushFrame(true, nil); err != nil {
    			return nil, err
    		}
    	}
    
    	if !isControl(messageType) && !isData(messageType) {
    		return nil, errBadWriteOpCode
    	}
    
    	c.writeFrameType = messageType
    	return messageWriter{c, c.writeSeq}, nil
    }

注意注释说明。即：1，一个连接上最多可以有一个打开的的writer。 如果程序未关闭，则NextWriter将关闭前一个writer，如下：

if c.writeErr != nil {
     	return nil, c.writeErr
       }

一次不能由多个goroutine访问NextWriter方法和从该方法返回的writer（多个的话将产生多个writer）

再看看reader，一样的：

// NextReader returns the next data message received from the peer. The
    // returned messageType is either TextMessage or BinaryMessage.
    //
    // There can be at most one open reader on a connection. NextReader discards
    // the previous message if the application has not already consumed it.
    //
    // The NextReader method and the readers returned from the method cannot be
    // accessed by more than one goroutine at a time.
    func (c *Conn) NextReader() (messageType int, r io.Reader, err error) {
    
    	c.readSeq++
    	c.readLength = 0
    
    	for c.readErr == nil {
    		frameType, err := c.advanceFrame()
    		if err != nil {
    			c.readErr = hideTempErr(err)
    			break
    		}
    		if frameType == TextMessage || frameType == BinaryMessage {
    			return frameType, messageReader{c, c.readSeq}, nil
    		}
    	}
    	return noFrame, nil, c.readErr
    }

后续继续详细跟进。