Go 如何使得 Web 工作

古城微笑少年丶 2021-07-24 18:59 280阅读 0赞

# 一 web 工作方式的几个概念 #

以下几个均是服务器端的概念。

Request：用户请求的信息，用来解析用户的请求信息，包括 post、get、cookie、url 等信息。

Response：服务器需要反馈给客户端的信息。

Conn：用户的每次请求链接。

Handler：处理请求和生成返回信息的处理逻辑。

# 二 http 包运行机制 #

Go 实现 Web 服务的工作模式的流程图

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http包执行流程

1 创建 Listen Socket，监听指定的端口, 等待客户端请求到来。

2 Listen Socket 接受客户端的请求，得到 Client Socket，接下来通过 Client Socket 与客户端通信。

3 处理客户端的请求。

首先从 Client Socket 读取 HTTP 请求的协议头，如果是 POST 方法，还可能要读取客户端提交的数据，然后交给相应的 handler 处理请求，handler 处理完毕准备好客户端需要的数据。通过 Client Socket 写给客户端。

整个过程需要了解清楚下面三个问题。

*  如何监听端口？
 *  如何接收客户端请求？
 *  如何分配 handler？

Go 是通过一个函数 ListenAndServe 来处理这些事情的。

首先，初始化一个 server 对象，然后调用 ListenAndServe() 函数。

func ListenAndServe(addr string, handler Handler) error {
       server := &Server{Addr: addr, Handler: handler}
       return server.ListenAndServe()
    }

其次，调用了 net.Listen("tcp", addr)，也就是底层用 TCP 协议搭建了一个服务，然后监控我们设置的端口。然后调用 srv.Serve() 函数，对 http 请求进行处理。

func (srv *Server) ListenAndServe() error {
       addr := srv.Addr
       if addr == "" {
          addr = ":http"
       }
       ln, err := net.Listen("tcp", addr)
       if err != nil {
          return err
       }
       return srv.Serve(tcpKeepAliveListener{ln.(*net.TCPListener)})
    }

下面的代码可以看到整个的 http 处理过程。

func (srv *Server) Serve(l net.Listener) error {
       defer l.Close()
       if fn := testHookServerServe; fn != nil {
          fn(srv, l)
       }
       var tempDelay time.Duration // how long to sleep on accept failure
    
       if err := srv.setupHTTP2_Serve(); err != nil {
          return err
       }
    
       srv.trackListener(l, true)
       defer srv.trackListener(l, false)
    
    
       baseCtx := context.Background() // base is always background, per Issue 16220
       ctx := context.WithValue(baseCtx, ServerContextKey, srv)
       for {
          rw, e := l.Accept()
          if e != nil {
             select {
             case <-srv.getDoneChan():
                return ErrServerClosed
             default:
             }
             if ne, ok := e.(net.Error); ok && ne.Temporary() {
                if tempDelay == 0 {
                   tempDelay = 5 * time.Millisecond
                } else {
                   tempDelay *= 2
                }
                if max := 1 * time.Second; tempDelay > max {
                   tempDelay = max
                }
                srv.logf("http: Accept error: %v; retrying in %v", e, tempDelay)
                time.Sleep(tempDelay)
                continue
             }
             return e
          }
          tempDelay = 0
          c := srv.newConn(rw)
          c.setState(c.rwc, StateNew) // before Serve can return
          go c.serve(ctx)
       }
    }

这个函数里面有一个for\{\}，首先通过 Listener 接收请求，其次创建一个 Conn，最后单独开了一个 goroutine，把这个请求的数据当做参数扔给这个 conn 去服务：go c.serve()。这个就是高并发体现了，用户的每一次请求都是在一个新的 goroutine 去服务，相互不影响。

那么如何具体分配到相应的函数来处理请求呢？见下面代码。

func (c *conn) serve(ctx context.Context) {
       c.remoteAddr = c.rwc.RemoteAddr().String()
       ctx = context.WithValue(ctx, LocalAddrContextKey, c.rwc.LocalAddr())
       defer func() {
          if err := recover(); err != nil && err != ErrAbortHandler {
             const size = 64 << 10
             buf := make([]byte, size)
             buf = buf[:runtime.Stack(buf, false)]
             c.server.logf("http: panic serving %v: %v\n%s", c.remoteAddr, err, buf)
          }
          if !c.hijacked() {
             c.close()
             c.setState(c.rwc, StateClosed)
          }
       }()
    
       if tlsConn, ok := c.rwc.(*tls.Conn); ok {
          if d := c.server.ReadTimeout; d != 0 {
             c.rwc.SetReadDeadline(time.Now().Add(d))
          }
          if d := c.server.WriteTimeout; d != 0 {
             c.rwc.SetWriteDeadline(time.Now().Add(d))
          }
          if err := tlsConn.Handshake(); err != nil {
             c.server.logf("http: TLS handshake error from %s: %v", c.rwc.RemoteAddr(), err)
             return
          }
          c.tlsState = new(tls.ConnectionState)
          *c.tlsState = tlsConn.ConnectionState()
          if proto := c.tlsState.NegotiatedProtocol; validNPN(proto) {
             if fn := c.server.TLSNextProto[proto]; fn != nil {
                h := initNPNRequest{tlsConn, serverHandler{c.server}}
                fn(c.server, tlsConn, h)
             }
             return
          }
       }
    
       // HTTP/1.x from here on.
    
       ctx, cancelCtx := context.WithCancel(ctx)
       c.cancelCtx = cancelCtx
       defer cancelCtx()
    
       c.r = &connReader{conn: c}
       c.bufr = newBufioReader(c.r)
       c.bufw = newBufioWriterSize(checkConnErrorWriter{c}, 4<<10)
       for {
          w, err := c.readRequest(ctx)
          if c.r.remain != c.server.initialReadLimitSize() {
             // If we read any bytes off the wire, we're active.
             c.setState(c.rwc, StateActive)
          }
          if err != nil {
             const errorHeaders = "\r\nContent-Type: text/plain; charset=utf-8\r\nConnection: close\r\n\r\n"
    
             if err == errTooLarge {
                // Their HTTP client may or may not be
                // able to read this if we're
                // responding to them and hanging up
                // while they're still writing their
                // request. Undefined behavior.
                const publicErr = "431 Request Header Fields Too Large"
                fmt.Fprintf(c.rwc, "HTTP/1.1 "+publicErr+errorHeaders+publicErr)
                c.closeWriteAndWait()
                return
             }
             if isCommonNetReadError(err) {
                return // don't reply
             }
    
             publicErr := "400 Bad Request"
             if v, ok := err.(badRequestError); ok {
                publicErr = publicErr + ": " + string(v)
             }
    
             fmt.Fprintf(c.rwc, "HTTP/1.1 "+publicErr+errorHeaders+publicErr)
             return
          }
    
          // Expect 100 Continue support
          req := w.req
          if req.expectsContinue() {
             if req.ProtoAtLeast(1, 1) && req.ContentLength != 0 {
                // Wrap the Body reader with one that replies on the connection
                req.Body = &expectContinueReader{readCloser: req.Body, resp: w}
             }
          } else if req.Header.get("Expect") != "" {
             w.sendExpectationFailed()
             return
          }
    
          c.curReq.Store(w)
    
          if requestBodyRemains(req.Body) {
             registerOnHitEOF(req.Body, w.conn.r.startBackgroundRead)
          } else {
             if w.conn.bufr.Buffered() > 0 {
                w.conn.r.closeNotifyFromPipelinedRequest()
             }
             w.conn.r.startBackgroundRead()
          }
    
          // HTTP cannot have multiple simultaneous active requests.[*]
          // Until the server replies to this request, it can't read another,
          // so we might as well run the handler in this goroutine.
          // [*] Not strictly true: HTTP pipelining. We could let them all process
          // in parallel even if their responses need to be serialized.
          // But we're not going to implement HTTP pipelining because it
          // was never deployed in the wild and the answer is HTTP/2.
          serverHandler{c.server}.ServeHTTP(w, w.req)
          w.cancelCtx()
          if c.hijacked() {
             return
          }
          w.finishRequest()
          if !w.shouldReuseConnection() {
             if w.requestBodyLimitHit || w.closedRequestBodyEarly() {
                c.closeWriteAndWait()
             }
             return
          }
          c.setState(c.rwc, StateIdle)
          c.curReq.Store((*response)(nil))
    
          if !w.conn.server.doKeepAlives() {
             // We're in shutdown mode. We might've replied
             // to the user without "Connection: close" and
             // they might think they can send another
             // request, but such is life with HTTP/1.1.
             return
          }
    
          if d := c.server.idleTimeout(); d != 0 {
             c.rwc.SetReadDeadline(time.Now().Add(d))
             if _, err := c.bufr.Peek(4); err != nil {
                return
             }
          }
          c.rwc.SetReadDeadline(time.Time{})
       }
    }

处理器分配代码。

func (sh serverHandler) ServeHTTP(rw ResponseWriter, req *Request) {
       handler := sh.srv.Handler
       if handler == nil {
          handler = DefaultServeMux
       }
       if req.RequestURI == "*" && req.Method == "OPTIONS" {
          handler = globalOptionsHandler{}
       }
       handler.ServeHTTP(rw, req)
    }

conn 首先会解析 request：c.readRequest()，然后获取相应的handler：handler := sh.srv.Handler。也就是函数 ListenAndServe 的第二个参数，当传递的是 nil，也就是为空时，那么默认获取 handler = DefaultServeMux。那么这个变量用来做什么的呢？这个变量就是一个路由器，它用来匹配 url 跳转到其相应的 handle 函数，那么这个有设置过吗？有，当我们调用 http.HandleFunc("/", sayhelloName) 时设置的。它的作用就是注册请求/的路由规则，当请求uri为"/"，路由就会转到函数 sayhelloName，DefaultServeMux会调用 ServeHTTP 方法，这个方法内部其实就是调用 sayhelloName 本身，最后通过写入response的信息反馈到客户端。

整个流程如下。

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