GO-net/http源码阅读

net/http处理Http请求的基本流程

基本流程

Http包的三个关键类型

Handler接口：所有请求的处理器、路由ServeMux都满足该接口。

// A Handler responds to an HTTP request.
//
// ServeHTTP should write reply headers and data to the ResponseWriter
// and then return. Returning signals that the request is finished; it
// is not valid to use the ResponseWriter or read from the
// Request.Body after or concurrently with the completion of the
// ServeHTTP call.
//
// Depending on the HTTP client software, HTTP protocol version, and
// any intermediaries between the client and the Go server, it may not
// be possible to read from the Request.Body after writing to the
// ResponseWriter. Cautious handlers should read the Request.Body
// first, and then reply.
//
// Except for reading the body, handlers should not modify the
// provided Request.
//
// If ServeHTTP panics, the server (the caller of ServeHTTP) assumes
// that the effect of the panic was isolated to the active request.
// It recovers the panic, logs a stack trace to the server error log,
// and either closes the network connection or sends an HTTP/2
// RST_STREAM, depending on the HTTP protocol. To abort a handler so
// the client sees an interrupted response but the server doesn't log
// an error, panic with the value ErrAbortHandler.
type Handler interface {
    ServeHTTP(ResponseWriter, *Request) //路由具体实现
}

ServeMux结构体：HTTP请求的多路转接器（路由），它负责将每一个接收到的请求的URL与一个注册模式的列表进行匹配，并调用和URL最匹配的模式的处理器。它内部用一个map来保存所有处理器Handler。
```
type ServeMux struct {
    mu sync.RWMutex   //锁，由于请求涉及到并发处理，因此这里需要一个锁机制
    m  map[string]muxEntry  // 路由规则，一个string对应一个mux实体，这里的string就是注册的路由表达式
    hosts bool // 是否在任意的规则中带有host信息
} 
```
其中的muxEntry结构体类型，则是保存了Handler请求处理器和匹配的模式字符串。
```
type muxEntry struct {
    h        Handler // 这个路由表达式对应哪个handler
    pattern  string  //匹配字符串
}
```
- http包有一个包级别变量DefaultServeMux，表示默认路由：var DefaultServeMux = NewServeMux()，使用包级别的http.Handle()、http.HandleFunc()方法注册处理器时都是注册到该路由中。
```
// NewServeMux allocates and returns a new ServeMux.
func NewServeMux() *ServeMux { return new(ServeMux) }

// DefaultServeMux is the default ServeMux used by Serve.
var DefaultServeMux = &defaultServeMux

var defaultServeMux ServeMux
```
- ServeMux结构体有ServeHTTP()方法（满足Handler接口），主要用于间接调用它所保存的muxEntry中保存的Handler处理器的ServeHTTP()方法。
关注了上面两个结构体后就产生了一个问题，我们的请求处理函数并没有显式实现ServeHTTP(ResponseWriter, *Request)，它是怎么能转换为Handler类型的对象？这里就涉及了第三个重要类型，HandlerFunc适配器：
```
// The HandlerFunc type is an adapter to allow the use of
// ordinary functions as HTTP handlers. If f is a function
// with the appropriate signature, HandlerFunc(f) is a
// Handler that calls f.
type HandlerFunc func(ResponseWriter, *Request)

// ServeHTTP calls f(w, r).
func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
    f(w, r)
}
```
- 自行定义的处理函数转换为Handler类型就是HandlerFunc调用之后的结果，这个类型默认就实现了ServeHTTP这个接口，即我们调用了HandlerFunc(f),强制类型转换f成为HandlerFunc类型，这样f就拥有了ServeHTTP方法。

HTTP服务器的执行流程

通过http.ListenAndServe(addr string, handler Handler)启动服务，通过给定函数构造Server类型对象，然后调用Server对象的ListenAndServer方法，并将该方法的返回值error返回给调用方。

// ListenAndServe listens on the TCP network address addr and then calls
// Serve with handler to handle requests on incoming connections.
// Accepted connections are configured to enable TCP keep-alives.
//
// The handler is typically nil, in which case the DefaultServeMux is used.
//
// ListenAndServe always returns a non-nil error.
func ListenAndServe(addr string, handler Handler) error {
    server := &Server{Addr: addr, Handler: handler}
    return server.ListenAndServe()
}

server.ListenAndServe()内部调用net.Listen("tcp", addr)，该方法内部又调用net.ListenTCP()创建并返回一个net.Listener监听器ln。

// ListenAndServe listens on the TCP network address srv.Addr and then
// calls Serve to handle requests on incoming connections.
// Accepted connections are configured to enable TCP keep-alives.
//
// If srv.Addr is blank, ":http" is used.
//
// ListenAndServe always returns a non-nil error. After Shutdown or Close,
// the returned error is ErrServerClosed.
func (srv *Server) ListenAndServe() error {
    if srv.shuttingDown() {
        return ErrServerClosed
    }
    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)})
}

ln通过断言转换为了net.TCPListener类型，并将转换后的类型作为参数转换为tcpKeepAliveListener对象，然后将tcpKeepAliveListener对象传给srv.Serve()函数作为参数。

TCPListener实现了Listener接口，此处tcpKeepAliveListener重写了Accept()方法从而实现了Listener接口。

// tcpKeepAliveListener sets TCP keep-alive timeouts on accepted
// connections. It's used by ListenAndServe and ListenAndServeTLS so
// dead TCP connections (e.g. closing laptop mid-download) eventually
// go away.
type tcpKeepAliveListener struct {
    *net.TCPListener
}

func (ln tcpKeepAliveListener) Accept() (net.Conn, error) {
    tc, err := ln.AcceptTCP()
    if err != nil {
        return nil, err
    }
    tc.SetKeepAlive(true) //发送心跳
    tc.SetKeepAlivePeriod(3 * time.Minute) //设置发送周期
    return tc, nil
}

Accept()函数首先调用TCPListener对象的AcceptTCP()函数获取一个TCP连接对象tc，然后tc调用SetKeepAlive(true)，让操作系统为收到的每一个连接启动发送keepalive消息(心跳，为了保持连接不断开)。

func (srv *Server) Serve(l net.Listener) error函数处理接收到的客户端的请求信息。这个函数里有一个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。

// Serve accepts incoming connections on the Listener l, creating a
// new service goroutine for each. The service goroutines read requests and
// then call srv.Handler to reply to them.
//
// HTTP/2 support is only enabled if the Listener returns *tls.Conn
// connections and they were configured with "h2" in the TLS
// Config.NextProtos.
//
// Serve always returns a non-nil error and closes l.
// After Shutdown or Close, the returned error is ErrServerClosed.
func (srv *Server) Serve(l net.Listener) error {
    if fn := testHookServerServe; fn != nil {
        fn(srv, l) // call hook with unwrapped listener
    }

    l = &onceCloseListener{Listener: l}
    defer l.Close()

    if err := srv.setupHTTP2_Serve(); err != nil {
        return err
    }

    if !srv.trackListener(&l, true) {
        return ErrServerClosed
    }
    defer srv.trackListener(&l, false)

    var tempDelay time.Duration     // how long to sleep on accept failure
    baseCtx := context.Background() // base is always background, per Issue 16220
    ctx := context.WithValue(baseCtx, ServerContextKey, srv) //新建一个context来管理每个连接conn的Go程
    for {
        rw, e := l.Accept() //调用tcpKeepAliveListener对象的 Accept() 方法
        if e != nil {
            select {
            case <-srv.getDoneChan():
                return ErrServerClosed //退出Serve方法，并执行延迟调用（从缓存中删除当前监听器）
            default:
            }
            //如果发生了net.Error错误，则隔一段时间就重试一次，间隔时间每次翻倍，最大为1秒
            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) //该方法根据net.Conn、srv构造了一个新的http.conn类型
        c.setState(c.rwc, StateNew) // before Serve can return
        go c.serve(ctx)
    }
}

func (srv *Server) newConn(rwc net.Conn) *conn创建一个conn对象，如果debugServerConnections为真，则通过newLoggingConn将rwc中的loggingConn的name信息包装一下，添加一部分信息。

// debugServerConnections controls whether all server connections are wrapped
// with a verbose logging wrapper.
const debugServerConnections = false

// Create new connection from rwc.
func (srv *Server) newConn(rwc net.Conn) *conn {
    c := &conn{
        server: srv,
        rwc:    rwc,
    }
    if debugServerConnections {
        c.rwc = newLoggingConn("server", c.rwc)
    }
    return c
}

func (c *conn) setState(nc net.Conn, state ConnState)通过传入一个连接和状态，根据状态的值改变服务器中该连接的追踪情况。

func (c *conn) setState(nc net.Conn, state ConnState) {
    srv := c.server
    switch state {
    case StateNew:
        srv.trackConn(c, true)
    case StateHijacked, StateClosed:
        srv.trackConn(c, false)
    }
    if state > 0xff || state < 0 {
        panic("internal error")
    }
    packedState := uint64(time.Now().Unix()<<8) | uint64(state)
    atomic.StoreUint64(&c.curState.atomic, packedState)
    if hook := srv.ConnState; hook != nil {
        hook(nc, state)
    }
}

c.serve(ctx)调用func (c *conn) serve(ctx context.Context)读取请求，然后根据conn内保存的server来构造一个serverHandler类型，并调用它的ServeHTTP()方法：serverHandler{c.server}.ServeHTTP(w, w.req)。ServeHTTP路由器接收到请求之后进行判断，如果是*那么关闭链接，不然调用mux.Handler(r)返回对应设置路由的处理Handler，然后执行h.ServeHTTP(w, r)。
```
// ServeHTTP dispatches the request to the handler whose
// pattern most closely matches the request URL.
func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request) {
    if r.RequestURI == "*" {
        if r.ProtoAtLeast(1, 1) {
            w.Header().Set("Connection", "close")
        }
        w.WriteHeader(StatusBadRequest)
        return
    }
    h, _ := mux.Handler(r)
    h.ServeHTTP(w, r)
}
```

h.ServeHTTP(w, r)调用对应路由的handler的ServerHTTP接口,handler的ServerHTTP接口根据用户请求的URL和路由器里面存储的map去匹配的，当匹配到之后返回存储的handler，调用这个handler的ServeHTTP接口就可以执行到相应的函数。

func (mux *ServeMux) Handler(r *Request) (h Handler, pattern string) {
    if r.Method != "CONNECT" {
        if p := cleanPath(r.URL.Path); p != r.URL.Path {
            _, pattern = mux.handler(r.Host, p)
            return RedirectHandler(p, StatusMovedPermanently), pattern
        }
    }    
    return mux.handler(r.Host, r.URL.Path)
}

func (mux *ServeMux) handler(host, path string) (h Handler, pattern string) {
    mux.mu.RLock()
    defer mux.mu.RUnlock()

    // Host-specific pattern takes precedence over generic ones
    if mux.hosts {
        h, pattern = mux.match(host + path)
    }
    if h == nil {
        h, pattern = mux.match(path)
    }
    if h == nil {
        h, pattern = NotFoundHandler(), ""
    }
    return
}

http包自带了几个创建常用处理器的函数：FileServer，NotFoundHandler、RedirectHandler、StripPrefix、TimeoutHandler。而RedirectHandler函数就是用来重定向的：它返回一个请求处理器，该处理器会对每个请求都使用状态码code重定向到网址url。

// RedirectHandler returns a request handler that redirects
// each request it receives to the given url using the given
// status code.
//
// The provided code should be in the 3xx range and is usually
// StatusMovedPermanently, StatusFound or StatusSeeOther.
func RedirectHandler(url string, code int) Handler {
    return &redirectHandler{url, code}
}

GO Http执行流程

首先调用Http.HandleFunc
按顺序做了几件事：

1 调用了DefaultServeMux的HandleFunc

2 调用了DefaultServeMux的Handle

3 往DefaultServeMux的map[string]muxEntry中增加对应的handler和路由规则
其次调用http.ListenAndServe(":9090", nil) - nil使用默认路由器

按顺序做了几件事情：

1 实例化Server

2 调用Server的ListenAndServe()

3 调用net.Listen("tcp", addr)监听端口

4 启动一个for循环，在循环体中Accept请求

5 对每个请求实例化一个Conn，并且开启一个goroutine为这个请求进行服务go c.serve()

6 读取每个请求的内容w, err := c.readRequest()

7 判断handler是否为空，如果没有设置handler（这个例子就没有设置handler），handler就设置为DefaultServeMux

8 调用handler的ServeHttp

9 在这个例子中，下面就进入到DefaultServeMux.ServeHttp

10 根据request选择handler，并且进入到这个handler的ServeHTTP
```
mux.handler(r).ServeHTTP(w, r)
```
11 选择handler：

A 判断是否有路由能满足这个request（循环遍历ServeMux的muxEntry）

B 如果有路由满足，调用这个路由handler的ServeHTTP

C 如果没有路由满足，调用NotFoundHandler的ServeHTTP

GO-net/http源码阅读