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Question

I am creating a simple web server using Go using net/HTTP package. The simple code is like this:

package main
import (
    "fmt"
    "net/http"
)
type hotdog int
func (m hotdog) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintln(w, "Any code you want in this func")
}
func main() {
    var d hotdog
    http.ListenAndServe(":8080", d)
}

So, I want to know what is happening under the hood? If I am not wrong

ListenAndServe() takes the address and handler and this function runs inside the library

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

and after this function calls the ListenAndServe

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)})
  }

Now the Serve Function is called. But I am not able to understand how this parameter (tcpKeepAliveListener{ln.(*net.TCPListener)}) is converted to (l net.Listener) in Serve Function

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)
    }
  }

Where is the handler called?

Answer 1

As they've already commented, the net.Listener type is an interface, and tcpKeepAliveListener satisfies that interface, so the type is correct.

Then, to answer your question about where is the Handler called, you need to go deeper in the code flow:

The line go c.serve(ctx) from the last piece of code you posted, is calling conn.serve():

// Serve a new connection.
func (c *conn) serve(ctx context.Context) {
    c.remoteAddr = c.rwc.RemoteAddr().String()
    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
%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 = "
Content-Type: text/plain; charset=utf-8
Connection: close

"

            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{})
    }
}

and there, it calls serverHandler{c.server}.ServeHTTP(w, w.req):

// serverHandler delegates to either the server's Handler or
// DefaultServeMux and also handles "OPTIONS *" requests.
type serverHandler struct {
    srv *Server
}

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)
}

which is the function wrapper that then calls ServeHTTP for the corresponding handler set before:

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)
}