How to limit the connections count of an HTTP Server implemented in Go?

You can use the netutil.LimitListener function to wrap around net.Listener if you don't want to implement your own wrapper:-

connectionCount := 20

l, err := net.Listen("tcp", ":8000")

if err != nil {
    log.Fatalf("Listen: %v", err)
}

defer l.Close()

l = netutil.LimitListener(l, connectionCount)

log.Fatal(http.Serve(l, nil))

The trick with this is to implement your own net.Listener. I have an example of a listener here (see waitConn and WaitListener) that tracks connections (but doesn't limit them), which you can use as the inspiration for an implementation. It will be shaped something like this:

type LimitedListener struct {
    sync.Mutex
    net.Listener
    sem chan bool
}

func NewLimitedListener(count int, l net.Listener) *net.LimitedListener {
    sem := make(chan bool, count)
    for i := 0; i < count; i++ {
        sem <- true
    }
    return &net.LimitedListener{
        Listener: l,
        sem:      sem,
    }
}

func (l *LimitedListener) Addr() net.Addr { /* ... */ }
func (l *LimitedListener) Close() error { /* ... */ }
func (l *LimitedListener) Accept() (net.Conn, err) {
    <-l.sem // acquire
    // l.Listener.Accept (on error, release before returning)
    // wrap LimitedConn
    return c, nil
}

type LimitedConn struct { /* ... */ }

func (c *LimitedConn) Close() error {
    /* ... */
    c.sem <- true // release
}

Essentially what this is doing is creating your own implementation of net.Listener that you can give to Serve that only calls the underlying Accept when it can acquire the semaphore; the semaphore so acquired is only released when the (suitably wrapped) net.Conn is Closed. Note that technically this use of the semaphore is correct with respect to the go1.2 memory model; a simpler semaphore will be legal in future versions of Go.