caddy/listeners.go

package caddy

import (
	"fmt"
	"net"
	"sync"
	"sync/atomic"
	"time"
)

// Listen returns a listener suitable for use in a Caddy module.
// Always be sure to close listeners when you are done with them.
func Listen(network, addr string) (net.Listener, error) {
	lnKey := network + "/" + addr

	listenersMu.Lock()
	defer listenersMu.Unlock()

	// if listener already exists, increment usage counter, then return listener
	if lnUsage, ok := listeners[lnKey]; ok {
		atomic.AddInt32(&lnUsage.usage, 1)
		return &fakeCloseListener{usage: &lnUsage.usage, key: lnKey, Listener: lnUsage.ln}, nil
	}

	// or, create new one and save it
	ln, err := net.Listen(network, addr)
	if err != nil {
		return nil, err
	}

	// make sure to start its usage counter at 1
	lnUsage := &listenerUsage{usage: 1, ln: ln}
	listeners[lnKey] = lnUsage

	return &fakeCloseListener{usage: &lnUsage.usage, key: lnKey, Listener: ln}, nil
}

// fakeCloseListener's Close() method is a no-op. This allows
// stopping servers that are using the listener without giving
// up the socket; thus, servers become hot-swappable while the
// listener remains running. Listeners should be re-wrapped in
// a new fakeCloseListener each time the listener is reused.
type fakeCloseListener struct {
	closed int32  // accessed atomically
	usage  *int32 // accessed atomically
	key    string
	net.Listener
}

// Accept accepts connections until Close() is called.
func (fcl *fakeCloseListener) Accept() (net.Conn, error) {
	// if the listener is already "closed", return error
	if atomic.LoadInt32(&fcl.closed) == 1 {
		return nil, fcl.fakeClosedErr()
	}

	// wrap underlying accept
	conn, err := fcl.Listener.Accept()
	if err == nil {
		return conn, nil
	}

	if atomic.LoadInt32(&fcl.closed) == 1 {
		// clear the deadline
		switch ln := fcl.Listener.(type) {
		case *net.TCPListener:
			ln.SetDeadline(time.Time{})
		case *net.UnixListener:
			ln.SetDeadline(time.Time{})
		}

		// if we cancelled the Accept() by setting a deadline
		// on the listener, we need to make sure any callers of
		// Accept() think the listener was actually closed;
		// if we return the timeout error instead, callers might
		// simply retry, leaking goroutines for longer
		if netErr, ok := err.(net.Error); ok && netErr.Timeout() {
			return nil, fcl.fakeClosedErr()
		}
	}

	return nil, err
}

// Close stops accepting new connections without
// closing the underlying listener, unless no one
// else is using it.
func (fcl *fakeCloseListener) Close() error {
	if atomic.CompareAndSwapInt32(&fcl.closed, 0, 1) {
		// unfortunately, there is no way to cancel any
		// currently-blocking calls to Accept() that are
		// awaiting connections since we're not actually
		// closing the listener; so we cheat by setting
		// a deadline in the past, which forces it to
		// time out; note that this only works for
		// certain types of listeners...
		switch ln := fcl.Listener.(type) {
		case *net.TCPListener:
			ln.SetDeadline(time.Now().Add(-1 * time.Minute))
		case *net.UnixListener:
			ln.SetDeadline(time.Now().Add(-1 * time.Minute))
		}

		// since we're no longer using this listener,
		// decrement the usage counter and, if no one
		// else is using it, close underlying listener
		if atomic.AddInt32(fcl.usage, -1) == 0 {
			listenersMu.Lock()
			delete(listeners, fcl.key)
			listenersMu.Unlock()
			err := fcl.Listener.Close()
			if err != nil {
				return err
			}
		}

	}

	return nil
}

func (fcl *fakeCloseListener) fakeClosedErr() error {
	return &net.OpError{
		Op:   "accept",
		Net:  fcl.Listener.Addr().Network(),
		Addr: fcl.Listener.Addr(),
		Err:  errFakeClosed,
	}
}

// ErrFakeClosed is the underlying error value returned by
// fakeCloseListener.Accept() after Close() has been called,
// indicating that it is pretending to be closed so that the
// server using it can terminate, while the underlying
// socket is actually left open.
var errFakeClosed = fmt.Errorf("listener 'closed' 😉")

// listenerUsage pairs a net.Listener with a
// count of how many servers are using it.
type listenerUsage struct {
	usage int32 // accessed atomically
	ln    net.Listener
}

var (
	listeners   = make(map[string]*listenerUsage)
	listenersMu sync.Mutex
)