From 59a5d0db28e36b9732bc9ad2b73bcde9188e207c Mon Sep 17 00:00:00 2001
From: Matthew Holt <mholt@users.noreply.github.com>
Date: Tue, 2 Apr 2019 15:31:02 -0600
Subject: [PATCH] Close listeners which are no longer used

---
 caddy.go     | 17 +++++++++++++++++
 listeners.go | 35 +++++++++++++++++++++++------------
 2 files changed, 40 insertions(+), 12 deletions(-)

diff --git a/caddy.go b/caddy.go
index b4af632e..b9d7022b 100644
--- a/caddy.go
+++ b/caddy.go
@@ -6,6 +6,7 @@ import (
 	"log"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
@@ -24,6 +25,7 @@ func Start(cfg Config) error {
 		cfg.runners[modName] = val.(Runner)
 	}
 
+	// start the new runners
 	for name, r := range cfg.runners {
 		err := r.Run()
 		if err != nil {
@@ -32,6 +34,7 @@ func Start(cfg Config) error {
 		}
 	}
 
+	// shut down down the old ones
 	currentCfgMu.Lock()
 	if currentCfg != nil {
 		for _, r := range currentCfg.runners {
@@ -44,6 +47,20 @@ func Start(cfg Config) error {
 	currentCfg = &cfg
 	currentCfgMu.Unlock()
 
+	// shut down listeners that are no longer being used
+	listenersMu.Lock()
+	for key, info := range listeners {
+		if atomic.LoadInt32(&info.usage) == 0 {
+			err := info.ln.Close()
+			if err != nil {
+				log.Printf("[ERROR] closing listener %s: %v", info.ln.Addr(), err)
+				continue
+			}
+			delete(listeners, key)
+		}
+	}
+	listenersMu.Unlock()
+
 	return nil
 }
 
diff --git a/listeners.go b/listeners.go
index 6849319b..7102e766 100644
--- a/listeners.go
+++ b/listeners.go
@@ -15,9 +15,10 @@ func Listen(network, addr string) (net.Listener, error) {
 	listenersMu.Lock()
 	defer listenersMu.Unlock()
 
-	// if listener already exists, return it
-	if ln, ok := listeners[lnKey]; ok {
-		return &fakeCloseListener{Listener: ln}, nil
+	// if listener already exists, increment usage counter, then return listener
+	if lnInfo, ok := listeners[lnKey]; ok {
+		atomic.AddInt32(&lnInfo.usage, 1)
+		return &fakeCloseListener{usage: &lnInfo.usage, Listener: lnInfo.ln}, nil
 	}
 
 	// or, create new one and save it
@@ -25,9 +26,12 @@ func Listen(network, addr string) (net.Listener, error) {
 	if err != nil {
 		return nil, err
 	}
-	listeners[lnKey] = ln
 
-	return &fakeCloseListener{Listener: ln}, nil
+	// make sure to start its usage counter at 1
+	lnInfo := &listenerUsage{usage: 1, ln: ln}
+	listeners[lnKey] = lnInfo
+
+	return &fakeCloseListener{usage: &lnInfo.usage, Listener: ln}, nil
 }
 
 // fakeCloseListener's Close() method is a no-op. This allows
@@ -36,7 +40,8 @@ func Listen(network, addr string) (net.Listener, error) {
 // listener remains running. Listeners should be re-wrapped in
 // a new fakeCloseListener each time the listener is reused.
 type fakeCloseListener struct {
-	closed int32
+	closed int32  // accessed atomically
+	usage  *int32 // accessed atomically
 	net.Listener
 }
 
@@ -92,16 +97,15 @@ func (fcl *fakeCloseListener) Close() error {
 		case *net.UnixListener:
 			ln.SetDeadline(time.Now().Add(-1 * time.Minute))
 		}
+
+		// since we're no longer using this listener,
+		// decrement the usage counter
+		atomic.AddInt32(fcl.usage, -1)
 	}
 
 	return nil
 }
 
-// CloseUnderlying actually closes the underlying listener.
-func (fcl *fakeCloseListener) CloseUnderlying() error {
-	return fcl.Listener.Close()
-}
-
 func (fcl *fakeCloseListener) fakeClosedErr() error {
 	return &net.OpError{
 		Op:   "accept",
@@ -118,7 +122,14 @@ func (fcl *fakeCloseListener) fakeClosedErr() error {
 // 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]net.Listener)
+	listeners   = make(map[string]*listenerUsage)
 	listenersMu sync.Mutex
 )