core: Eliminate unnecessary shutdown delay on Unix (#5413)

* core: Eliminate unnecessary shutdown delay on Unix

Fix #5393, alternate to #5405

* Comments, cleanup, adjust logs

* Fix build constraint

4 files changed, 36 insertions, 13 deletions

diff --git a/go.mod b/go.mod
index 4a5dc86..1b6dbb9 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,6 @@
 module github.com/caddyserver/caddy/v2
 
-go 1.18
+go 1.19
 
 require (
 	github.com/BurntSushi/toml v1.2.1
diff --git a/listen.go b/listen.go
index 1ca1687..e0d67c6 100644
--- a/listen.go
+++ b/listen.go
@@ -12,10 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// TODO: Go 1.19 introduced the "unix" build tag. We have to support Go 1.18 until Go 1.20 is released.
-// When Go 1.19 is our minimum, change this build tag to simply "!unix".
-// (see similar change needed in listen_unix.go)
-//go:build !(aix || android || darwin || dragonfly || freebsd || hurd || illumos || ios || linux || netbsd || openbsd || solaris)
+//go:build !unix
 
 package caddy
 
diff --git a/listen_unix.go b/listen_unix.go
index 7ea6745..50d9a66 100644
--- a/listen_unix.go
+++ b/listen_unix.go
@@ -12,10 +12,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// TODO: Go 1.19 introduced the "unix" build tag. We have to support Go 1.18 until Go 1.20 is released.
-// When Go 1.19 is our minimum, remove this build tag, since "_unix" in the filename will do this.
-// (see also change needed in listen.go)
-//go:build aix || android || darwin || dragonfly || freebsd || hurd || illumos || ios || linux || netbsd || openbsd || solaris
+// Even though the filename ends in _unix.go, we still have to specify the
+// build constraint here, because the filename convention only works for
+// literal GOOS values, and "unix" is a shortcut unique to build tags.
+//go:build unix
 
 package caddy
 
@@ -98,7 +98,19 @@ func listenTCPOrUnix(ctx context.Context, lnKey string, network, address string,
 		}
 		return reusePort(network, address, c)
 	}
-	return config.Listen(ctx, network, address)
+
+	// even though SO_REUSEPORT lets us bind the socket multiple times,
+	// we still put it in the listenerPool so we can count how many
+	// configs are using this socket; necessary to ensure we can know
+	// whether to enforce shutdown delays, for example (see #5393).
+	ln, err := config.Listen(ctx, network, address)
+	if err == nil {
+		listenerPool.LoadOrStore(lnKey, nil)
+	}
+
+	// lightly wrap the listener so that when it is closed,
+	// we can decrement the usage pool counter
+	return deleteListener{ln, lnKey}, err
 }
 
 // reusePort sets SO_REUSEPORT. Ineffective for unix sockets.
@@ -116,3 +128,17 @@ func reusePort(network, address string, conn syscall.RawConn) error {
 		}
 	})
 }
+
+// deleteListener is a type that simply deletes itself
+// from the listenerPool when it closes. It is used
+// solely for the purpose of reference counting (i.e.
+// counting how many configs are using a given socket).
+type deleteListener struct {
+	net.Listener
+	lnKey string
+}
+
+func (dl deleteListener) Close() error {
+	_, _ = listenerPool.Delete(dl.lnKey)
+	return dl.Listener.Close()
+}
diff --git a/modules/caddyhttp/app.go b/modules/caddyhttp/app.go
index 36a4011..670185a 100644
--- a/modules/caddyhttp/app.go
+++ b/modules/caddyhttp/app.go
@@ -517,7 +517,7 @@ func (app *App) Stop() error {
 
 	// honor scheduled/delayed shutdown time
 	if delay {
-		app.logger.Debug("shutdown scheduled",
+		app.logger.Info("shutdown scheduled",
 			zap.Duration("delay_duration", time.Duration(app.ShutdownDelay)),
 			zap.Time("time", scheduledTime))
 		time.Sleep(time.Duration(app.ShutdownDelay))
@@ -528,9 +528,9 @@ func (app *App) Stop() error {
 		var cancel context.CancelFunc
 		ctx, cancel = context.WithTimeout(ctx, time.Duration(app.GracePeriod))
 		defer cancel()
-		app.logger.Debug("servers shutting down; grace period initiated", zap.Duration("duration", time.Duration(app.GracePeriod)))
+		app.logger.Info("servers shutting down; grace period initiated", zap.Duration("duration", time.Duration(app.GracePeriod)))
 	} else {
-		app.logger.Debug("servers shutting down with eternal grace period")
+		app.logger.Info("servers shutting down with eternal grace period")
 	}
 
 	// goroutines aren't guaranteed to be scheduled right away,