core: Reuse unix sockets (UDS) and don't try to serve HTTP/3 over UDS (#5063)

* core: Reuse unix sockets

* Don't serve HTTP/3 over unix sockets

This requires upstream support, if even useful

* Don't use unix build tag... yet

* Fix build tag

* Allow ErrNotExist when unlinking socket

4 files changed, 134 insertions, 36 deletions

diff --git a/listen.go b/listen.go
index 2c4a0b2..268785a 100644
--- a/listen.go
+++ b/listen.go
@@ -1,4 +1,21 @@
-//go:build !linux
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// 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)
 
 package caddy
 
diff --git a/listen_linux.go b/listen_linux.go
deleted file mode 100644
index b1220ce..0000000
--- a/listen_linux.go
+++ /dev/null
@@ -1,34 +0,0 @@
-package caddy
-
-import (
-	"context"
-	"net"
-	"syscall"
-	"time"
-
-	"go.uber.org/zap"
-	"golang.org/x/sys/unix"
-)
-
-// ListenTimeout is the same as Listen, but with a configurable keep-alive timeout duration.
-func ListenTimeout(network, addr string, keepalivePeriod time.Duration) (net.Listener, error) {
-	// check to see if plugin provides listener
-	if ln, err := getListenerFromPlugin(network, addr); err != nil || ln != nil {
-		return ln, err
-	}
-
-	config := &net.ListenConfig{Control: reusePort, KeepAlive: keepalivePeriod}
-	return config.Listen(context.Background(), network, addr)
-}
-
-func reusePort(network, address string, conn syscall.RawConn) error {
-	return conn.Control(func(descriptor uintptr) {
-		if err := unix.SetsockoptInt(int(descriptor), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1); err != nil {
-			Log().Error("setting SO_REUSEPORT",
-				zap.String("network", network),
-				zap.String("address", address),
-				zap.Uintptr("descriptor", descriptor),
-				zap.Error(err))
-		}
-	})
-}
diff --git a/listen_unix.go b/listen_unix.go
new file mode 100644
index 0000000..f7b6279
--- /dev/null
+++ b/listen_unix.go
@@ -0,0 +1,115 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// 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
+
+package caddy
+
+import (
+	"context"
+	"errors"
+	"io/fs"
+	"net"
+	"sync"
+	"syscall"
+	"time"
+
+	"go.uber.org/zap"
+	"golang.org/x/sys/unix"
+)
+
+// ListenTimeout is the same as Listen, but with a configurable keep-alive timeout duration.
+func ListenTimeout(network, addr string, keepalivePeriod time.Duration) (net.Listener, error) {
+	// check to see if plugin provides listener
+	if ln, err := getListenerFromPlugin(network, addr); err != nil || ln != nil {
+		return ln, err
+	}
+
+	socketKey := listenerKey(network, addr)
+	if isUnixNetwork(network) {
+		unixSocketsMu.Lock()
+		defer unixSocketsMu.Unlock()
+
+		socket, exists := unixSockets[socketKey]
+		if exists {
+			// make copy of file descriptor
+			socketFile, err := socket.File() // dup() deep down
+			if err != nil {
+				return nil, err
+			}
+
+			// use copy to make new listener
+			ln, err := net.FileListener(socketFile)
+			if err != nil {
+				return nil, err
+			}
+
+			// the old socket fd will likely be closed soon, so replace it in the map
+			unixSockets[socketKey] = ln.(*net.UnixListener)
+
+			return ln.(*net.UnixListener), nil
+		}
+
+		// from what I can tell after some quick research, it's quite common for programs to
+		// leave their socket file behind after they close, so the typical pattern is to
+		// unlink it before you bind to it -- this is often crucial if the last program using
+		// it was killed forcefully without a chance to clean up the socket, but there is a
+		// race, as the comment in net.UnixListener.close() explains... oh well?
+		if err := syscall.Unlink(addr); err != nil && !errors.Is(err, fs.ErrNotExist) {
+			return nil, err
+		}
+	}
+
+	config := &net.ListenConfig{Control: reusePort, KeepAlive: keepalivePeriod}
+
+	ln, err := config.Listen(context.Background(), network, addr)
+	if err != nil {
+		return nil, err
+	}
+
+	if uln, ok := ln.(*net.UnixListener); ok {
+		// TODO: ideally, we should unlink the socket once we know we're done using it
+		// (i.e. either on exit or a new config that doesn't use this socket; in UsagePool
+		// terms, when the reference count reaches 0), but given that we unlink existing
+		// socket before we create the new one anyway (see above), we don't necessarily
+		// need to clean up after ourselves; still, doing so would probably be more tidy
+		uln.SetUnlinkOnClose(false)
+		unixSockets[socketKey] = uln
+	}
+
+	return ln, nil
+}
+
+// reusePort sets SO_REUSEPORT. Ineffective for unix sockets.
+func reusePort(network, address string, conn syscall.RawConn) error {
+	return conn.Control(func(descriptor uintptr) {
+		if err := unix.SetsockoptInt(int(descriptor), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1); err != nil {
+			Log().Error("setting SO_REUSEPORT",
+				zap.String("network", network),
+				zap.String("address", address),
+				zap.Uintptr("descriptor", descriptor),
+				zap.Error(err))
+		}
+	})
+}
+
+// unixSockets keeps track of the currently-active unix sockets
+// so we can transfer their FDs gracefully during reloads.
+var (
+	unixSockets   = make(map[string]*net.UnixListener)
+	unixSocketsMu sync.Mutex
+)
diff --git a/modules/caddyhttp/app.go b/modules/caddyhttp/app.go
index 84b0b94..c9a5543 100644
--- a/modules/caddyhttp/app.go
+++ b/modules/caddyhttp/app.go
@@ -409,7 +409,7 @@ func (app *App) Start() error {
 					ln = tls.NewListener(ln, tlsCfg)
 
 					// enable HTTP/3 if configured
-					if srv.protocol("h3") {
+					if srv.protocol("h3") && !listenAddr.IsUnixNetwork() {
 						app.logger.Info("enabling HTTP/3 listener", zap.String("addr", hostport))
 						if err := srv.serveHTTP3(hostport, tlsCfg); err != nil {
 							return err