Add support for systemd socket activation (#704)

* feat: add support for systemd socket activation

If webhook has been launched via systemd socket activation, simply use the systemd-provided socket rather than opening our own.

* docs: documentation for the systemd socket activation mode

* refactor: moved setuid and setgid flags into platform-specific section

The setuid and setgid flags do not work on Windows, so moved them to platform_unix so they are only added to the flag set on compatible platforms.

Also disallow the use of setuid and setgid in combination with -socket, since a setuid webhook process would not be able to clean up a socket that was created while running as root.  If you _need_ to have the socket owned by root but the webhook process running as a normal user, you can achieve the same effect with systemd socket activation.

README.md

@@ -117,6 +117,9 @@ Note that when running in this mode the [`ip-whitelist`](docs/Hook-Rules.md#matc
 ## CORS Headers
 If you want to set CORS headers, you can use the `-header name=value` flag while starting [webhook][w] to set the appropriate CORS headers that will be returned with each response.
 
+## Running under `systemd`
+On platforms that use [systemd](https://systemd.io), [webhook][w] supports the _socket activation_ mechanism.  If [webhook][w] detects that it has been launched from a systemd-managed socket it will automatically use that instead of opening its own listening port.  See [the systemd page](docs/Systemd-Activation.md) for full details.
+
 ## Interested in running webhook inside of a Docker container?
 You can use one of the following Docker images, or create your own (please read [this discussion](https://github.com/adnanh/webhook/issues/63)):
 - [almir/webhook](https://github.com/almir/docker-webhook)

docs/Systemd-Activation.md

@@ -0,0 +1,61 @@
+# Using systemd socket activation
+
+_New in v2.9.0_
+
+On platforms that use [systemd](https://systemd.io), [webhook][w] 
+supports the _socket activation_ mechanism.  In this mode, systemd itself is responsible for managing the listening socket, and it launches [webhook][w] the first time it receives a request on the socket.  This has a number of advantages over the standard mode:
+
+- [webhook][w] can run as a normal user while still being able to use a port number like 80 or 443 that would normally require root privilege
+- if the [webhook][w] process dies and is restarted, pending connections are not dropped - they just keep waiting until the restarted [webhook][w] is ready
+
+No special configuration is necessary to tell [webhook][w] that socket activation is being used - socket activation sets specific environment variables when launching the activated service, if [webhook][w] detects these variables it will ignore the `-port` and `-socket` options and simply use the systemd-provided socket instead of opening its own.
+
+## Configuration
+To run [webhook][w] with socket activation you need to create _two_ separate unit files in your systemd configuration directory (typically `/etc/systemd/system`), one for the socket and one for the service.  They must have matching names; in this example we use `webhook.socket` and `webhook.service`.  At their simplest, these files should look like:
+
+**webhook.socket**
+```
+[Unit]
+Description=Webhook server socket
+
+[Socket]
+# Listen on all network interfaces, port 9000
+ListenStream=9000
+
+# Alternatives:
+
+## Listen on one specific interface only
+# ListenStream=10.0.0.1:9000
+# FreeBind=true
+
+## Listen on a Unix domain socket
+# ListenStream=/tmp/webhook.sock
+
+[Install]
+WantedBy=multi-user.target
+```
+
+**webhook.service**
+```
+[Unit]
+Description=Webhook server
+
+[Service]
+Type=exec
+ExecStart=webhook -nopanic -hooks /etc/webhook/hooks.yml
+
+# Which user should the webhooks run as?
+User=nobody
+Group=nogroup
+```
+
+You should enable and start the _socket_, but it is not necessary to enable the _service_ - this will be started automatically when the socket receives its first request.
+
+```sh
+sudo systemctl enable webhook.socket
+sudo systemctl start webhook.socket
+```
+
+Systemd unit files support many other options, see the [systemd.socket](https://www.freedesktop.org/software/systemd/man/latest/systemd.socket.html) and [systemd.service](https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html) manual pages for full details.
+
+[w]: https://github.com/adnanh/webhook

go.mod

@@ -7,6 +7,7 @@ toolchain go1.22.0
 require (
 	github.com/Microsoft/go-winio v0.6.2
 	github.com/clbanning/mxj/v2 v2.7.0
+	github.com/coreos/go-systemd/v22 v22.5.0
 	github.com/dustin/go-humanize v1.0.1
 	github.com/fsnotify/fsnotify v1.7.0
 	github.com/ghodss/yaml v1.0.0

go.sum

@@ -2,6 +2,8 @@ github.com/Microsoft/go-winio v0.6.2 h1:F2VQgta7ecxGYO8k3ZZz3RS8fVIXVxONVUPlNERo
 github.com/Microsoft/go-winio v0.6.2/go.mod h1:yd8OoFMLzJbo9gZq8j5qaps8bJ9aShtEA8Ipt1oGCvU=
 github.com/clbanning/mxj/v2 v2.7.0 h1:WA/La7UGCanFe5NpHF0Q3DNtnCsVoxbPKuyBNHWRyME=
 github.com/clbanning/mxj/v2 v2.7.0/go.mod h1:hNiWqW14h+kc+MdF9C6/YoRfjEJoR3ou6tn/Qo+ve2s=
+github.com/coreos/go-systemd/v22 v22.5.0 h1:RrqgGjYQKalulkV8NGVIfkXQf6YYmOyiJKk8iXXhfZs=
+github.com/coreos/go-systemd/v22 v22.5.0/go.mod h1:Y58oyj3AT4RCenI/lSvhwexgC+NSVTIJ3seZv2GcEnc=
 github.com/dustin/go-humanize v1.0.1 h1:GzkhY7T5VNhEkwH0PVJgjz+fX1rhBrR7pRT3mDkpeCY=
 github.com/dustin/go-humanize v1.0.1/go.mod h1:Mu1zIs6XwVuF/gI1OepvI0qD18qycQx+mFykh5fBlto=
 github.com/fsnotify/fsnotify v1.7.0 h1:8JEhPFa5W2WU7YfeZzPNqzMP6Lwt7L2715Ggo0nosvA=
@@ -10,6 +12,7 @@ github.com/ghodss/yaml v1.0.0 h1:wQHKEahhL6wmXdzwWG11gIVCkOv05bNOh+Rxn0yngAk=
 github.com/ghodss/yaml v1.0.0/go.mod h1:4dBDuWmgqj2HViK6kFavaiC9ZROes6MMH2rRYeMEF04=
 github.com/go-chi/chi/v5 v5.0.12 h1:9euLV5sTrTNTRUU9POmDUvfxyj6LAABLUcEWO+JJb4s=
 github.com/go-chi/chi/v5 v5.0.12/go.mod h1:DslCQbL2OYiznFReuXYUmQ2hGd1aDpCnlMNITLSKoi8=
+github.com/godbus/dbus/v5 v5.0.4/go.mod h1:xhWf0FNVPg57R7Z0UbKHbJfkEywrmjJnf7w5xrFpKfA=
 github.com/gofrs/uuid/v5 v5.0.0 h1:p544++a97kEL+svbcFbCQVM9KFu0Yo25UoISXGNNH9M=
 github.com/gofrs/uuid/v5 v5.0.0/go.mod h1:CDOjlDMVAtN56jqyRUZh58JT31Tiw7/oQyEXZV+9bD8=
 github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=

platform_unix.go

@@ -6,15 +6,40 @@ package main
 import (
 	"flag"
 	"fmt"
+	"github.com/coreos/go-systemd/v22/activation"
 	"net"
 )
 
 func platformFlags() {
 	flag.StringVar(&socket, "socket", "", "path to a Unix socket (e.g. /tmp/webhook.sock) to use instead of listening on an ip and port; if specified, the ip and port options are ignored")
+	flag.IntVar(&setGID, "setgid", 0, "set group ID after opening listening port; must be used with setuid, not permitted with -socket")
+	flag.IntVar(&setUID, "setuid", 0, "set user ID after opening listening port; must be used with setgid, not permitted with -socket")
 }
 
 func trySocketListener() (net.Listener, error) {
+	// first check whether we have any sockets from systemd
+	listeners, err := activation.Listeners()
+	if err != nil {
+		return nil, fmt.Errorf("failed to retrieve sockets from systemd: %w", err)
+	}
+	numListeners := len(listeners)
+	if numListeners > 1 {
+		return nil, fmt.Errorf("received %d sockets from systemd, but only 1 is supported", numListeners)
+	}
+	if numListeners == 1 {
+		sockAddr := listeners[0].Addr()
+		if sockAddr.Network() == "tcp" {
+			addr = sockAddr.String()
+		} else {
+			addr = fmt.Sprintf("{%s:%s}", sockAddr.Network(), sockAddr.String())
+		}
+		return listeners[0], nil
+	}
+	// if we get to here, we got no sockets from systemd, so check -socket flag
 	if socket != "" {
+		if setGID != 0 || setUID != 0 {
+			return nil, fmt.Errorf("-setuid and -setgid options are not compatible with -socket.  If you need to bind a socket as root but run webhook as a different user, consider using systemd activation")
+		}
 		addr = fmt.Sprintf("{unix:%s}", socket)
 		return net.Listen("unix", socket)
 	}