feat: add support for systemd listener activation sockets

To support starting a network service on demand and to
support a "least privilege-approach" with regards to the
permission a network service process needs to have, systemd
supports opening a network socket on behalf of the service
and passing it as an open file descriptor.

The service gets notified about open file descriptors for
this purpose as well as metadata such as named listeners
via environment variables.

This patch adds support for prefixing the listen address
passed with --address with "sd-listen-fd:" to access these
file descriptors, taking either a listener name passed using
the `LISTEN_FDNAMES` environment variable or `LISTEN_FD_$n`
for unnamed file descriptiors where `n` is the id of the
descriptor starting at 3 (LISTEN_FD_3).

See sd_listen_fds(3)

cmd/root.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 	"syscall"
 
+	"github.com/coreos/go-systemd/v22/activation"
 	"github.com/hacdias/webdav/v5/lib"
 	"github.com/spf13/cobra"
 	"go.uber.org/zap"
@@ -115,7 +116,21 @@ func getListener(cfg *lib.Config) (net.Listener, error) {
 		network string
 	)
 
-	if strings.HasPrefix(cfg.Address, "unix:") {
+	if strings.HasPrefix(cfg.Address, "sd-listen-fd:") {
+		listeners, err := activation.ListenersWithNames()
+		if err != nil {
+			return nil, err
+		}
+
+		address := cfg.Address[13:]
+		listener, ok := listeners[address]
+
+		if !ok || len(listener) < 1 {
+			return nil, errors.New("unknown sd-listen-fd address '" + address + "'")
+		}
+
+		return listener[0], nil
+	} else if strings.HasPrefix(cfg.Address, "unix:") {
 		address = cfg.Address[5:]
 		network = "unix"
 	} else {

go.mod

@@ -3,6 +3,7 @@ module github.com/hacdias/webdav/v5
 go 1.23
 
 require (
+	github.com/coreos/go-systemd/v22 v22.3.2
 	github.com/go-viper/mapstructure/v2 v2.2.1
 	github.com/rs/cors v1.11.1
 	github.com/spf13/cobra v1.8.1

go.sum

@@ -1,3 +1,5 @@
+github.com/coreos/go-systemd/v22 v22.3.2 h1:D9/bQk5vlXQFZ6Kwuu6zaiXJ9oTPe68++AzAJc1DzSI=
+github.com/coreos/go-systemd/v22 v22.3.2/go.mod h1:Y58oyj3AT4RCenI/lSvhwexgC+NSVTIJ3seZv2GcEnc=
 github.com/cpuguy83/go-md2man/v2 v2.0.4/go.mod h1:tgQtvFlXSQOSOSIRvRPT7W67SCa46tRHOmNcaadrF8o=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc h1:U9qPSI2PIWSS1VwoXQT9A3Wy9MM3WgvqSxFWenqJduM=
@@ -8,6 +10,7 @@ github.com/fsnotify/fsnotify v1.8.0 h1:dAwr6QBTBZIkG8roQaJjGof0pp0EeF+tNV7YBP3F/
 github.com/fsnotify/fsnotify v1.8.0/go.mod h1:8jBTzvmWwFyi3Pb8djgCCO5IBqzKJ/Jwo8TRcHyHii0=
 github.com/go-viper/mapstructure/v2 v2.2.1 h1:ZAaOCxANMuZx5RCeg0mBdEZk7DZasvvZIxtHqx8aGss=
 github.com/go-viper/mapstructure/v2 v2.2.1/go.mod h1:oJDH3BJKyqBA2TXFhDsKDGDTlndYOZ6rGS0BRZIxGhM=
+github.com/godbus/dbus/v5 v5.0.4/go.mod h1:xhWf0FNVPg57R7Z0UbKHbJfkEywrmjJnf7w5xrFpKfA=
 github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
 github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/hashicorp/hcl v1.0.0 h1:0Anlzjpi4vEasTeNFn2mLJgTSwt0+6sfsiTG8qcWGx4=