support systemd activation for api serve

systemd has a feature called socket activation where initially systemd
manages and listens on ports/uds and only invokes a service when traffic
appears. to then hand over the involved sockets, systemd will pass the
relevant FDs into the invoked process and defines them in the environment.

use coreos/go-systemd to grab the active listeners passed by systemd and
use them to serve the api routes. only one listener may be specified right
now as we also only support one -listen argument for the binary.

this allows admins to craft a systemd socket and service file for aptly
where systemd manages the socket, its permission and its live time, and
lazy start aptly when needed.

cmd/api_serve.go

@@ -8,6 +8,7 @@ import (
 	"os"
 
 	"github.com/smira/aptly/api"
+	"github.com/smira/aptly/systemd/activation"
 	"github.com/smira/aptly/utils"
 	"github.com/smira/commander"
 	"github.com/smira/flag"
@@ -34,8 +35,24 @@ func aptlyAPIServe(cmd *commander.Command, args []string) error {
 		return err
 	}
 
-	listen := context.Flags().Lookup("listen").Value.String()
+	// Try to recycle systemd fds for listening
+	listeners, err := activation.Listeners(true)
+	if len(listeners) > 1 {
+		panic("Got more than 1 listener from systemd. This is currently not supported!")
+	}
+	if err == nil && len(listeners) == 1 {
+		listener := listeners[0]
+		defer listener.Close()
+		fmt.Printf("\nTaking over web server at: %s (press Ctrl+C to quit)...\n", listener.Addr().String())
+		err = http.Serve(listener, api.Router(context))
+		if err != nil {
+			return fmt.Errorf("unable to serve: %s", err)
+		}
+		return nil
+	}
 
+	// If there are none: use the listen argument.
+	listen := context.Flags().Lookup("listen").Value.String()
 	fmt.Printf("\nStarting web server at: %s (press Ctrl+C to quit)...\n", listen)
 
 	listenURL, err := url.Parse(listen)
@@ -70,7 +87,8 @@ func makeCmdAPIServe() *commander.Command {
 		Long: `
 Start HTTP server with aptly REST API. The server can listen to either a port
 or Unix domain socket. When using a socket, Aptly will fully manage the socket
-file.
+file. This command also supports taking over from a systemd file descriptors to
+enable systemd socket activation.
 
 Example: