Support socket-based, on-demand activation of the Nix daemon with systemd

Systemd can start the Nix daemon on demand when the Nix daemon socket
is first accessed.  This is signalled through the LISTEN_FDS
environment variable, so all we need to do is check for that and then
use file descriptor 3 as the listen socket instead of creating one
ourselves.

src/nix-worker/nix-worker.cc

@@ -695,26 +695,39 @@ static void setSigChldAction(bool autoReap)
 }
 
 
+#define SD_LISTEN_FDS_START 3
+
+
 static void daemonLoop()
 {
     /* Get rid of children automatically; don't let them become
        zombies. */
     setSigChldAction(true);
 
+    AutoCloseFD fdSocket;
+
+    /* Handle socket-based activation by systemd. */
+    if (getEnv("LISTEN_FDS") != "") {
+        if (getEnv("LISTEN_PID") != int2String(getpid()) || getEnv("LISTEN_FDS") != "1")
+            throw Error("unexpected systemd environment variables");
+        fdSocket = SD_LISTEN_FDS_START;
+    }
+
+    /* Otherwise, create and bind to a Unix domain socket. */
+    else {
+    
         /* Create and bind to a Unix domain socket. */
-    AutoCloseFD fdSocket = socket(PF_UNIX, SOCK_STREAM, 0);
+        fdSocket = socket(PF_UNIX, SOCK_STREAM, 0);
         if (fdSocket == -1)
             throw SysError("cannot create Unix domain socket");
 
-    closeOnExec(fdSocket);
-    
         string socketPath = nixStateDir + DEFAULT_SOCKET_PATH;
 
         createDirs(dirOf(socketPath));
 
-    /* Urgh, sockaddr_un allows path names of only 108 characters.  So
-       chdir to the socket directory so that we can pass a relative
-       path name. */
+        /* Urgh, sockaddr_un allows path names of only 108 characters.
+           So chdir to the socket directory so that we can pass a
+           relative path name. */
         chdir(dirOf(socketPath).c_str());
         Path socketPathRel = "./" + baseNameOf(socketPath);
     
@@ -739,6 +752,9 @@ static void daemonLoop()
 
         if (listen(fdSocket, 5) == -1)
             throw SysError(format("cannot listen on socket `%1%'") % socketPath);
+    }
+
+    closeOnExec(fdSocket);
         
     /* Loop accepting connections. */
     while (1) {