postgresql: Enable systemd integration for 9.6+

This allows, finally, proper detection when postgresql is ready to
accept connections. Until now, it was possible that services depending
on postgresql would fail in a race condition trying to connect
to postgresql.

nixos/modules/services/databases/postgresql.nix

@@ -238,6 +238,9 @@ in
             User = "postgres";
             Group = "postgres";
             PermissionsStartOnly = true;
+            Type = if lib.versionAtLeast cfg.package.version "9.6"
+                   then "notify"
+                   else "simple";
 
             # Shut down Postgres using SIGINT ("Fast Shutdown mode").  See
             # http://www.postgresql.org/docs/current/static/server-shutdown.html

pkgs/servers/sql/postgresql/default.nix

@@ -1,10 +1,11 @@
-{ lib, stdenv, glibc, fetchurl, zlib, readline, libossp_uuid, openssl, libxml2, makeWrapper, tzdata }:
+{ lib, stdenv, glibc, fetchurl, zlib, readline, libossp_uuid, openssl, libxml2, makeWrapper, tzdata, systemd }:
 
 let
 
   common = { version, sha256, psqlSchema }:
    let atLeast = lib.versionAtLeast version; in stdenv.mkDerivation (rec {
     name = "postgresql-${version}";
+    inherit version;
 
     src = fetchurl {
       url = "mirror://postgresql/source/v${version}/${name}.tar.bz2";
@@ -15,7 +16,7 @@ let
     setOutputFlags = false; # $out retains configureFlags :-/
 
     buildInputs =
-      [ zlib readline openssl libxml2 makeWrapper ]
+      [ zlib readline openssl libxml2 makeWrapper systemd ]
       ++ lib.optionals (!stdenv.isDarwin) [ libossp_uuid ];
 
     enableParallelBuilding = true;
@@ -33,6 +34,7 @@ let
       "--sysconfdir=/etc"
       "--libdir=$(lib)/lib"
       "--with-system-tzdata=${tzdata}/share/zoneinfo"
+      (lib.optionalString (atLeast "9.6" && !stdenv.isDarwin) "--with-systemd")
       (if stdenv.isDarwin then "--with-uuid=e2fs" else "--with-ossp-uuid")
     ];