add "waitup", s6 readiness helper for network interfaces

run e.g. "waitup wlan0 10" to wait until wlan0 is operationally
up and running, and then send a newline to file descriptor 10

overlay.nix

@@ -86,6 +86,9 @@ final: prev: {
     };
   };
 
+  netlink-lua = final.callPackage ./pkgs/netlink-lua {};
+  waitup = final.callPackage ./pkgs/waitup {};
+
   # these are packages for the build system not the host/target
 
   tufted = final.callPackage ./pkgs/tufted {};

pkgs/netlink-lua/default.nix

@@ -0,0 +1,23 @@
+{ lua, lib, fetchpatch, fetchFromGitHub, libmnl }:
+let pname = "netlink";
+in lua.pkgs.buildLuaPackage {
+  inherit pname;
+  version = "0.1.1-1";
+
+  buildInputs = [ libmnl ];
+
+  src = fetchFromGitHub {
+    repo = "lua-netlink";
+    owner = "chris2511";
+    rev = "v0.1.1";
+    hash = "sha256:1833naskl4p7rz5kk0byfgngvw1mvf6cnz64sr3ny7i202wv7s52";
+  };
+
+  buildPhase = "$CC -shared -l mnl -l lua -o netlink.so src/*.c";
+
+  installPhase = ''
+    mkdir -p "$out/lib/lua/${lua.luaversion}"
+    cp  netlink.so "$out/lib/lua/${lua.luaversion}/"
+  '';
+
+}

pkgs/waitup/default.nix

@@ -0,0 +1,31 @@
+{
+  lua5_3
+, netlink-lua
+, stdenv
+, makeWrapper
+}:
+let
+  lua = lua5_3;
+  netlink = netlink-lua.override {inherit lua;};
+  fennel = lua.pkgs.fennel;
+in stdenv.mkDerivation rec {
+  pname = "waitup";
+  version = "1";
+
+  buildInputs = [ lua netlink-lua ];
+  nativeBuildInputs = [ makeWrapper  fennel ];
+
+  src = ./.;
+
+  installPhase = ''
+    mkdir -p $out/bin $out/lib
+    fennel --compile  ${./waitup.fnl} > $out/lib/waitup.lua
+
+    makeWrapper ${lua}/bin/lua $out/bin/${pname} \
+      --prefix LUA_CPATH ";" ${netlink}/lib/lua/${lua.luaversion}/\?.so \
+      --add-flags $out/lib/waitup.lua
+  '';
+}
+
+# to use fennel.view,
+#  --prefix LUA_PATH ";" ${fennel}/share/lua/5.2/\?.lua \

pkgs/waitup/waitup.fnl

@@ -0,0 +1,36 @@
+(local netlink (require :netlink))
+(local sock (netlink.socket))
+
+(when  (< (# arg) 2)
+  (print "usage: waitup ifname fd")
+  (os.exit 1))
+
+(local ifname (. arg 1))
+(local fd (tonumber (. arg 2)))
+(local stream (io.open (.. "/proc/self/fd/" fd) "w"))
+
+(fn notify-ready []
+  (stream:write "\n")
+  (print (.. (. arg 0) ": received netlink operstate up for " ifname))
+  (stream:close))
+
+(fn run-events [evs]
+  (each [_ v (ipairs evs)]
+    (print :event v.event v.name)
+
+    (match v
+      ;; - up: Reflects the administrative state of the interface (IFF_UP)
+      ;; - running: Reflects the operational state (IFF_RUNNING).
+      {:event "newlink" :name ifname :up :yes :running :yes}
+      (notify-ready)
+
+      {:event "newlink" :name ifname :up :no}
+      (os.exit 0))))
+
+(run-events (sock:query {:link true}))
+
+(print (.. (. arg 0) ": waiting for netlink NEWLINK " ifname))
+
+(while (sock:poll)
+  (let [ev (sock:event)]
+    (run-events ev)))