feat(nix/emptyDerivation) add emptyDerivation

The most trivial of all derivations. It is more useful than it looks.

Can be used to bind nix expressions (e.g. test suites) to a
derivation, so that `nix-build` does not crap itself.

Change-Id: I61c24d8c129c9505733161207f3c30e820f5b15e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/665
Reviewed-by: tazjin <mail@tazj.in>

nix/emptyDerivation/emptyDerivation.nix

@@ -0,0 +1,34 @@
+{ stdenv, pkgs, getBins }:
+
+# The empty derivation. All it does is touch $out.
+# Basically the unit value for derivations.
+#
+# In addition to simple test situations which require
+# a derivation, we set __functor, so you can call it
+# as a function and pass an attrset. The set you pass
+# is `//`-merged with the attrset before calling derivation,
+# so you can use this to add more fields.
+
+let
+  bins = getBins pkgs.s6-portable-utils [ "s6-touch" ]
+      // getBins pkgs.execline [ "importas" "exec" ];
+
+  emptiness = {
+    name = "empty-derivation";
+
+    # TODO(Profpatsch): can we get system from tvl?
+    inherit (stdenv) system;
+
+    builder = bins.exec;
+    args = [
+      bins.importas "out" "out"
+      bins.s6-touch "$out"
+    ];
+  };
+
+in (derivation emptiness) // {
+  # This allows us to call the empty derivation
+  # like a function and override fields/add new fields.
+  __functor = _: overrides:
+    derivation (emptiness // overrides);
+}