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/OWNERS

@@ -0,0 +1,3 @@
+inherited: true
+owners:
+  - Profpatsch

nix/emptyDerivation/default.nix

@@ -0,0 +1,20 @@
+{ depot, pkgs, ... }:
+
+let
+  emptyDerivation = import ./emptyDerivation.nix {
+    inherit pkgs;
+    inherit (pkgs) stdenv;
+    inherit (depot.nix) getBins;
+  };
+
+  tests = import ./tests.nix {
+    inherit emptyDerivation;
+    inherit pkgs;
+    inherit (depot.nix) writeExecline getBins;
+    inherit (depot.nix.runTestsuite) runTestsuite it assertEq;
+  };
+
+in {
+  __functor = _: emptyDerivation;
+  inherit tests;
+}

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);
+}

nix/emptyDerivation/tests.nix

@@ -0,0 +1,32 @@
+{ emptyDerivation, getBins, pkgs, writeExecline, runTestsuite, it, assertEq }:
+
+let
+  bins = getBins pkgs.s6-portable-utils [ "s6-echo" ];
+
+  empty = it "is just an empty path" [
+    (assertEq "path empty"
+      (builtins.readFile emptyDerivation)
+      "")
+  ];
+
+  fooOut = emptyDerivation {
+    builder = writeExecline "foo-builder" {} [
+      "importas" "out" "out"
+      "redirfd" "-w" "1" "$out"
+      bins.s6-echo "-n" "foo"
+    ];
+  };
+
+  overrideBuilder = it "can override the builder" [
+    (assertEq "output is foo"
+      (builtins.readFile fooOut)
+      "foo")
+    (assertEq "can add new drv variables"
+      (emptyDerivation { foo = "bar"; }).foo
+      "bar")
+  ];
+
+in runTestsuite "emptyDerivation" [
+  empty
+  overrideBuilder
+]

third_party/default.nix

@@ -120,6 +120,7 @@ let
       runCommandLocal
       rustPlatform
       rustc
+      s6-portable-utils
       sbcl
       sqlite
       stern