feat(nix/getBins): add getBins r/1107

This is a simple-stupid “unix import system” for nix, for referencing
binaries in `/bin/` by their name and lifting them to a Nix attrset.

Allows for simple aliasing of executable names.

Change-Id: Ifa23cb377201c3b08050c5026e9751e736afaf56
Reviewed-on: https://cl.tvl.fyi/c/depot/+/664
Reviewed-by: tazjin <mail@tazj.in>

3 files changed, 91 insertions, 0 deletions

diff --git a/nix/getBins/OWNERS b/nix/getBins/OWNERS
new file mode 100644
index 000000000000..a742d0d22bf6
--- /dev/null
+++ b/nix/getBins/OWNERS
@@ -0,0 +1,3 @@
+inherited: true
+owners:
+  - Profpatsch
diff --git a/nix/getBins/default.nix b/nix/getBins/default.nix
new file mode 100644
index 000000000000..b0c1135fc87a
--- /dev/null
+++ b/nix/getBins/default.nix
@@ -0,0 +1,48 @@
+{ lib, pkgs, depot, ... }:
+
+# Takes a derivation and a list of binary names
+# and returns an attribute set of `name -> path`.
+# The list can also contain renames in the form of
+# `{ use, as }`, which goes `as -> usePath`.
+#
+# It is usually used to construct an attrset `bins`
+# containing all the binaries required in a file,
+# similar to a simple import system.
+#
+# Example:
+#
+#   bins = getBins pkgs.hello [ "hello" ]
+#       // getBins pkgs.coreutils [ "printf" "ln" "echo" ]
+#       // getBins pkgs.execline
+#            [ { use = "if"; as = "execlineIf" } ]
+#       // getBins pkgs.s6-portable-utils
+#            [ { use = "s6-test"; as = "test" }
+#              { use = "s6-cat"; as = "cat" }
+#            ];
+#
+#   provides
+#     bins.{hello,printf,ln,echo,execlineIf,test,cat}
+#
+
+let
+  getBins = drv: xs:
+    let f = x:
+      # TODO(Profpatsch): typecheck
+      let x' = if builtins.isString x then { use = x; as = x; } else x;
+      in {
+        name = x'.as;
+        value = "${lib.getBin drv}/bin/${x'.use}";
+      };
+    in builtins.listToAttrs (builtins.map f xs);
+
+
+  tests = import ./tests.nix {
+    inherit getBins;
+    inherit (pkgs) writeScriptBin;
+    inherit (depot.nix.runTestsuite) assertEq it runTestsuite;
+  };
+
+in {
+  __functor = _: getBins;
+  inherit tests;
+}
diff --git a/nix/getBins/tests.nix b/nix/getBins/tests.nix
new file mode 100644
index 000000000000..ff81deb5f1ec
--- /dev/null
+++ b/nix/getBins/tests.nix
@@ -0,0 +1,40 @@
+{ writeScriptBin, assertEq, it, runTestsuite, getBins }:
+
+let
+  drv = writeScriptBin "hello" "it’s me";
+  drv2 = writeScriptBin "goodbye" "tschau";
+
+  bins = getBins drv [
+            "hello"
+            { use = "hello"; as = "also-hello"; }
+          ]
+      // getBins drv2 [ "goodbye" ]
+      ;
+
+  simple = it "path is equal to the executable name" [
+    (assertEq "path"
+      bins.hello
+      "${drv}/bin/hello")
+    (assertEq "content"
+      (builtins.readFile bins.hello)
+      "it’s me")
+  ];
+
+  useAs = it "use/as can be used to rename attributes" [
+    (assertEq "path"
+      bins.also-hello
+      "${drv}/bin/hello")
+  ];
+
+  secondDrv = it "by merging attrsets you can build up bins" [
+    (assertEq "path"
+      bins.goodbye
+      "${drv2}/bin/goodbye")
+  ];
+
+in
+  runTestsuite "getBins" [
+    simple
+    useAs
+    secondDrv
+  ]