feat(nix/runExecline): add runExecline

runExecline is a primitive that just does not care.

It’s similar to `runCommand`, but instead of concatenating bash
scripts left and right, it actually *uses* the features of
`derivation`, passing things to `args` and making it possible to
overwrite the `builder` in a sensible manner.

Additionally, it provides a way to pass a nix string to `stdin` of the
build script.

Similar to `writeExecline`, the passed script is not a string, but a
nested list of nix lists representing execline blocks. Escaping is
done by the implementation, the user can just use normal nix strings.

Change-Id: I890d9e5d921207751cdc8cc4309381395d92742f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/701
Reviewed-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

nix/runExecline/tests.nix

@@ -0,0 +1,108 @@
+{ stdenv, pkgs, runExecline, getBins
+# https://www.mail-archive.com/skaware@list.skarnet.org/msg01256.html
+, coreutils }:
+
+let
+
+  bins = getBins coreutils [ "mv" ]
+      // getBins pkgs.execline [
+           "execlineb"
+           { use = "if"; as = "execlineIf"; }
+           "redirfd"
+           "importas"
+         ]
+      // getBins pkgs.s6-portable-utils [
+           "s6-chmod"
+           "s6-grep"
+           "s6-touch"
+           "s6-cat"
+           "s6-test"
+         ];
+
+  # lol
+  writeScript = name: script: runExecline name {
+    derivationArgs = {
+      inherit script;
+      passAsFile = [ "script" ];
+      preferLocalBuild = true;
+      allowSubstitutes = false;
+    };
+  } [
+      "importas" "-ui" "s" "scriptPath"
+      "importas" "-ui" "out" "out"
+      "foreground" [
+        bins.mv "$s" "$out"
+      ]
+      bins.s6-chmod "0755" "$out"
+  ];
+
+  # execline block of depth 1
+  block = args: builtins.map (arg: " ${arg}") args ++ [ "" ];
+
+  # derivation that tests whether a given line exists
+  # in the given file. Does not use runExecline, because
+  # that should be tested after all.
+  fileHasLine = line: file: derivation {
+    name = "run-execline-test-file-${file.name}-has-line";
+    inherit (stdenv) system;
+    builder = bins.execlineIf;
+    args =
+      (block [
+        bins.redirfd "-r" "0" file   # read file to stdin
+        bins.s6-grep "-F" "-q" line   # and grep for the line
+      ])
+      ++ [
+        # if the block succeeded, touch $out
+        bins.importas "-ui" "out" "out"
+        bins.s6-touch "$out"
+      ];
+    preferLocalBuild = true;
+    allowSubstitutes = false;
+  };
+
+  # basic test that touches out
+  basic = runExecline "run-execline-test-basic" {
+    derivationArgs = {
+      preferLocalBuild = true;
+      allowSubstitutes = false;
+    };
+  } [
+      "importas" "-ui" "out" "out"
+      "${bins.s6-touch}" "$out"
+  ];
+
+  # whether the stdin argument works as intended
+  stdin = fileHasLine "foo" (runExecline "run-execline-test-stdin" {
+    stdin = "foo\nbar\nfoo";
+    derivationArgs = {
+      preferLocalBuild = true;
+      allowSubstitutes = false;
+    };
+  } [
+      "importas" "-ui" "out" "out"
+      # this pipes stdout of s6-cat to $out
+      # and s6-cat redirects from stdin to stdout
+      "redirfd" "-w" "1" "$out" bins.s6-cat
+  ]);
+
+  wrapWithVar = runExecline "run-execline-test-wrap-with-var" {
+    builderWrapper = writeScript "var-wrapper" ''
+      #!${bins.execlineb} -S0
+      export myvar myvalue $@
+    '';
+    derivationArgs = {
+      preferLocalBuild = true;
+      allowSubstitutes = false;
+    };
+  } [
+    "importas" "-ui" "v" "myvar"
+    "if" [ bins.s6-test "myvalue" "=" "$v" ]
+      "importas" "out" "out"
+      bins.s6-touch "$out"
+  ];
+
+in [
+  basic
+  stdin
+  wrapWithVar
+]