refactor(nix/escapeExecline): move into own directory

The escaping functions are going to be used by both `writeExecline`
and `runExecline`, so let’s move them to their own namespace.

Change-Id: Iccf69eaeca3062573e0751a17c548b7def86196d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/706
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: Kane York <rikingcoding@gmail.com>

nix/escapeExecline/OWNERS

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

nix/escapeExecline/default.nix

@@ -0,0 +1,29 @@
+{ lib, ... }:
+let
+  # replaces " and \ to \" and \\ respectively and quote with "
+  # e.g.
+  #   a"b\c -> "a\"b\\c"
+  #   a\"bc -> "a\\\"bc"
+  escapeExeclineArg = arg:
+    ''"${builtins.replaceStrings [ ''"'' ''\'' ] [ ''\"'' ''\\'' ] (toString arg)}"'';
+
+  # Escapes an execline (list of execline strings) to be passed to execlineb
+  # Give it a nested list of strings. Nested lists are interpolated as execline
+  # blocks ({}).
+  # Everything is quoted correctly.
+  #
+  # Example:
+  #   escapeExecline [ "if" [ "somecommand" ] "true" ]
+  #   == ''"if" { "somecommand" } "true"''
+  escapeExecline = execlineList: lib.concatStringsSep " "
+    (let
+      go = arg:
+        if      builtins.isString arg then [(escapeExeclineArg arg)]
+        else if builtins.isPath arg then [(escapeExeclineArg "${arg}")]
+        else if lib.isDerivation arg then [(escapeExeclineArg arg)]
+        else if builtins.isList arg then [ "{" ] ++ builtins.concatMap go arg ++ [ "}" ]
+        else abort "escapeExecline can only hande nested lists of strings, was ${lib.generators.toPretty {} arg}";
+     in builtins.concatMap go execlineList);
+
+in
+escapeExecline