feat(nix): add basic netstring nix generation functions

Moving to toplevel so I can use them with `runExecline`. They should
be pretty atomic, and are proven to work (tests are still in my user
dir, since they test the producers indirectly via the python parser
and I don’t want to pull it out right now).

Change-Id: Id0baa3adcb2ec646458a104c7868c2889b8c64f5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3054
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

nix/netstring/attrsToKeyValList.nix

@@ -0,0 +1,33 @@
+{ depot, lib, ... }:
+
+# Convert an attrset of strings to a list of key/value netstring pairs.
+# A good minimally viable json replacement if all you need is to iterate.
+# You can use e.g. `forstdin -Ed '' item` in execline to split the items
+# and then get the key and value via `multidefine -d '' $item { key value }`
+#
+# Example:
+#   { foo = "bar"; x = "abc"; }
+#   => "12:3:foo,3:bar,,10:1:x,3:abc,,"
+#
+# Example with runExecline:
+#   nix.runExecline "test" {
+#     stdin = nix.netstring.attrsToKeyValList {
+#       foo = "bar";
+#       x = "abc";
+#     };
+#   } [
+#     "forstdin" "-Ed" "" "item"
+#     "multidefine" "-d" "" "$item" [ "key" "value" ]
+#     "${pkgs.coreutils}/bin/echo" "\${key} -> \${value}"
+#   ]
+
+#   will print:
+#     foo -> bar
+#     x -> abc
+attrs:
+lib.concatStrings
+  (lib.mapAttrsToList
+    (k: v: depot.nix.netstring.fromString
+       ( depot.nix.netstring.fromString k
+       + depot.nix.netstring.fromString v))
+    attrs)

nix/netstring/fromString.nix

@@ -0,0 +1,10 @@
+{ ... }:
+# convert any nix string into a netstring
+# (prefixed by its length) according to https://en.wikipedia.org/wiki/Netstring
+#
+# Examples:
+#   netstring.fromString "foo"
+#   => "3:foo,"
+#   netstring.fromString ""
+#   => "0:,"
+s: "${toString (builtins.stringLength s)}:${s},"