gn: 20190403 -> 2020-03-09

This updates gn to the required version for chromiumDev (the recommended
version for the stable release of Chromium isn't sufficient [0]).

[0]: The Chromium build fails during the configuration phase:
ERROR at //mojo/public/tools/bindings/mojom.gni:393:16: Undefined identifier
               "cpp_typemaps",
               ^-------------

pkgs/development/tools/build-managers/gn/default.nix

@@ -1,31 +1,37 @@
 { stdenv, lib, fetchgit, darwin, writeText
-, git, ninja, python3 }:
+, ninja, python3
+}:
 
 let
-  rev = "64b846c96daeb3eaf08e26d8a84d8451c6cb712b";
-  sha256 = "1v2kzsshhxn0ck6gd5w16gi2m3higwd9vkyylmsczxfxnw8skgpy";
+  # Note: Please use the recommended version for Chromium, e.g.:
+  # https://git.archlinux.org/svntogit/packages.git/tree/trunk/chromium-gn-version.sh?h=packages/gn
+  rev = "fd3d768bcfd44a8d9639fe278581bd9851d0ce3a";
+  revNum = "1718"; # git describe HEAD --match initial-commit | cut -d- -f3
+  version = "2020-03-09";
+  sha256 = "1asc14y8by7qcn10vbk467hvx93s30pif8r0brissl0sihsaqazr";
 
-  shortRev = builtins.substring 0 7 rev;
+  revShort = builtins.substring 0 7 rev;
   lastCommitPosition = writeText "last_commit_position.h" ''
     #ifndef OUT_LAST_COMMIT_POSITION_H_
     #define OUT_LAST_COMMIT_POSITION_H_
 
-    #define LAST_COMMIT_POSITION "(${shortRev})"
+    #define LAST_COMMIT_POSITION_NUM ${revNum}
+    #define LAST_COMMIT_POSITION "${revNum} (${revShort})"
 
     #endif  // OUT_LAST_COMMIT_POSITION_H_
   '';
 
-in
-stdenv.mkDerivation {
-  pname = "gn";
-  version = "20190403";
+in stdenv.mkDerivation {
+  pname = "gn-unstable";
+  inherit version;
 
   src = fetchgit {
+    # Note: The TAR-Archives (+archive/${rev}.tar.gz) are not deterministic!
     url = "https://gn.googlesource.com/gn";
     inherit rev sha256;
   };
 
-  nativeBuildInputs = [ ninja python3 git ];
+  nativeBuildInputs = [ ninja python3 ];
   buildInputs = lib.optionals stdenv.isDarwin (with darwin; with apple_sdk.frameworks; [
     libobjc
     cctools
@@ -49,8 +55,8 @@ stdenv.mkDerivation {
   setupHook = ./setup-hook.sh;
 
   meta = with lib; {
-    description = "A meta-build system that generates NinjaBuild files";
-    homepage = https://gn.googlesource.com/gn;
+    description = "A meta-build system that generates build files for Ninja";
+    homepage = "https://gn.googlesource.com/gn";
     license = licenses.bsd3;
     platforms = platforms.unix;
     maintainers = with maintainers; [ stesie matthewbauer ];