fix git module in nix

default.nix

@@ -59,6 +59,9 @@
 # Forces all warnings as errors
 , forceError ? true
 
+# If we are in Hydra
+, declInput ? null
+
 }:
 
 with pkgs.lib;
@@ -84,7 +87,10 @@ let
         inherit pkgs;
         filter = localLib.isPlutus;
       };
-      customOverlays = optional forceError errorOverlay;
+      gitModuleOverlay = import ./nix/overlays/git-module.nix {
+        inherit pkgs declInput;
+      };
+      customOverlays = optional forceError errorOverlay ++ [gitModuleOverlay];
       # Filter down to local packages, except those named in the given list
       localButNot = nope:
         let okay = builtins.filter (name: !(builtins.elem name nope)) localLib.plutusPkgList;

nix/overlays/git-module.nix

@@ -0,0 +1,24 @@
+{ pkgs, declInput }:
+
+with pkgs.lib;
+
+let
+  git-rev = if isNull declInput then builtins.readFile ../../.git/HEAD else declInput.rev;
+  gitModulePatch = pkgs.writeText "gitModulePatch" ''
+    diff --git a/src/Git.hs b/src/Git.hs
+    index b0398a7..45a9066 100644
+    --- a/src/Git.hs
+    +++ b/src/Git.hs
+    @@ -1,4 +1,4 @@
+     module Git where
+    
+     gitHead :: String
+    -gitHead = "master"
+    +gitHead = "${pkgs.lib.removeSuffix "\n" git-rev}"
+  '';
+in
+self: super: {
+    plutus-playground-server = super.plutus-playground-server.overrideDerivation (oldAttrs: {
+      patches = [gitModulePatch];
+    });
+}

nix/overlays/required.nix

@@ -11,6 +11,7 @@ let
     postCheck = "./Setup doctest --doctest-options=\"${opts}\"";
   });
   doctestOpts = "-pgmL markdown-unlit -XTemplateHaskell -XDeriveFunctor -XScopedTypeVariables";
+
 in
 
 self: super: {