polkadot: fix commit hash fetch for version string

pkgs/applications/blockchains/polkadot/default.nix

@@ -4,6 +4,7 @@
 , llvmPackages
 , protobuf
 , rustPlatform
+, writeShellScriptBin
 }:
 rustPlatform.buildRustPackage rec {
   pname = "polkadot";
@@ -13,12 +14,38 @@ rustPlatform.buildRustPackage rec {
     owner = "paritytech";
     repo = "polkadot";
     rev = "v${version}";
-    sha256 = "sha256-TgpQ+nRKkbyQcCwecnhgF70FI+rlDuTy7XHUNhvYy64=";
+    sha256 = "sha256-NXuYUmo80rrBZCcuISKon48SKyyJrkzCEhggxaJNfBM=";
+
+    # see the comment below on fakeGit for how this is used
+    leaveDotGit = true;
+    postFetch = ''
+      ( cd $out; git rev-parse --short HEAD > .git_commit )
+      rm -rf $out/.git
+    '';
   };
 
   cargoSha256 = "sha256-PIORMTzQbMdlrKwuF4MiGrLlg2nQpgLRsaHHeiCbqrg=";
 
-  nativeBuildInputs = [ clang ];
+  nativeBuildInputs =
+    let
+      # the build process of polkadot requires a .git folder in order to determine
+      # the git commit hash that is being built and add it to the version string.
+      # since having a .git folder introduces reproducibility issues to the nix
+      # build, we check the git commit hash after fetching the source and save it
+      # into a .git_commit file, and then delete the .git folder. then we create a
+      # fake git command that will just return the contents of this file, which will
+      # be used when the polkadot build calls `git rev-parse` to fetch the commit
+      # hash.
+      fakeGit = writeShellScriptBin "git" ''
+        if [[ $@ = "rev-parse --short HEAD" ]]; then
+          cat /build/source/.git_commit
+        else
+          >&2 echo "Unknown command: $@"
+          exit 1
+        fi
+      '';
+    in
+    [ clang fakeGit ];
 
   LIBCLANG_PATH = "${llvmPackages.libclang.lib}/lib";
   PROTOC = "${protobuf}/bin/protoc";