Fix proto-lens ghc-9.12 build

The proto-lens source-repository-package build fails with:

    Error: [Cabal-6661]
    filepath wildcard 'proto-lens-imports/google/protobuf/descriptor.proto'
    refers to the directory 'proto-lens-imports/google/protobuf', which does not exist or is not a directory.
    filepath wildcard 'proto-lens-imports/google/protobuf/descriptor.proto'
    does not match any files.

However, SRP is currently required because the released version does not
support ghc-9.12.

Author: sgillespie

flake.nix

@@ -179,6 +179,27 @@
           isCross = pkgs: 
             with pkgs.haskell-nix.haskellLib; isNativeMusl || isCrossHost;
 
+          # Fetch proto-lens with submodules and fix symlinks for plan and build phases
+          protoLensSrc = nixpkgs.fetchgit {
+            url = "https://github.com/google/proto-lens";
+            rev = "20de5227947b0c37dd6852dcc6f2db1cd5889cee";
+            sha256 = "sha256-VUYU2swjU7L8Zdu6Zfz6jo2ulW5uPhAamt2GjH5hZRY=";
+            fetchSubmodules = true;
+          };
+
+          protoLensSrcFixed = nixpkgs.runCommand "proto-lens-fixed" {} ''
+            mkdir -p $out
+            cp -a ${protoLensSrc}/. $out/
+            chmod -R +w $out
+            # Fix proto-lens-imports symlink in proto-lens
+            rm -rf $out/proto-lens/proto-lens-imports/google
+            cp -r ${protoLensSrc}/google/protobuf/src/google $out/proto-lens/proto-lens-imports/
+            # Fix proto-src symlink in proto-lens-protobuf-types
+            rm -rf $out/proto-lens-protobuf-types/proto-src
+            cp -r ${protoLensSrc}/google/protobuf/src $out/proto-lens-protobuf-types/proto-src
+            chmod -R -w $out
+          '';
+
           project = (nixpkgs.haskell-nix.cabalProject' ({ config, lib, pkgs, ... }: rec {
             src = ./.;
             name = "cardano-db-sync";
@@ -202,6 +223,7 @@
 
             inputMap = {
               "https://chap.intersectmbo.org/" = inputs.CHaP;
+              "https://github.com/google/proto-lens/20de5227947b0c37dd6852dcc6f2db1cd5889cee" = protoLensSrcFixed;
             };
 
             shell = {