Workaround for cabal trying to build dependencies in nix-shell

For source-repository-package's, cabal tries to build them on its own, even
when all dependencies are already provided by Nix. Relevant issues:
- haskell/cabal#6049
- IntersectMBO/ouroboros-network#645
- haskell/cabal#5586 (comment)

This seems to be a problem even with a cabal that includes
haskell/cabal#6917 (see
input-output-hk/haskell.nix#720 (comment)
for how to test a cabal-install 3.4)

The only known workaround is to remove the source-repository-package
sections from cabal.project, but this should only be done for cabal when
used from a nix-shell, not from cabal without a nix-shell, and not outside
the nix-shell.

To make this work smoothly, the script `scripts/nix-setup` can be used,
which splits the source-repository-package sections into cabal.project.srcs,
which is then again included from here (to make the Nix setup still work).
Running the script again undoes it.

.gitignore

@@ -8,6 +8,7 @@ dist-newstyle
 .ghc.environment.*
 cabal.project.local*
 cabal.project.local~
+cabal.project.srcs
 dist/
 
 stack.yaml.local.lock

default.nix

@@ -16,8 +16,8 @@ let
       sha256 = "sha256-kFLfYQ+8ogz4uycvriAszwP3Af7yqRGrxH6l6HmnKuc=";
     })
     { inherit system; };
-  morphoPkgs = import ./nix/morpho-node.nix { inherit pkgs src haskellCompiler profile; };
-  shell = morphoPkgs.shellFor {
+  morphoPkgs = nixShell: import ./nix/morpho-node.nix { inherit pkgs src haskellCompiler nixShell profile; };
+  shell = (morphoPkgs true).shellFor {
     packages = ps: with ps; [
       morpho-checkpoint-node
     ];
@@ -39,8 +39,9 @@ let
       # Systemd won't build on darwin, checking first we're not on a
       # Darwin env.
       (pkgs.stdenv.lib.optional (!pkgs.stdenv.isDarwin) pkgs.systemd);
-    #exactDeps = false;
+    exactDeps = true;
   };
   # Instantiate a package set using the generated file.
-in
-morphoPkgs // { inherit shell pkgs mantis; }
+in morphoPkgs false // {
+  inherit shell pkgs mantis;
+}

nix/morpho-node.nix

@@ -2,13 +2,44 @@
 , src
 , haskellCompiler ? "ghc865"
 , profile
+, nixShell
 , ...
 }:
 
 pkgs.haskell-nix.cabalProject {
   inherit src;
   name = "morpho-checkpoint-node";
   compiler-nix-name = haskellCompiler;
+
+  # For source-repository-package's, cabal tries to build them on its own, even
+  # when all dependencies are already provided by Nix. Relevant issues:
+  # - https://github.com/haskell/cabal/issues/6049
+  # - https://github.com/input-output-hk/ouroboros-network/pull/645
+  # - https://github.com/haskell/cabal/issues/5586#issuecomment-479576084
+  #
+  # This seems to be a problem even with a cabal that includes
+  # https://github.com/haskell/cabal/pull/6917 (see
+  # https://github.com/input-output-hk/haskell.nix/issues/720#issuecomment-745397468
+  # for how to test a cabal-install 3.4)
+  #
+  # The only known workaround is to remove the source-repository-package
+  # sections from cabal.project, but this should only be done for cabal when
+  # used from a nix-shell, not from cabal without a nix-shell, and not outside
+  # the nix-shell.
+  #
+  # To make this work smoothly, the script `scripts/nix-setup` can be used,
+  # which splits the source-repository-package sections into cabal.project.srcs,
+  # which is then again included from here (to make the Nix setup still work).
+  # Running the script again undoes it.
+  cabalProject =
+    let
+      exists = builtins.pathExists (../cabal.project.srcs);
+      standard = builtins.readFile (src + "/cabal.project");
+      sources = builtins.readFile (../cabal.project.srcs);
+    in if exists then standard + "\n" + sources
+    else if nixShell then throw "You need to run scripts/nix-setup first"
+    else standard;
+
   pkg-def-extras = [
     (hackage: {
       packages = {

scripts/nix-setup

@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+tmp=$(mktemp -d)
+trap 'rm -rf "$tmp"' exit
+
+if [[ ! -f cabal.project.srcs ]]; then
+  # We later mark cabal.project as not having any changes, since we remove
+  # source-repository-package sections from it but don't want the user to commit that
+  # But if the user already has changes to it, that would make them not commit their own changes too
+  # So make the user either stage or commit the changes to prevent this
+  if ! git diff --exit-code --quiet cabal.project; then
+    echo "cabal.project has unstaged changes. Stage or commit them first to use this script" >&2
+    exit 1
+  fi
+  echo "Setting up cabal.project to work with nix-shell" >&2
+
+  # Splits the file at the first occurence of source-repository-package
+  csplit -s -f "$tmp/cabal.project.split." cabal.project /source-repository-package/
+  mv "$tmp/cabal.project.split.00" cabal.project
+  mv "$tmp/cabal.project.split.01" cabal.project.srcs
+
+  # Prevent modification of these files since we later mark them as not having any changes
+  chmod -w cabal.project cabal.project.srcs
+
+  # In order to prevent the user from committing the changes to cabal.project, tell git that it should assume it wasn't changed
+  git update-index --assume-unchanged cabal.project
+else
+  echo "Undoing cabal.project setup for nix-shell" >&2
+
+  # Combine the split files into one again
+  cat cabal.project cabal.project.srcs > "$tmp/cabal.project"
+  chmod +w cabal.project cabal.project.srcs
+  rm cabal.project.srcs
+  mv "$tmp/cabal.project" cabal.project
+
+  # Tell git to track changes to cabal.project again
+  git update-index --no-assume-unchanged cabal.project
+fi