`pkg-config` issue on `cabal.project` with no `source-repository-package` stanzas

[zeme-iohk]

After integrating CHaP with plutus (i.e. removing all source-repository-packages stanzas from cabal.project) the devshell fails to cabal build all.

I can only reproduce this on my M1 MacBook with rosetta (nixpkgs.stdenv.system == x86_64-darwin)

To reproduce, checkout the commit right after CHaP was introduced and run cabal:

git clone git@github.com:input-output-hk/plutus.git

cd plutus

git checkout 0785993024d11dc7b85ff9c9276adf838753dcb1

nix develop 

cabal build all --minimize-conflict-set

Warning: Requested index-state 2022-10-10T00:00:00Z is newer than
'cardano-haskell-packages'! Falling back to older state
(2022-09-27T00:17:36Z).
Resolving dependencies...
Error: cabal: Could not resolve dependencies:
[__0] trying: plutus-core-1.0.0.0 (user goal)
[__1] next goal: inline-r (dependency of plutus-core)
[__1] rejecting: inline-r-0.10.5, inline-r-0.10.4, inline-r-0.10.3,
inline-r-0.10.2, inline-r-0.10.1, inline-r-0.10, inline-r-0.9.2,
inline-r-0.9.1, inline-r-0.9.0.2, inline-r-0.9.0.1, inline-r-0.9.0.0,
inline-r-0.8.0.1, inline-r-0.8.0.0, inline-r-0.7.3.0, inline-r-0.7.2.0,
inline-r-0.7.1.2, inline-r-0.7.1.1, inline-r-0.7.1.0, inline-r-0.7.0.0
(conflict: pkg-config package libR>=3.0, not found in the pkg-config database)
[__1] fail (backjumping, conflict set: inline-r, plutus-core)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: inline-r, plutus-core

The revision right before CHaP was introduced works fine:

git checkout 69ddd0edf30dc19a1926c8d9eab694709468ec46
nix develop 
cabal build all
... works fine

Note that a "fix" has already been found: by adding pkg-config to the devshell, everything works again.
See this plutus PR: IntersectMBO/plutus#4906

However something doesn't add up.

Basically, none of these place a pkg-config in $PATH:

nix develop .#x86_64-darwin.plutus.library.plutus-project.shell
nix develop .#x86_64-darwin.plutus.library.plutus-project.devshell
nix-shell (see https://github.com/input-output-hk/plutus/blob/511611ce9234d3f34f50c322a0888fc2786c17e8/shell.nix#L113)

Also

If I run cabal build all -v | grep pkg-config

• In nix-shell
  • Before CHaP (finds pkg-config and works ✅)

  /nix/store/3vq6z9rz5ips7jlifna48mxx8wcj83xm-pkg-config-wrapper-0.29.2/bin/pkg-config --version
  /nix/store/3vq6z9rz5ips7jlifna48mxx8wcj83xm-pkg-config-wrapper-0.29.2/bin/pkg-config --variable pc_path pkg-config
  /nix/store/3vq6z9rz5ips7jlifna48mxx8wcj83xm-pkg-config-wrapper-0.29.2/bin/pkg-config --version
  /nix/store/3vq6z9rz5ips7jlifna48mxx8wcj83xm-pkg-config-wrapper-0.29.2/bin/pkg-config --list-all
  /nix/store/3vq6z9rz5ips7jlifna48mxx8wcj83xm-pkg-config-wrapper-0.29.2/bin/pkg-config --modversion formw python3 menu 'ncurses++' menuw panel libsodium python-3.10-embed python ncurses panelw zlib libR python3-embed libsecp256k1 ncursesw bzip2 'ncurses++w' form editorconfig python-3.10
  

  • After CHap (doesn't find pkg-config 🤔 but still works ✅)

    Failed to query pkg-config, Cabal will continue without solving for pkg-config
  constraints: Cannot find pkg-config program
  No pkg-config found
  No pkg-config found
  No pkg-config found
  No pkg-config found
  No pkg-config found
  No pkg-config found
  No pkg-config found
  

• In nix develop
  • Before CHaP (finds pkg-config and works ✅)

  /nix/store/54af7qasa616lcl8wswyqn1yplfr9zz4-devshell-dir/bin/pkg-config --version
  /nix/store/54af7qasa616lcl8wswyqn1yplfr9zz4-devshell-dir/bin/pkg-config --variable pc_path pkg-config
  /nix/store/54af7qasa616lcl8wswyqn1yplfr9zz4-devshell-dir/bin/pkg-config --version
  /nix/store/54af7qasa616lcl8wswyqn1yplfr9zz4-devshell-dir/bin/pkg-config --list-all
  /nix/store/54af7qasa616lcl8wswyqn1yplfr9zz4-devshell-dir/bin/pkg-config --modversion libsecp256k1 libR libsodium
  

  • After CHaP (doesn't find pkg-config and breaks ❌)

  Failed to query pkg-config, Cabal will continue without solving for pkg-config
  constraints: Cannot find pkg-config program
  CallStack (from HasCallStack):
    withMetadata, called at src/Distribution/Simple/Utils.hs:370:14 in Cabal-3.8.1.0-3n5GDPIVQh93OGMJzC2H2:Distribution.Simple.Utils
  Error: cabal: Could not resolve dependencies:
  [__0] trying: plutus-core-1.0.0.0 (user goal)
  [__1] next goal: inline-r (dependency of plutus-core)
  [__1] rejecting: inline-r-0.10.5, inline-r-0.10.4, inline-r-0.10.3,
  inline-r-0.10.2, inline-r-0.10.1, inline-r-0.10, inline-r-0.9.2,
  inline-r-0.9.1, inline-r-0.9.0.2, inline-r-0.9.0.1, inline-r-0.9.0.0,
  inline-r-0.8.0.1, inline-r-0.8.0.0, inline-r-0.7.3.0, inline-r-0.7.2.0,
  inline-r-0.7.1.2, inline-r-0.7.1.1, inline-r-0.7.1.0, inline-r-0.7.0.0
  (conflict: pkg-config package libR>=3.0, not found in the pkg-config database)
  [__1] fail (backjumping, conflict set: inline-r, plutus-core)
  After searching the rest of the dependency tree exhaustively, these were the
  goals I've had most trouble fulfilling: inline-r, plutus-core