Stack fails to build on Mac when src/ has two nested dirs named the same way as executable

[Martinsos]

Steps to reproduce

Create a project that has executable named "foo" and also has module Foo.Foo.Bar in src/ that executable calls.

I created a minimal example to reproduce it here: https://github.com/wasp-lang/wasp/tree/mac-build-error/waspc .
To try it out, check out https://github.com/wasp-lang/wasp, branch mac-build-error and then run stack build on Mac machine.

We confirmed this to happen on two different Macs + Github CI for Mac.

Ways to fix it: rename executable to something else, or remove one of the two directories that have the same name as executable.

Expected

I expect project to compile, and it does on Linux, but it does not on Mac.

Actual

What actually happened.

If you suspect that a stack command misbehaved, please include the output of that command in --verbose mode.
If the output is larger than a page please paste the output in a Gist.

$ stack build
Building all executables for `waspc' once. After a successful build of all of them, only specified executables will be rebuilt.
waspc> configure (lib + exe)
Configuring waspc-0.2.2.2...
waspc> build (lib + exe)
Preprocessing library for waspc-0.2.2.2..
Building library for waspc-0.2.2.2..
[1 of 2] Compiling Paths_waspc
[2 of 2] Compiling Wasp.Wasp.Hi
Preprocessing executable 'wasp' for waspc-0.2.2.2..
Building executable 'wasp' for waspc-0.2.2.2..
[1 of 2] Compiling Main
[2 of 2] Compiling Paths_waspc
Linking .stack-work/dist/x86_64-osx/Cabal-3.2.1.0/build/wasp/wasp ...
ld: can't open output file for writing: .stack-work/dist/x86_64-osx/Cabal-3.2.1.0/build/wasp/wasp, errno=21
clang: error: linker command failed with exit code 1 (use -v to see invocation)
`gcc' failed in phase `Linker'. (Exit code: 1)

--  While building package waspc-0.2.2.2 (scroll up to its section to see the error) using:
      /Users/matija/.stack/setup-exe-cache/x86_64-osx/Cabal-simple_mPHDZzAJ_3.2.1.0_ghc-8.10.7 --builddir=.stack-work/dist/x86_64-osx/Cabal-3.2.1.0 build lib:waspc exe:wasp --ghc-options " -fdiagnostics-color=always"
    Process exited with code: ExitFailure 1

$ stack build --verbose
...
2021-11-11 19:18:19.774230: [info] Preprocessing library for waspc-0.0.0.1..
2021-11-11 19:18:19.774365: [info] Building library for waspc-0.0.0.1..
2021-11-11 19:18:20.387682: [info] Preprocessing executable 'wasp' for waspc-0.0.0.1..
2021-11-11 19:18:20.387790: [info] Building executable 'wasp' for waspc-0.0.0.1..
2021-11-11 19:18:20.876276: [info] Linking .stack-work/dist/x86_64-osx/Cabal-3.2.1.0/build/wasp/wasp ...
2021-11-11 19:18:21.280408: [warn] ld: can't open output file for writing: .stack-work/dist/x86_64-osx/Cabal-3.2.1.0/build/wasp/wasp, errno=21
2021-11-11 19:18:21.285690: [warn] clang: error: linker command failed with exit code 1 (use -v to see invocation)
2021-11-11 19:18:21.287545: [warn] gcc' failed in phase Linker'. (Exit code: 1)
2021-11-11 19:18:21.319877: [debug] Start: getPackageFiles /Users/matija/git/wasp-lang/wasp/waspc/waspc.cabal
2021-11-11 19:18:21.322401: [debug] Failed to decode module interface: /Users/matija/git/wasp-lang/wasp/waspc/.stack-work/dist/x86_64-osx/Cabal-3.2.1.0/build/Paths_waspc.hi Decoding failure: Invalid magic: e49ceb0f
2021-11-11 19:18:21.322555: [debug] Failed to decode module interface: /Users/matija/git/wasp-lang/wasp/waspc/.stack-work/dist/x86_64-osx/Cabal-3.2.1.0/build/Wasp/Wasp/Hi.hi Decoding failure: Invalid magic: e49ceb0f
2021-11-11 19:18:21.323939: [debug] Failed to decode module interface: /Users/matija/git/wasp-lang/wasp/waspc/.stack-work/dist/x86_64-osx/Cabal-3.2.1.0/build/wasp/wasp-tmp/Paths_waspc.hi Decoding failure: Invalid magic: e49ceb0f
2021-11-11 19:18:21.324095: [debug] Failed to decode module interface: /Users/matija/git/wasp-lang/wasp/waspc/.stack-work/dist/x86_64-osx/Cabal-3.2.1.0/build/wasp/wasp-tmp/Main.hi Decoding failure: Invalid magic: e49ceb0f
2021-11-11 19:18:21.324205: [debug] Finished in 4ms: getPackageFiles /Users/matija/git/wasp-lang/wasp/waspc/waspc.cabal
2021-11-11 19:18:21.325389: [error]
--  While building package waspc-0.0.0.1 (scroll up to its section to see the error) using:
      /Users/matija/.stack/setup-exe-cache/x86_64-osx/Cabal-simple_mPHDZzAJ_3.2.1.0_ghc-8.10.7 --builddir=.stack-work/dist/x86_64-osx/Cabal-3.2.1.0 build lib:waspc exe:wasp --ghc-options " -fdiagnostics-color=always"
    Process exited with code: ExitFailure 1

Stack version

$ stack --version
Version 2.5.1, Git revision d6ab861544918185236cf826cb2028abb266d6d5 x86_64 hpack-0.33.0

Method of installation

homebrew

[brandon-leapyear]

✨ This is an old work account. Please reference @brandonchinn178 for all future communication ✨

---

Related: #3918

[mpilgrem]

@Martinsos, thanks for reporting. As @brandon-leapyear has indicated, I think this arises because the file system on macOs is case-insensitive and there is no distinction made between:

• .stack-work/dist/.../build/foo/foo (the executable foo for the package)
• .stack-work/dist/.../build/Foo/Foo (the directory for the build artefacts for module Foo.Foo.Something)

The problem is not limited to macOS. On Linux (case-sensitive) with executable Foo, there is no distinction between:

• .stack-work/dist/.../build/Foo/Foo (the executable Foo for the package)
• .stack-work/dist/.../build/Foo/Foo (the directory for the build artefacts for modules Foo.Foo.Something)

It seems to me to be a feature/bug of Cabal (the library). This Cabal issue (from 2016) appears to be related: haskell/cabal#3545.