Extend flag(os-string) handling to test-suites

[ulidtko]

GHC's pkgdb permits having multiple versions of a library installed side-by-side.

So, one can end up with:

$ ghc-pkg list -v 2>&1 | grep filepath
    filepath-1.4.300.1 (filepath-1.4.300.1)
    filepath-1.5.5.0 (filepath-1.5.5.0-4NuYd63ABenDOAyOjf3bpG)

— and I certainly did.

In an environment like this, file-io v0.2.0 succeeds build with --disable-tests, and fails build with --enable-tests. The build system gets confused which package links where, cabal starts making bizarre decisions in its Linked component graph, and the different PosixPath types stop aligning.

This patch prevents that issue, by ensuring that tests always speak the same PosixPath as the main lib, in any build configuration.

common stanzas and import are since cabal-version: 2.2 — thus no bump was required. Unfortunately, import is only allowed on the first line of a block.

---

TL;DR: this fixes build with --enable-tests in uncommon conditions during distro packaging.

[hasufell]

The build system gets confused which package links where, cabal starts making bizarre decisions

Do you have a reproducer? This sounds like something that needs investigation.

[hasufell]

We don't need filepath here anymore (same for the other tests)

[ulidtko]

I tried dropping the dependency: all tests are importing System.OsPath — and then emit member of the hidden package errors.

[ulidtko]

Do you have a reproducer?

Yes; minimized, here.

Dockerfile

FROM haskell:9.6.6-slim

RUN cabal update && \
    cabal v1-install --global filepath --constraint 'filepath >=1.5' && \
    echo "Confirm the environment, two versions expected:" && \
    ghc-pkg list -v | grep filepath

RUN echo "Test-deps" && \
    cabal v1-install --global tasty-hunit temporary

RUN cd /tmp && \
    git clone https://github.com/haskell/file-io && \
    echo 'module Main(main) where { import Distribution.Simple; main = defaultMain; }' > file-io/Setup.hs

WORKDIR /tmp/file-io

#-- NOTE: uncommenting the PR patch fixes build with tests
#RUN curl -fSsL https://github.com/haskell/file-io/pull/52.diff | patch -p1

RUN echo '=== WITHOUT TESTS ==='; \
    set -x; \
    runhaskell Setup configure --disable-tests --verbose && \
    runhaskell Setup build

RUN echo '=== WITH TESTS ==='; \
    set -x; \
    runhaskell Setup configure --enable-tests --verbose && \
    runhaskell Setup build

— @hasufell if you try docker build - < this.dockerfile ☝️ as is — (only) the last step fails. If you uncomment the curl | patch line — both succeed.

I know, I know, this is chokefull of unsavory "legacy" and "not recommended" approaches — but this faithfully replicates how entire distro lineages are installing haskell things.