fix(ci): fix macOS ci to use provided fmt

[slfan1989]

Summary

Fix the S3 build on macOS 26 / AppleClang by disabling fmt's consteval format-string checks for vendored avro-cpp.

Problem

The S3 CI job on AArch64 macOS 26 fails while compiling vendored avro-cpp:

fmt/format-inl.h: error: call to consteval function ... is not a constant expression

https://github.com/apache/iceberg-cpp/actions/runs/26262392739/job/77298497335?pr=669

[zhjwpku]

Can you try PRIVATE, so that FMT_CONSTEVAL= won't propagate. Not sure if that work :)

[slfan1989]

That makes sense. The current failure happens while compiling avrocpp_s itself, so PRIVATE may be sufficient and would avoid leaking the workaround to downstream targets. I’ll try switching it to PRIVATE and let CI verify whether Iceberg sources still compile without inheriting FMT_CONSTEVAL=.

[zhjwpku]

It seems PRIVATE won't work :(

[slfan1989]

I changed it back to PUBLIC.

[HuaHuaY]

I think a simpler fix would be to temporarily introduce a higher version of fmt in CI environment or CMake. Latest fmt has fixed this issue fmtlib/fmt#4396, but avro will fetch an outdated fmt if it doesn't find a fmt package. There is a PR apache/avro#3788 in avro which tries to replace fmt with std::format. We can remove the fmt after avro accepts the PR.

// lang/c++/CMakeLists.txt
find_package(fmt QUIET)
if (NOT fmt_FOUND)
    include(FetchContent)
    FetchContent_Declare(
            fmt
            GIT_REPOSITORY  https://github.com/fmtlib/fmt.git
            GIT_TAG         10.2.1
            GIT_PROGRESS    TRUE
            USES_TERMINAL_DOWNLOAD TRUE
    )
    FetchContent_MakeAvailable(fmt)
endif ()

[wgtmac]

I think a simpler fix would be to temporarily introduce a higher version of fmt in CI environment or CMake. Latest fmt has fixed this issue fmtlib/fmt#4396, but avro will fetch an outdated fmt if it doesn't find a fmt package. There is a PR in avro which tries to replace fmt with std::format. We can remove the fmt after avro accepts the PR.

// lang/c++/CMakeLists.txt
find_package(fmt QUIET)
if (NOT fmt_FOUND)
    include(FetchContent)
    FetchContent_Declare(
            fmt
            GIT_REPOSITORY  https://github.com/fmtlib/fmt.git
            GIT_TAG         10.2.1
            GIT_PROGRESS    TRUE
            USES_TERMINAL_DOWNLOAD TRUE
    )
    FetchContent_MakeAvailable(fmt)
endif ()

I think this is a better approach. We can see that homebrew is available on the macos-26 image: https://github.com/actions/runner-images/blob/main/images/macos/macos-26-arm64-Readme.md. Homebrew has fmt 12.1.0 as the stable version: https://formulae.brew.sh/formula/fmt. We can add a step to call brew install fmt in the macOS related CI.

[wgtmac]

Thank you for fixing it, @slfan1989!

[slfan1989]

Thank you for fixing it, @slfan1989!

@wgtmac @zhjwpku @HuaHuaY Thanks for the review and suggestion!