[Bug]: 1.15.7 pre-release subproject libglnx build failure in opensuse rpmbuild environment using meson due to flaw in feature detection code

[nerditation]

Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a bug that matches the one I want to file, without success.
• If this is an issue with a particular app, I have tried filing it in the appropriate issue tracker for the app (e.g. under https://github.com/flathub/) and determined that it is an issue with Flatpak itself.
• This issue is not a report of a security vulnerability (see here if you need to report a security issue).

Flatpak version

1.15.7 pre-release

What Linux distribution are you using?

openSUSE

Linux distribution version

tumbleweed

What architecture are you using?

x86_64

How to reproduce

1. set CFLAGS as opensuse build service would use, specifically:
   • export CFLAGS='-Werror=return-type'
2. build flatpak using meson

Expected Behavior

build succeeds

Actual Behavior

build failed. here's an example log file of a failed build:

• NOTE: this is the log from an empty dummy project with libglnx as its subproject, but flatpak 1.15.7 will give the exact same error message.

https://build.opensuse.org/package/live_build_log/home:nerditation/libglnx-testing/openSUSE_Tumbleweed/x86_64

Additional Information

I was testing the fix for fcitx/fcitx5#1017, which requires a new feature in flatpak 1.15.7. opensuse tumbleweed still has 1.15.6 in it's repository, so I forked it and updated the rpm spec to use the 1.15.7 pre-release tarball as source code.

because 1.15.7 removed support for autotools, I also adapted the spec to use meson. but the build failed when compiling libglnx. so I tried to chroot into the build environment to do some digging, but if I manually call meson setup and meson compile with the same command line flags, it builds just fine.

after going through tons of log messages, I finally figured it out: opensuse's build service wants set some compiler flags before actually building the package, one of such flags is -Werror=return-type. it turns out this flag will change the behavior of libglnx's custom feature detection code snippet in its meson.build:

https://github.com/GNOME/libglnx/blob/202b294e6079e23242e65e0426f8639841d1210b/meson.build#L32-L49

because the return type at line 43 is int, -Werror=return-type will reject such code, thus the detected feature set is incorrect and later compilation fails due to duplicated function definitions.

currently, I managed to work around this with a simple patch, but it's better to be fixed upstream, since 1.15.7 is still in pre-release anyway.

-    int func (void) {
+    void func (void) {

in the long run thoug, I think we should add more feature detections to meson (similar to autoconf) so we can avoid this kind of ad-hoc feature detection in the package.

[smcv]

it's better to be fixed upstream

The upstream for the libglnx copylib is https://gitlab.gnome.org/GNOME/libglnx.

[smcv]

Please review/test https://gitlab.gnome.org/GNOME/libglnx/-/merge_requests/57. This is not going to be fixed as a Flatpak-specific change, but it can be fixed by fixing libglnx and then updating Flatpak's copy.