qt module: look for qt tools only on build machine #14381
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Qt tools may be needed at build time, let's make their detection based on the build machine rather than the host machine as currently done. This incidentally fixes cross-compilation search for moc, rcc, uic and lrelease Qt tools because they are installed in libexec dir and that path is somehow not properly looked for otherwise. Before: """ Detecting Qt6 tools Run-time dependency qt6 (modules: Core) found: YES 6.8.1 (pkg-config) Program /usr/bin/moc found: NO Program /usr/lib/qt6/libexec/moc found: NO Program moc6 found: NO Program moc-qt6 found: NO Program moc found: NO """ After: """ Detecting Qt6 tools Found pkg-config: YES (/home/qschulz/work/upstream/buildroot/output/host/bin/pkg-config) 2.3.0 Build-time dependency qt6 (modules: Core) found: YES 6.8.1 (pkg-config) Program /home/qschulz/work/upstream/buildroot/output/host/bin/moc found: NO Program /home/qschulz/work/upstream/buildroot/output/host/libexec/moc found: YES 6.8.1 6.8.1 (/home/qschulz/work/upstream/buildroot/output/host/libexec/moc) """ Suggested-by: Ross Burton <[email protected]> Signed-off-by: Quentin Schulz <[email protected]>
| @@ -270,6 +270,9 @@ def _detect_tools(self, state: ModuleState, method: str, required: bool = True) | |||
| self._tools_detected = True | |||
| mlog.log(f'Detecting Qt{self.qt_version} tools') | |||
| kwargs = {'required': required, 'modules': 'Core', 'method': method} | |||
| # Tools are for compile-time steps, so force native check for | |||
| # compatibility with cross-compile scenarios | |||
| kwargs["native"] = True | |||
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This is potentially awkward if e.g. the tools can emit output which is platform-specific or Qt version-specific. In particular I'm worried about moc (as the other tools handle resource files in ways where this shouldn't really matter, or for translations, cannot really matter).
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I'm not sure to understand how this differs from the current state? Whatever is found by meson will be used, we already have some special handling for different qt versions (e.g. qt5+ seems to use --version, while qt4 uses -v if I read compilers_detect properly?), so that likely won't change anything there would it?
There's also some oddities handled in _detect_tools based on the version returned by compilers_detect.
What are you suggesting we do here?
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When build Qt6, buildroot compiles two sets of packages. First, Qt6 is compiled for the buildroot host (which is the build machine, in meson terms). This creates pkgconfig files that are installed in host/lib/pkgconfig/ (all relative paths here are relative to the buildroot root build directory). In particular, host/lib/pkgconfig/Qt6Core.pc contains
prefix=host
libexecdir=${prefix}/libexec
That is the location where buildroot installs the Qt6 host compilation tools (moc, rcc, ...).
Then, buildroot cross-compiles Qt6 for the target (which is the host in meson terms). This creates pkgconfig files that are installed in host/aarch64-buildroot-linux-gnu/sysroot/usr/lib/pkgconfig/. Those are what meson needs to pick to find the Qt6 headers and libraries for cross-compilation. Those packages do not install the build tools. Looking at the same Qt6Core.pc for the target package (host/aarch64-buildroot-linux-gnu/sysroot/usr/lib/pkgconfig/Qt6Core.pc), we see
prefix=/usr
libexecdir=${prefix}/lib/qt6/libexec
Does meson have the ability to use different pkg-config libdirs, one for build tools and one for host (in meson terms) headers and libraries ?
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Does meson have the ability to use different pkg-config libdirs, one for build tools and one for host (in meson terms) headers and libraries ?
If I'm not mistaken, yes. c.f. https://mesonbuild.com/Builtin-options.html#specifying-options-per-machine
build.pkg_config_path controls the paths pkg-config will search for native: true (build machine) dependencies.
which seems to be used by Buildroot already, c.f. https://git.busybox.net/buildroot/tree/package/pkg-meson.mk#n169
$$(MESON) setup [...] -Dbuild.pkg_config_path=$$(HOST_DIR)/lib/pkgconfig
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Adding build.pkg_config_path = br_host_dir / 'lib/pkgconfig' to the [built-in options] section of the cross-file I used to build libcamera helps finding the Qt build tools, but meson then picks incorrect include paths for the QtCore and QtGui headers. I haven't investigated why, it could be a bad interaction between pkg_config_libdir and pkg_config_path.
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I agree with @eli-schwartz that picking the native tools is dangerous.
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Adding
build.pkg_config_path = br_host_dir / 'lib/pkgconfig'to the[built-in options][...] helps finding the Qt build tools, but meson then picks incorrect include paths for the QtCore and QtGui headers.
Can you please provide logs, I don't understand what you're talking about. Which incorrect include paths are you talking about? In which context?
I agree with @eli-schwartz that picking the native tools is dangerous.
With regard to what? I'm not familiar with either Qt or meson so I'm probably missing something here but I'm not seeing arguments in either your answer or eli's so I cannot discuss them or look for a better implementation as I simply do not understand what the issue is (I am not saying this patch is the answer :) ). So please elaborate on what your concerns are :) Thanks!
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Data point: applying this patch fixes my cross compilation woes. I'm a distro maintainer at NixOS, where we do a lot of cross compiling, and this patch enables compiling appstream-qt, a package that uses qt6 moc so far as I can tell, from x86_64-linux to x86_64-freebsd. I am by no means a meson or qt expert, so feel free to ask questions, but I hope this patch does land :) |
Qt tools may be needed at build time, let's make their detection based on the build machine rather than the host machine as currently done.
This incidentally fixes cross-compilation search for moc, rcc, uic and lrelease Qt tools because they are installed in libexec dir and that path is somehow not properly looked for otherwise.
Before:
After:
Cc @rossburton who suggested the fix in #13018. I am not entirely sure this is enough to fix #13018 hence why I omitted it in the commit log. Cc @cordlandwehr for awareness.
Cc @pinchartl and @kbingham maintainers of https://libcamera.org/ for which this patch is needed.
This was written for building the qcam (qt6-based) test application of libcamera in Buildroot 2025.02 where the build machine sysroot (provided by Buildroot) differs from the build distribution. I have not tested this patch in any other context.