Re: changing tests/qtest/meson.build causes unnecessary rebuilding

[Paolo Bonzini]

On 21/01/21 12:56, Peter Maydell wrote:
> $ make -C build/arm-clang/ -j8
> make: Entering directory
> '/home/petmay01/linaro/qemu-from-laptop/qemu/build/arm-clang'
> [1/23] Generating qemu-version.h with a meson_exe.py custom command
> make: Leaving directory
> '/home/petmay01/linaro/qemu-from-laptop/qemu/build/arm-clang'
> $ touch tests/qtest/meson.build
> $ make -C build/arm-clang/ -j8
> make: Entering directory
> '/home/petmay01/linaro/qemu-from-laptop/qemu/build/arm-clang'
> /usr/bin/ninja  build.ninja && touch build.ninja.stamp
> [0/1] Regenerating build files.
> The Meson build system
> [...]
>
> It then goes on to rebuild hundreds or thousands of files, most of
> which are not even in tests/. (Oddly, the exact set of files recompiled
> seems to vary from run to run.)
>
> Forcing a full rebuild of all of QEMU seems rather excessive when
> the change was likely "add a new test case"...

This is "avoid build.ninja changes due to order of hash table iteration" 
(https://github.com/mesonbuild/meson/pull/7900/).  I think Meson 0.57 
(with the fix) should be out soon, hopefully before 6.0.

Alternatively you can try to bug your distro to include the patches, 
they are pretty safe.

Paolo