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Add WESTON_EXPORT_FOR_TESTS and convert vertex-clip

Pekka Paalanen requested to merge pq/weston:mr/test-export into main

This is a new function exporting macro that intends to make writing unit tests in the Weston test suite easier.

A test needs to access a private function to be able to verify its behavior. Previously we have used things like putting such functions in a separate .c file and then building that file into the corresponding test. That is a bit awkward and can lead to proliferation of arbitrary .c files for no good reason. It may also require pre-processor magic, and sometimes copying chunks of code causing a risk of deviating the code being tested from the code actually used.

This patch proposes another approach: a private export from a DSO. Except, private exports do not really exist, and this is just a normal export with a specific C macro, and omitting the function from public headers.

Once exported, a test program can link the DSO during build, be that a shared library or even a plugin, use the private header declaring the function, and simply call the function in the test.

The declaration of WESTON_EXPORT_FOR_TESTS is in shared/helpers.h so that it is available to all components equally while still not being in a public header. Other places that were considered:

  • include/libweston/libweston.h is a public header, but external users should not know about the macro.

  • libweston/libweston-private.h is too private and not available to all components, particularly color-lcms plugin.

  • libweston/backend.h is not appropriate for color-lcms plugin either.

vertex-clip test is converted as an example.

I did have a look at the matrix stuff, but there would be more to untangle there.

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