Always check if found libraries are linkable

We don't always check if a library is actually linkable because of concerns
that a library may not be standalone, so linking against it may still have
unresolved references. We can workaround that by building a shared library
as a test rather than an executable, and in fact we already do that in a
bunch of cases since bb5f2ca.

This comes up in particular with Fedora and dependency('atomic') because
on x86_64, they provide a libatomic.so linker script (so our file existence
check passes), but trying to use it later on will fail if the backing package
isn't installed.

The _test_all_naming unittest has been tweaked because we were using a
blank file rather than an actual shared library, which (as expected),
now fails.

I've also added a test for dependency('atomic') providing a result that
can actually be linked against. I've not made it check generally whether
dependency('atomic') finds something when we expect to (at least for now)
as it'll involve some CI whack-a-mole.

_get_file_from_list has not been deleted because the Ninja backend's
guess_library_absolute_path uses it. We might be able to change it to
also use a link test but I've left that alone.

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=2352531
Bug: #14946
Closes: #10936

Author: thesamesam

mesonbuild/compilers/mixins/clike.py

@@ -1090,10 +1090,8 @@ def _get_trials_from_pattern(cls, pattern: str, directory: str, libname: str) ->
     @staticmethod
     def _get_file_from_list(env: Environment, paths: T.List[str]) -> T.Optional[Path]:
         '''
-        We just check whether the library exists. We can't do a link check
-        because the library might have unresolved symbols that require other
-        libraries. On macOS we check if the library matches our target
-        architecture.
+        Check whether the library exists by filename. On macOS, we also
+        check if the library matches our target architecture.
         '''
         for p in paths:
             if os.path.isfile(p):
@@ -1152,12 +1150,21 @@ def _find_library_real(self, libname: str, env: 'Environment', extra_dirs: T.Lis
                 trials = self._get_trials_from_pattern(p, d, libname)
                 if not trials:
                     continue
-                trial = self._get_file_from_list(env, trials)
-                if not trial:
+
+                trial_result = ""
+                for trial in trials:
+                    largs = self.get_linker_always_args() + self.get_allow_undefined_link_args()
+                    extra_args = [trial] + self.linker_to_compiler_args(largs)
+
+                    if self.links(code, env, extra_args=extra_args, disable_cache=True)[0]:
+                        trial_result = trial
+                        break
+
+                if not trial_result:
                     continue
-                if libname.startswith('lib') and trial.name.startswith(libname) and lib_prefix_warning:
+                if libname.startswith('lib') and trial_result.startswith(libname) and lib_prefix_warning:
                     mlog.warning(f'find_library({libname!r}) starting in "lib" only works by accident and is not portable')
-                return [trial.as_posix()]
+                return [Path(trial_result).as_posix()]
         return None
 
     def _find_library_impl(self, libname: str, env: 'Environment', extra_dirs: T.List[str],

test cases/common/285 atomic/a.c

@@ -0,0 +1,3 @@
+int main(void) {
+  return 0;
+}

test cases/common/285 atomic/meson.build

@@ -0,0 +1,23 @@
+project('meson system dependency', 'c', meson_version: '>=1.7.0')
+
+cc = meson.get_compiler('c')
+
+# We could check if dependency('atomic') actually finds something when
+# we 'know' it exists (MESON_SKIP_TEST) but that's likely to be brittle,
+# so don't bother (for now, at least).
+atomic = dependency('atomic', required : false)
+
+# If the dependency provider says it found something, make sure it can
+# be linked against (https://github.com/mesonbuild/meson/issues/14946).
+dependencies = [
+  atomic
+]
+
+exe = executable(
+  'a',
+  'a.c',
+  dependencies : dependencies,
+  install : false,
+)
+
+test('basic', exe)

unittests/internaltests.py

@@ -551,10 +551,14 @@ def _test_all_naming(self, cc, env, patterns, platform):
             for i in ['libfoo.so.6.0', 'libfoo.so.5.0', 'libfoo.so.54.0', 'libfoo.so.66a.0b', 'libfoo.so.70.0.so.1',
                       'libbar.so.7.10', 'libbar.so.7.9', 'libbar.so.7.9.3']:
                 libpath = Path(tmpdir) / i
-                libpath.write_text('', encoding='utf-8')
-            found = cc._find_library_real('foo', env, [tmpdir], '', LibType.PREFER_SHARED, lib_prefix_warning=True, ignore_system_dirs=False)
+                src = libpath.with_suffix('.c')
+                with src.open('w', encoding='utf-8') as f:
+                    f.write('int meson_foobar (void) { return 0; }')
+                subprocess.check_call(['gcc', src, '-o', libpath, '-shared'])
+
+            found = cc._find_library_real('foo', env, [tmpdir], 'int main(void) { return 0; };', LibType.PREFER_SHARED, lib_prefix_warning=True, ignore_system_dirs=False)
             self.assertEqual(os.path.basename(found[0]), 'libfoo.so.54.0')
-            found = cc._find_library_real('bar', env, [tmpdir], '', LibType.PREFER_SHARED, lib_prefix_warning=True, ignore_system_dirs=False)
+            found = cc._find_library_real('bar', env, [tmpdir], 'int main(void) { return 0; };', LibType.PREFER_SHARED, lib_prefix_warning=True, ignore_system_dirs=False)
             self.assertEqual(os.path.basename(found[0]), 'libbar.so.7.10')
 
     def test_find_library_patterns(self):