unit tests: Fix sdl2 failure test on macOS

Also ensure that the test's no-pkg-config codepath will always be run,
even on the CI where we always have pkg-config available.

This counts as a test case for #4728

run_tests.py

@@ -213,6 +213,13 @@ def run_mtest_inprocess(commandlist):
         sys.stderr = old_stderr
     return returncode, mystdout.getvalue(), mystderr.getvalue()
 
+def clear_meson_configure_class_caches():
+    mesonbuild.compilers.CCompiler.library_dirs_cache = {}
+    mesonbuild.compilers.CCompiler.program_dirs_cache = {}
+    mesonbuild.compilers.CCompiler.find_library_cache = {}
+    mesonbuild.dependencies.PkgConfigDependency.pkgbin_cache = {}
+    mesonbuild.dependencies.PkgConfigDependency.class_pkgbin = mesonlib.PerMachine(None, None, None)
+
 def run_configure_inprocess(commandlist):
     old_stdout = sys.stdout
     sys.stdout = mystdout = StringIO()
@@ -223,6 +230,7 @@ def run_configure_inprocess(commandlist):
     finally:
         sys.stdout = old_stdout
         sys.stderr = old_stderr
+        clear_meson_configure_class_caches()
     return returncode, mystdout.getvalue(), mystderr.getvalue()
 
 def run_configure_external(full_command):

run_unittests.py

@@ -230,6 +230,32 @@ def temp_filename():
         except OSError:
             pass
 
+@contextmanager
+def no_pkgconfig():
+    '''
+    A context manager that overrides shutil.which and ExternalProgram to force
+    them to return None for pkg-config to simulate it not existing.
+    '''
+    old_which = shutil.which
+    old_search = ExternalProgram._search
+
+    def new_search(self, name, search_dir):
+        if name == 'pkg-config':
+            return [None]
+        return old_search(self, name, search_dir)
+
+    def new_which(cmd, *kwargs):
+        if cmd == 'pkg-config':
+            return None
+        return old_which(cmd, *kwargs)
+
+    shutil.which = new_which
+    ExternalProgram._search = new_search
+    try:
+        yield
+    finally:
+        shutil.which = old_which
+        ExternalProgram._search = old_search
 
 class PatchModule:
     '''
@@ -3412,7 +3438,7 @@ class FailureTests(BasePlatformTests):
     and slows down testing.
     '''
     dnf = "[Dd]ependency.*not found(:.*)?"
-    nopkg = '[Pp]kg-config not found'
+    nopkg = '[Pp]kg-config.*not found'
 
     def setUp(self):
         super().setUp()
@@ -3499,10 +3525,9 @@ class FailureTests(BasePlatformTests):
             raise unittest.SkipTest('sdl2-config found')
         self.assertMesonRaises("dependency('sdl2', method : 'sdlconfig')", self.dnf)
         if shutil.which('pkg-config'):
-            errmsg = self.dnf
-        else:
-            errmsg = self.nopkg
-        self.assertMesonRaises("dependency('sdl2', method : 'pkg-config')", errmsg)
+            self.assertMesonRaises("dependency('sdl2', method : 'pkg-config')", self.dnf)
+        with no_pkgconfig():
+            self.assertMesonRaises("dependency('sdl2', method : 'pkg-config')", self.nopkg)
 
     def test_gnustep_notfound_dependency(self):
         # Want to test failure, so skip if available