tests: Make monkey patching smarter

There are two problems here. The first is that we're doing manual monkey
patching which is fragile and verbose, when unittest.mock is designed
specifically to solve this exact problem. The second is that we're
monkey patching os.environ at two different levels with the same
information. So let's only do it once.

run_tests.py

@@ -26,6 +26,7 @@
 from enum import Enum
 from glob import glob
 from pathlib import Path
+from unittest import mock
 from mesonbuild import compilers
 from mesonbuild import dependencies
 from mesonbuild import mesonlib
@@ -255,16 +256,11 @@ def ensure_backend_detects_changes(backend):
         time.sleep(1)
 
 def run_mtest_inprocess(commandlist):
-    old_stdout = sys.stdout
-    sys.stdout = mystdout = StringIO()
-    old_stderr = sys.stderr
-    sys.stderr = mystderr = StringIO()
-    try:
+    stderr = StringIO()
+    stdout = StringIO()
+    with mock.patch.object(sys, 'stdout', stdout), mock.patch.object(sys, 'stderr', stderr):
         returncode = mtest.run_with_args(commandlist)
-    finally:
-        sys.stdout = old_stdout
-        sys.stderr = old_stderr
-    return returncode, mystdout.getvalue(), mystderr.getvalue()
+    return returncode, stdout.getvalue(), stderr.getvalue()
 
 def clear_meson_configure_class_caches():
     compilers.CCompiler.library_dirs_cache = {}
@@ -275,22 +271,14 @@ def clear_meson_configure_class_caches():
     dependencies.PkgConfigDependency.class_pkgbin = mesonlib.PerMachine(None, None)
 
 def run_configure_inprocess(commandlist, env=None):
-    old_stdout = sys.stdout
-    sys.stdout = mystdout = StringIO()
-    old_stderr = sys.stderr
-    sys.stderr = mystderr = StringIO()
-    old_environ = os.environ.copy()
-    if env is not None:
-        os.environ.update(env)
-    try:
-        returncode = mesonmain.run(commandlist, get_meson_script())
-    finally:
-        sys.stdout = old_stdout
-        sys.stderr = old_stderr
-        clear_meson_configure_class_caches()
-        os.environ.clear()
-        os.environ.update(old_environ)
-    return returncode, mystdout.getvalue(), mystderr.getvalue()
+    stderr = StringIO()
+    stdout = StringIO()
+    with mock.patch.dict(os.environ, env or {}), mock.patch.object(sys, 'stdout', stdout), mock.patch.object(sys, 'stderr', stderr):
+        try:
+            returncode = mesonmain.run(commandlist, get_meson_script())
+        finally:
+            clear_meson_configure_class_caches()
+    return returncode, stdout.getvalue(), stderr.getvalue()
 
 def run_configure_external(full_command, env=None):
     pc, o, e = mesonlib.Popen_safe(full_command, env=env)

run_unittests.py

@@ -1657,13 +1657,7 @@ def init(self, srcdir, *,
         self.privatedir = os.path.join(self.builddir, 'meson-private')
         if inprocess:
             try:
-                if override_envvars is not None:
-                    old_envvars = os.environ.copy()
-                    os.environ.update(override_envvars)
-                (returncode, out, err) = run_configure_inprocess(self.meson_args + args + extra_args)
-                if override_envvars is not None:
-                    os.environ.clear()
-                    os.environ.update(old_envvars)
+                (returncode, out, err) = run_configure_inprocess(self.meson_args + args + extra_args, override_envvars)
                 if 'MESON_SKIP_TEST' in out:
                     raise unittest.SkipTest('Project requested skipping.')
                 if returncode != 0:
@@ -1707,15 +1701,8 @@ def run_tests(self, *, inprocess=False, override_envvars=None):
         if not inprocess:
             self._run(self.test_command, workdir=self.builddir, override_envvars=override_envvars)
         else:
-            if override_envvars is not None:
-                old_envvars = os.environ.copy()
-                os.environ.update(override_envvars)
-            try:
+            with mock.patch.dict(os.environ, override_envvars):
                 run_mtest_inprocess(['-C', self.builddir])
-            finally:
-                if override_envvars is not None:
-                    os.environ.clear()
-                    os.environ.update(old_envvars)
 
     def install(self, *, use_destdir=True, override_envvars=None):
         if self.backend is not Backend.ninja: