Issue #12333: run tests on the new module in a subprocess

It is not possible to unload a module written in C, so use a subprocess to run
the tests on the module compiled by test_build_ext(). Using a subprocess, we
don't have to unload the module, save/restore sys.path, and the test can be run
more than once.

This commit fixes also an access error on rmtree() on Windows: because the
module was not really unloaded, it was not possible to remove the temporary
directory (it is not possible to remove a directory on Windows if it still
contains an open file).

Lib/packaging/tests/test_command_build_ext.py

@@ -8,13 +8,10 @@ from packaging.dist import Distribution
 from packaging.errors import UnknownFileError, CompileError
 from packaging.command.build_ext import build_ext
 from packaging.compiler.extension import Extension
+from test.script_helper import assert_python_ok
 
 from packaging.tests import support, unittest, verbose, unload
 
-# http://bugs.python.org/issue4373
-# Don't load the xx module more than once.
-ALREADY_TESTED = False
-
 
 def _get_source_filename():
     srcdir = sysconfig.get_config_var('srcdir')
@@ -29,8 +26,6 @@ class BuildExtTestCase(support.TempdirManager,
         # Note that we're making changes to sys.path
         super(BuildExtTestCase, self).setUp()
         self.tmp_dir = self.mkdtemp()
-        self.sys_path = sys.path, sys.path[:]
-        sys.path.append(self.tmp_dir)
         filename = _get_source_filename()
         if os.path.exists(filename):
             shutil.copy(filename, self.tmp_dir)
@@ -58,7 +53,6 @@ class BuildExtTestCase(support.TempdirManager,
                 cmd.library_dirs = value.split(os.pathsep)
 
     def test_build_ext(self):
-        global ALREADY_TESTED
         xx_c = os.path.join(self.tmp_dir, 'xxmodule.c')
         if not os.path.exists(xx_c):
             # skipping if we cannot find it
@@ -86,29 +80,27 @@ class BuildExtTestCase(support.TempdirManager,
         finally:
             sys.stdout = old_stdout
 
-        if ALREADY_TESTED:
-            return
-        else:
-            ALREADY_TESTED = True
+        code = """if 1:
+            import sys
+            sys.path.insert(0, %r)
 
-        import xx
+            import xx
 
-        for attr in ('error', 'foo', 'new', 'roj'):
-            self.assertTrue(hasattr(xx, attr))
+            for attr in ('error', 'foo', 'new', 'roj'):
+                assert hasattr(xx, attr)
 
-        self.assertEqual(xx.foo(2, 5), 7)
-        self.assertEqual(xx.foo(13, 15), 28)
-        self.assertEqual(xx.new().demo(), None)
-        doc = 'This is a template module just for instruction.'
-        self.assertEqual(xx.__doc__, doc)
-        self.assertTrue(isinstance(xx.Null(), xx.Null))
-        self.assertTrue(isinstance(xx.Str(), xx.Str))
+            assert xx.foo(2, 5) == 7
+            assert xx.foo(13, 15) == 28
+            assert xx.new().demo() is None
+            doc = 'This is a template module just for instruction.'
+            assert xx.__doc__ == doc
+            assert isinstance(xx.Null(), xx.Null)
+            assert isinstance(xx.Str(), xx.Str)"""
+        code = code % self.tmp_dir
+        assert_python_ok('-c', code)
 
     def tearDown(self):
         # Get everything back to normal
-        unload('xx')
-        sys.path = self.sys_path[0]
-        sys.path[:] = self.sys_path[1]
         if sys.version > "2.6":
             site.USER_BASE = self.old_user_base
             build_ext.USER_BASE = self.old_user_base