bpo-35755: shutil.which() uses os.confstr("CS_PATH") (GH-12858)

shutil.which() and distutils.spawn.find_executable() now use
os.confstr("CS_PATH") if available instead of os.defpath, if the PATH
environment variable is not set.

Don't use os.confstr("CS_PATH") nor os.defpath if the PATH
environment variable is set to an empty string to mimick Unix 'which'
command behavior.

Changes:

* find_executable() now starts by checking for the executable in the
  current working directly case. Add an explicit
  "if not path: return None".
* Add tests for PATH='' (empty string), PATH=':' and for PATHEXT.

Lib/test/test_shutil.py

@@ -1619,6 +1619,57 @@ class TestWhich(unittest.TestCase):
             rv = shutil.which(self.file)
             self.assertEqual(rv, self.temp_file.name)
 
+    def test_environ_path_empty(self):
+        # PATH='': no match
+        with support.EnvironmentVarGuard() as env:
+            env['PATH'] = ''
+            with unittest.mock.patch('os.confstr', return_value=self.dir, \
+                                     create=True), \
+                 support.swap_attr(os, 'defpath', self.dir), \
+                 support.change_cwd(self.dir):
+                rv = shutil.which(self.file)
+                self.assertIsNone(rv)
+
+    def test_environ_path_cwd(self):
+        expected_cwd = os.path.basename(self.temp_file.name)
+        if sys.platform == "win32":
+            curdir = os.curdir
+            if isinstance(expected_cwd, bytes):
+                curdir = os.fsencode(curdir)
+            expected_cwd = os.path.join(curdir, expected_cwd)
+
+        # PATH=':': explicitly looks in the current directory
+        with support.EnvironmentVarGuard() as env:
+            env['PATH'] = os.pathsep
+            with unittest.mock.patch('os.confstr', return_value=self.dir, \
+                                     create=True), \
+                 support.swap_attr(os, 'defpath', self.dir):
+                rv = shutil.which(self.file)
+                self.assertIsNone(rv)
+
+                # look in current directory
+                with support.change_cwd(self.dir):
+                    rv = shutil.which(self.file)
+                    self.assertEqual(rv, expected_cwd)
+
+    def test_environ_path_missing(self):
+        with support.EnvironmentVarGuard() as env:
+            env.pop('PATH', None)
+
+            # without confstr
+            with unittest.mock.patch('os.confstr', side_effect=ValueError, \
+                                     create=True), \
+                 support.swap_attr(os, 'defpath', self.dir):
+                rv = shutil.which(self.file)
+            self.assertEqual(rv, self.temp_file.name)
+
+            # with confstr
+            with unittest.mock.patch('os.confstr', return_value=self.dir, \
+                                     create=True), \
+                 support.swap_attr(os, 'defpath', ''):
+                rv = shutil.which(self.file)
+            self.assertEqual(rv, self.temp_file.name)
+
     def test_empty_path(self):
         base_dir = os.path.dirname(self.dir)
         with support.change_cwd(path=self.dir), \
@@ -1633,6 +1684,23 @@ class TestWhich(unittest.TestCase):
             rv = shutil.which(self.file)
             self.assertIsNone(rv)
 
+    @unittest.skipUnless(sys.platform == "win32", 'test specific to Windows')
+    def test_pathext(self):
+        ext = ".xyz"
+        temp_filexyz = tempfile.NamedTemporaryFile(dir=self.temp_dir,
+                                                   prefix="Tmp2", suffix=ext)
+        os.chmod(temp_filexyz.name, stat.S_IXUSR)
+        self.addCleanup(temp_filexyz.close)
+
+        # strip path and extension
+        program = os.path.basename(temp_filexyz.name)
+        program = os.path.splitext(program)[0]
+
+        with support.EnvironmentVarGuard() as env:
+            env['PATHEXT'] = ext
+            rv = shutil.which(program, path=self.temp_dir)
+            self.assertEqual(rv, temp_filexyz.name)
+
 
 class TestWhichBytes(TestWhich):
     def setUp(self):