pythongh-109590 - Fix shutil.which to more closely match CMD on win32

An extensionless file will on be attempted if '.' is in PATHEXT

Fix up the tests to make better use of the bytes testing of shutil.which

Lib/shutil.py

@@ -1551,11 +1551,17 @@ def which(cmd, mode=os.F_OK | os.X_OK, path=None):
         pathext_source = os.getenv("PATHEXT") or _WIN_DEFAULT_PATHEXT
         pathext = [ext for ext in pathext_source.split(os.pathsep) if ext]
 
+        dot = '.'
         if use_bytes:
             pathext = [os.fsencode(ext) for ext in pathext]
-
-        # Always try checking the originally given cmd, if it doesn't match, try pathext
-        files = [cmd] + [cmd + ext for ext in pathext]
+            dot = b'.'
+
+        # Attempt to match CMD behavior:
+        # Only try the given cmd if it has an extension (therefore has a dot)
+        # or a dot is a pathext in PATHEXT.
+        # Otherwise use PATHEXT to formulate paths to check.
+        files = (([cmd] if (dot in cmd or dot in pathext) else []) +
+                 [cmd + ext for ext in pathext])
     else:
         # On other platforms you don't have things like PATHEXT to tell you
         # what file suffixes are executable, so just pass on cmd as-is.

Lib/test/test_shutil.py

@@ -2067,6 +2067,14 @@ def setUp(self):
         self.curdir = os.curdir
         self.ext = ".EXE"
 
+    def to_text_type(self, s):
+        '''
+        In this class we're testing with str, so convert s to a str
+        '''
+        if isinstance(s, bytes):
+            return s.decode()
+        return s
+
     def test_basic(self):
         # Given an EXE in a directory, it should be returned.
         rv = shutil.which(self.file, path=self.dir)
@@ -2254,9 +2262,9 @@ def test_empty_path_no_PATH(self):
 
     @unittest.skipUnless(sys.platform == "win32", 'test specific to Windows')
     def test_pathext(self):
-        ext = ".xyz"
+        ext = self.to_text_type(".xyz")
         temp_filexyz = tempfile.NamedTemporaryFile(dir=self.temp_dir,
-                                                   prefix="Tmp2", suffix=ext)
+                                                   prefix=self.to_text_type("Tmp2"), suffix=ext)
         os.chmod(temp_filexyz.name, stat.S_IXUSR)
         self.addCleanup(temp_filexyz.close)
 
@@ -2265,38 +2273,39 @@ def test_pathext(self):
         program = os.path.splitext(program)[0]
 
         with os_helper.EnvironmentVarGuard() as env:
-            env['PATHEXT'] = ext
+            env['PATHEXT'] = ext if isinstance(ext, str) else ext.decode()
             rv = shutil.which(program, path=self.temp_dir)
             self.assertEqual(rv, temp_filexyz.name)
 
     # Issue 40592: See https://bugs.python.org/issue40592
     @unittest.skipUnless(sys.platform == "win32", 'test specific to Windows')
     def test_pathext_with_empty_str(self):
-        ext = ".xyz"
+        ext = self.to_text_type(".xyz")
         temp_filexyz = tempfile.NamedTemporaryFile(dir=self.temp_dir,
-                                                   prefix="Tmp2", suffix=ext)
+                                                   prefix=self.to_text_type("Tmp2"), suffix=ext)
         self.addCleanup(temp_filexyz.close)
 
         # strip path and extension
         program = os.path.basename(temp_filexyz.name)
         program = os.path.splitext(program)[0]
 
         with os_helper.EnvironmentVarGuard() as env:
-            env['PATHEXT'] = f"{ext};"  # note the ;
+            env['PATHEXT'] = f"{ext if isinstance(ext, str) else ext.decode()};"  # note the ;
             rv = shutil.which(program, path=self.temp_dir)
             self.assertEqual(rv, temp_filexyz.name)
 
     # See GH-75586
     @unittest.skipUnless(sys.platform == "win32", 'test specific to Windows')
     def test_pathext_applied_on_files_in_path(self):
         with os_helper.EnvironmentVarGuard() as env:
-            env["PATH"] = self.temp_dir
+            env["PATH"] = self.temp_dir if isinstance(self.temp_dir, str) else self.temp_dir.decode()
             env["PATHEXT"] = ".test"
 
-            test_path = pathlib.Path(self.temp_dir) / "test_program.test"
-            test_path.touch(mode=0o755)
+            test_path = os.path.join(self.temp_dir, self.to_text_type("test_program.test"))
+            open(test_path, 'w').close()
+            os.chmod(test_path, 0o755)
 
-            self.assertEqual(shutil.which("test_program"), str(test_path))
+            self.assertEqual(shutil.which(self.to_text_type("test_program")), test_path)
 
     # See GH-75586
     @unittest.skipUnless(sys.platform == "win32", 'test specific to Windows')
@@ -2312,16 +2321,56 @@ def test_win_path_needs_curdir(self):
             self.assertFalse(shutil._win_path_needs_curdir('dontcare', os.X_OK))
             need_curdir_mock.assert_called_once_with('dontcare')
 
+    # See GH-109590
+    @unittest.skipUnless(sys.platform == "win32", 'test specific to Windows')
+    def test_extensionless_file_resolution_no_dot_in_pathext(self):
+        with os_helper.EnvironmentVarGuard() as env:
+            env['PATHEXT'] = ".test;"
+            env['PATH'] = self.temp_dir if isinstance(self.temp_dir, str) else self.temp_dir.decode()
+
+            extensionless_file_in_path = os.path.join(self.temp_dir, self.to_text_type("file"))
+            open(extensionless_file_in_path, 'w').close()
+
+            extensioned_file_in_path = os.path.join(self.temp_dir, self.to_text_type("file.test"))
+            open(extensioned_file_in_path, 'w').close()
+
+
+            self.assertEqual(shutil.which(self.to_text_type('file'), os.F_OK), extensioned_file_in_path)
+
+    # See GH-109590
+    @unittest.skipUnless(sys.platform == "win32", 'test specific to Windows')
+    def test_extensionless_file_resolution_dot_in_pathext(self):
+        with os_helper.EnvironmentVarGuard() as env:
+            env['PATHEXT'] = ".test;.;"
+            env['PATH'] = self.temp_dir if isinstance(self.temp_dir, str) else self.temp_dir.decode()
+
+            extensionless_file_in_path = os.path.join(self.temp_dir, self.to_text_type("file"))
+            open(extensionless_file_in_path, 'w').close()
+
+            extensioned_file_in_path = os.path.join(self.temp_dir, self.to_text_type("file.test"))
+            open(extensioned_file_in_path, 'w').close()
+
+            self.assertEqual(shutil.which(self.to_text_type('file')), extensionless_file_in_path)
+
 
 class TestWhichBytes(TestWhich):
     def setUp(self):
         TestWhich.setUp(self)
         self.dir = os.fsencode(self.dir)
         self.file = os.fsencode(self.file)
         self.temp_file.name = os.fsencode(self.temp_file.name)
+        self.temp_dir = os.fsencode(self.temp_dir)
         self.curdir = os.fsencode(self.curdir)
         self.ext = os.fsencode(self.ext)
 
+    def to_text_type(self, s):
+        '''
+        In this class we're testing with bytes, so convert s to a bytes
+        '''
+        if isinstance(s, str):
+            return s.encode()
+        return s
+
 
 class TestMove(BaseTest, unittest.TestCase):