pythongh-127001: Fix conflict with extensionless files in shutil.whic…

…h() on Windows

Name without a PATHEXT extension is only searched if the mode does not
include X_OK.

Lib/shutil.py

@@ -1555,16 +1555,16 @@ def which(cmd, mode=os.F_OK | os.X_OK, path=None):
         if use_bytes:
             pathext = [os.fsencode(ext) for ext in pathext]
 
-        files = ([cmd] + [cmd + ext for ext in pathext])
+        files = [cmd + ext for ext in pathext]
 
         # gh-109590. If we are looking for an executable, we need to look
         # for a PATHEXT match. The first cmd is the direct match
         # (e.g. python.exe instead of python)
         # Check that direct match first if and only if the extension is in PATHEXT
         # Otherwise check it last
-        suffix = os.path.splitext(files[0])[1].upper()
-        if mode & os.X_OK and not any(suffix == ext.upper() for ext in pathext):
-            files.append(files.pop(0))
+        normcmd = cmd.upper()
+        if not (mode & os.X_OK) or any(normcmd.endswith(ext.upper()) for ext in pathext):
+            files.insert(0, cmd)
     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.
@@ -1573,7 +1573,7 @@ def which(cmd, mode=os.F_OK | os.X_OK, path=None):
     seen = set()
     for dir in path:
         normdir = os.path.normcase(dir)
-        if not normdir in seen:
+        if normdir not in seen:
             seen.add(normdir)
             for thefile in files:
                 name = os.path.join(dir, thefile)

Lib/test/test_shutil.py

@@ -2592,8 +2592,8 @@ def test_pathext_preferred_for_execute(self):
             open(exe, 'w').close()
             os.chmod(exe, 0o755)
 
-            # default behavior allows a direct match if nothing in PATHEXT matches
-            self.assertEqual(shutil.which(self.to_text_type("test.exe")), exe)
+            self.assertIsNone(shutil.which(self.to_text_type("test.exe")))
+            self.assertEqual(shutil.which(self.to_text_type("test.exe"), mode=os.F_OK), exe)
 
             dot_test = os.path.join(self.temp_dir, self.to_text_type("test.exe.test"))
             open(dot_test, 'w').close()