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.

spawn.py

@@ -172,21 +172,32 @@ def find_executable(executable, path=None):
     A string listing directories separated by 'os.pathsep'; defaults to
     os.environ['PATH'].  Returns the complete filename or None if not found.
     """
-    if path is None:
-        path = os.environ.get('PATH', os.defpath)
-
-    paths = path.split(os.pathsep)
-    base, ext = os.path.splitext(executable)
-
+    _, ext = os.path.splitext(executable)
     if (sys.platform == 'win32') and (ext != '.exe'):
         executable = executable + '.exe'
 
-    if not os.path.isfile(executable):
-        for p in paths:
-            f = os.path.join(p, executable)
-            if os.path.isfile(f):
-                # the file exists, we have a shot at spawn working
-                return f
-        return None
-    else:
+    if os.path.isfile(executable):
         return executable
+
+    if path is None:
+        path = os.environ.get('PATH', None)
+        if path is None:
+            try:
+                path = os.confstr("CS_PATH")
+            except (AttributeError, ValueError):
+                # os.confstr() or CS_PATH is not available
+                path = os.defpath
+        # bpo-35755: Don't use os.defpath if the PATH environment variable is
+        # set to an empty string to mimick Unix which command behavior
+
+    # PATH='' doesn't match, whereas PATH=':' looks in the current directory
+    if not path:
+        return None
+
+    paths = path.split(os.pathsep)
+    for p in paths:
+        f = os.path.join(p, executable)
+        if os.path.isfile(f):
+            # the file exists, we have a shot at spawn working
+            return f
+    return None

tests/test_spawn.py

@@ -87,11 +87,52 @@ class SpawnTestCase(support.TempdirManager,
             rv = find_executable(dont_exist_program , path=tmp_dir)
             self.assertIsNone(rv)
 
-            # test os.defpath: missing PATH environment variable
+            # PATH='': no match, except in the current directory
             with test_support.EnvironmentVarGuard() as env:
-                with mock.patch('distutils.spawn.os.defpath', tmp_dir):
-                    env.pop('PATH')
+                env['PATH'] = ''
+                with unittest.mock.patch('distutils.spawn.os.confstr',
+                                         return_value=tmp_dir, create=True), \
+                     unittest.mock.patch('distutils.spawn.os.defpath',
+                                         tmp_dir):
+                    rv = find_executable(program)
+                    self.assertIsNone(rv)
+
+                    # look in current directory
+                    with test_support.change_cwd(tmp_dir):
+                        rv = find_executable(program)
+                        self.assertEqual(rv, program)
+
+            # PATH=':': explicitly looks in the current directory
+            with test_support.EnvironmentVarGuard() as env:
+                env['PATH'] = os.pathsep
+                with unittest.mock.patch('distutils.spawn.os.confstr',
+                                         return_value='', create=True), \
+                     unittest.mock.patch('distutils.spawn.os.defpath', ''):
+                    rv = find_executable(program)
+                    self.assertIsNone(rv)
+
+                    # look in current directory
+                    with test_support.change_cwd(tmp_dir):
+                        rv = find_executable(program)
+                        self.assertEqual(rv, program)
+
+            # missing PATH: test os.confstr("CS_PATH") and os.defpath
+            with test_support.EnvironmentVarGuard() as env:
+                env.pop('PATH', None)
+
+                # without confstr
+                with unittest.mock.patch('distutils.spawn.os.confstr',
+                                         side_effect=ValueError,
+                                         create=True), \
+                     unittest.mock.patch('distutils.spawn.os.defpath',
+                                         tmp_dir):
+                    rv = find_executable(program)
+                    self.assertEqual(rv, filename)
 
+                # with confstr
+                with unittest.mock.patch('distutils.spawn.os.confstr',
+                                         return_value=tmp_dir, create=True), \
+                     unittest.mock.patch('distutils.spawn.os.defpath', ''):
                     rv = find_executable(program)
                     self.assertEqual(rv, filename)