Fix regression with sys.path filter (#4258)

* Fix regression with sys.path filter

3 files changed, 141 insertions, 30 deletions

diff --git a/ChangeLog b/ChangeLog
index 407b79b3c..c85071544 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -84,6 +84,10 @@ Release date: TBA
 
   Closes #3202
 
+* Fix regression with plugins on PYTHONPATH if latter is cwd
+
+  Closes #4252
+
 
 What's New in Pylint 2.7.2?
 ===========================
diff --git a/pylint/__init__.py b/pylint/__init__.py
index 20bb554d2..7d2341396 100644
--- a/pylint/__init__.py
+++ b/pylint/__init__.py
@@ -55,16 +55,22 @@ def modify_sys_path() -> None:
     CPython issue: https://bugs.python.org/issue33053
 
     - Remove the first entry. This will always be either "" or the working directory
-    - Remove the working directory from the second and third entries. This can
-      occur if PYTHONPATH includes a ":" at the beginning or the end.
+    - Remove the working directory from the second and third entries
+      if PYTHONPATH includes a ":" at the beginning or the end.
       https://github.com/PyCQA/pylint/issues/3636
+      Don't remove it if PYTHONPATH contains the cwd or '.' as the entry will
+      only be added once.
     - Don't remove the working directory from the rest. It will be included
       if pylint is installed in an editable configuration (as the last item).
       https://github.com/PyCQA/pylint/issues/4161
     """
-    sys.path = [
-        p for i, p in enumerate(sys.path) if i > 0 and not (i < 3 and p == os.getcwd())
-    ]
+    sys.path.pop(0)
+    env_pythonpath = os.environ.get("PYTHONPATH", "")
+    cwd = os.getcwd()
+    if env_pythonpath.startswith(":") and env_pythonpath not in (f":{cwd}", ":."):
+        sys.path.pop(0)
+    elif env_pythonpath.endswith(":") and env_pythonpath not in (f"{cwd}:", ".:"):
+        sys.path.pop(1)
 
 
 __all__ = ["__version__"]
diff --git a/tests/test_self.py b/tests/test_self.py
index b3d8927b9..af78f6f3c 100644
--- a/tests/test_self.py
+++ b/tests/test_self.py
@@ -45,7 +45,7 @@ import warnings
 from copy import copy
 from io import StringIO
 from os.path import abspath, dirname, join
-from typing import Generator
+from typing import Generator, Optional
 from unittest import mock
 from unittest.mock import patch
 
@@ -724,61 +724,131 @@ class TestRunTC:
             finally:
                 sys.path = original_path
 
+        @contextlib.contextmanager
+        def test_environ_pythonpath(
+            new_pythonpath: Optional[str],
+        ) -> Generator[None, None, None]:
+            original_pythonpath = os.environ.get("PYTHONPATH")
+            if new_pythonpath:
+                os.environ["PYTHONPATH"] = new_pythonpath
+            elif new_pythonpath is None and original_pythonpath is not None:
+                # If new_pythonpath is None, make sure to delete PYTHONPATH if present
+                del os.environ["PYTHONPATH"]
+            try:
+                yield
+            finally:
+                if original_pythonpath:
+                    os.environ["PYTHONPATH"] = original_pythonpath
+                elif new_pythonpath is not None:
+                    # Only delete PYTHONPATH if new_pythonpath wasn't None
+                    del os.environ["PYTHONPATH"]
+
         with test_sys_path(), patch("os.getcwd") as mock_getcwd:
             cwd = "/tmp/pytest-of-root/pytest-0/test_do_not_import_files_from_0"
             mock_getcwd.return_value = cwd
-
-            paths = [
-                cwd,
-                cwd,
+            default_paths = [
                 "/usr/local/lib/python39.zip",
                 "/usr/local/lib/python3.9",
                 "/usr/local/lib/python3.9/lib-dynload",
                 "/usr/local/lib/python3.9/site-packages",
             ]
+
+            paths = [
+                cwd,
+                *default_paths,
+            ]
             sys.path = copy(paths)
-            modify_sys_path()
-            assert sys.path == paths[2:]
+            with test_environ_pythonpath(None):
+                modify_sys_path()
+            assert sys.path == paths[1:]
+
+            paths = [
+                cwd,
+                cwd,
+                *default_paths,
+            ]
+            sys.path = copy(paths)
+            with test_environ_pythonpath("."):
+                modify_sys_path()
+            assert sys.path == paths[1:]
 
             paths = [
                 cwd,
                 "/custom_pythonpath",
+                *default_paths,
+            ]
+            sys.path = copy(paths)
+            with test_environ_pythonpath("/custom_pythonpath"):
+                modify_sys_path()
+            assert sys.path == paths[1:]
+
+            paths = [
                 cwd,
-                "/usr/local/lib/python39.zip",
-                "/usr/local/lib/python3.9",
-                "/usr/local/lib/python3.9/lib-dynload",
-                "/usr/local/lib/python3.9/site-packages",
+                "/custom_pythonpath",
+                cwd,
+                *default_paths,
             ]
             sys.path = copy(paths)
-            modify_sys_path()
+            with test_environ_pythonpath("/custom_pythonpath:"):
+                modify_sys_path()
             assert sys.path == [paths[1]] + paths[3:]
 
             paths = [
                 "",
                 cwd,
                 "/custom_pythonpath",
-                "/usr/local/lib/python39.zip",
-                "/usr/local/lib/python3.9",
-                "/usr/local/lib/python3.9/lib-dynload",
-                "/usr/local/lib/python3.9/site-packages",
+                *default_paths,
             ]
             sys.path = copy(paths)
-            modify_sys_path()
+            with test_environ_pythonpath(":/custom_pythonpath"):
+                modify_sys_path()
             assert sys.path == paths[2:]
 
             paths = [
-                "",
                 cwd,
-                "/usr/local/lib/python39.zip",
-                "/usr/local/lib/python3.9",
-                "/usr/local/lib/python3.9/lib-dynload",
-                "/usr/local/lib/python3.9/site-packages",
                 cwd,
+                "/custom_pythonpath",
+                *default_paths,
             ]
             sys.path = copy(paths)
-            modify_sys_path()
+            with test_environ_pythonpath(":/custom_pythonpath:"):
+                modify_sys_path()
             assert sys.path == paths[2:]
 
+            paths = [
+                cwd,
+                cwd,
+                *default_paths,
+            ]
+            sys.path = copy(paths)
+            with test_environ_pythonpath(":."):
+                modify_sys_path()
+            assert sys.path == paths[1:]
+            sys.path = copy(paths)
+            with test_environ_pythonpath(f":{cwd}"):
+                modify_sys_path()
+            assert sys.path == paths[1:]
+
+            sys.path = copy(paths)
+            with test_environ_pythonpath(".:"):
+                modify_sys_path()
+            assert sys.path == paths[1:]
+            sys.path = copy(paths)
+            with test_environ_pythonpath(f"{cwd}:"):
+                modify_sys_path()
+            assert sys.path == paths[1:]
+
+            paths = [
+                "",
+                cwd,
+                *default_paths,
+                cwd,
+            ]
+            sys.path = copy(paths)
+            with test_environ_pythonpath(cwd):
+                modify_sys_path()
+            assert sys.path == paths[1:]
+
     @staticmethod
     def test_do_not_import_files_from_local_directory(tmpdir):
         p_astroid = tmpdir / "astroid.py"
@@ -838,7 +908,7 @@ class TestRunTC:
         # https://github.com/PyCQA/pylint/issues/3636
         with tmpdir.as_cwd():
             orig_pythonpath = os.environ.get("PYTHONPATH")
-            os.environ["PYTHONPATH"] = os.environ.get("PYTHONPATH", "") + ":"
+            os.environ["PYTHONPATH"] = f"{(orig_pythonpath or '').strip(':')}:"
             subprocess.check_output(
                 [
                     sys.executable,
@@ -849,8 +919,39 @@ class TestRunTC:
                 ],
                 cwd=str(tmpdir),
             )
-            if orig_pythonpath is not None:
+            if orig_pythonpath:
+                os.environ["PYTHONPATH"] = orig_pythonpath
+            else:
+                del os.environ["PYTHONPATH"]
+
+    @staticmethod
+    def test_import_plugin_from_local_directory_if_pythonpath_cwd(tmpdir):
+        p_plugin = tmpdir / "plugin.py"
+        p_plugin.write("# Some plugin content")
+
+        with tmpdir.as_cwd():
+            orig_pythonpath = os.environ.get("PYTHONPATH")
+            os.environ["PYTHONPATH"] = "."
+            process = subprocess.run(
+                [
+                    sys.executable,
+                    "-m",
+                    "pylint",
+                    "--load-plugins",
+                    "plugin",
+                ],
+                cwd=str(tmpdir),
+                stderr=subprocess.PIPE,
+                check=False,
+            )
+            assert (
+                "AttributeError: module 'plugin' has no attribute 'register'"
+                in process.stderr.decode()
+            )
+            if orig_pythonpath:
                 os.environ["PYTHONPATH"] = orig_pythonpath
+            else:
+                del os.environ["PYTHONPATH"]
 
     def test_allow_import_of_files_found_in_modules_during_parallel_check(self, tmpdir):
         test_directory = tmpdir / "test_directory"