Add more idiomatic safe realpath helper

Author: johnslavik

Lib/pdb.py

@@ -65,44 +65,44 @@
 # NOTE: the actual command documentation is collected from docstrings of the
 # commands and is appended to __doc__ after the class has been defined.
 
-import os
-import io
-import re
-import sys
-import cmd
+import asyncio
+import atexit
 import bdb
-import dis
+import builtins
+import cmd
 import code
+import codeop
+import dis
 import glob
+import inspect
+import io
+import itertools
 import json
-import stat
-import token
-import types
-import atexit
-import codeop
+import linecache
+import os
 import pprint
+import re
+import selectors
 import signal
 import socket
-import typing
-import asyncio
-import inspect
-import weakref
-import builtins
+import stat
+import sys
 import tempfile
 import textwrap
+import threading
+import token
 import tokenize
-import itertools
 import traceback
-import linecache
-import selectors
-import threading
-import _colorize
-import _pyrepl.utils
-
+import types
+import typing
+import weakref
 from contextlib import ExitStack, closing, contextmanager
 from types import CodeType
 from warnings import deprecated
 
+import _colorize
+import _pyrepl.utils
+
 
 class Restart(Exception):
     """Causes a debugger to be restarted for the debugged python program."""
@@ -184,25 +184,34 @@ class _ExecutableTarget:
 class _ScriptTarget(_ExecutableTarget):
     def __init__(self, target):
         if not os.path.exists(target):
-            print(f'Error: {target} does not exist')
+            print(f"Error: {target} does not exist")
             sys.exit(1)
         if os.path.isdir(target):
-            print(f'Error: {target} is a directory')
+            print(f"Error: {target} is a directory")
             sys.exit(1)
 
-        # Use the original path for files like anonymous pipes
-        # from process substitution (GH-142315).
-        realpath = os.path.realpath(target)
-        self._target = realpath if os.path.exists(realpath) else target
+        self._target = self._safe_realpath(target)
 
-        # If safe_path(-P) is not set, sys.path[0] is the directory
+        # If PYTHONSAFEPATH (-P) is not set, sys.path[0] is the directory
         # of pdb, and we should replace it with the directory of the script
         if not sys.flags.safe_path:
             sys.path[0] = os.path.dirname(self._target)
 
     def __repr__(self):
         return self._target
 
+    @staticmethod
+    def _safe_realpath(path):
+        """
+        Return the canonical path (realpath) if it is accessible from the userspace.
+        Otherwise (for example, if the path is a symlink to an anonymous pipe),
+        return the original path.
+
+        See GH-142315.
+        """
+        realpath = os.path.realpath(path)
+        return realpath if os.path.exists(realpath) else path
+
     @property
     def filename(self):
         return self._target