pkg_resources: do not call stat() and access()

The current code in find_on_path is doing a lot of stat() calls which are
actually useless and prone to race conditions.

As described in Python documentation
(https://docs.python.org/3/library/os.html#os.access), os.access must not be
used before opening a file. Same goes for a directory.

This patch removes those checks by handling exceptions correctly when using
os.listdir() instead, which improves pkg_resources import time.

pkg_resources/__init__.py

@@ -34,6 +34,7 @@ import platform
 import collections
 import plistlib
 import email.parser
+import errno
 import tempfile
 import textwrap
 import itertools
@@ -80,6 +81,11 @@ __import__('pkg_resources.extern.packaging.markers')
 if (3, 0) < sys.version_info < (3, 3):
     raise RuntimeError("Python 3.3 or later is required")
 
+if six.PY2:
+    # Those builtin exceptions are only defined in Python 3
+    PermissionError = None
+    NotADirectoryError = None
+
 # declare some globals that will be defined later to
 # satisfy the linters.
 require = None
@@ -2008,7 +2014,6 @@ def find_on_path(importer, path_item, only=False):
     """Yield distributions accessible on a sys.path directory"""
     path_item = _normalize_cached(path_item)
 
-    if os.path.isdir(path_item) and os.access(path_item, os.R_OK):
     if _is_unpacked_egg(path_item):
         yield Distribution.from_filename(
             path_item, metadata=PathMetadata(
@@ -2016,8 +2021,20 @@ def find_on_path(importer, path_item, only=False):
             )
         )
     else:
+        try:
+            entries = os.listdir(path_item)
+        except (PermissionError, NotADirectoryError):
+            return
+        except OSError as e:
+            # Ignore the directory if does not exist, not a directory or we
+            # don't have permissions
+            if (e.errno in (errno.ENOTDIR, errno.EACCES, errno.ENOENT)
+                # Python 2 on Windows needs to be handled this way :(
+               or hasattr(e, "winerror") and e.winerror == 267):
+                return
+            raise
         # scan for .egg and .egg-info in directory
-            path_item_entries = _by_version_descending(os.listdir(path_item))
+        path_item_entries = _by_version_descending(entries)
         for entry in path_item_entries:
             lower = entry.lower()
             if lower.endswith('.egg-info') or lower.endswith('.dist-info'):