mitogen: Workaround CPython importlib PermissionError when cwd is unreadable

On macOS when using a become plugin as an unprivileged user, to another
unprivileged user it is likely that the current working directory can't be
read. In this case os.cwd() raises PermissionError.

On versions of Python currently in the wild (March 2024, CPython <= 3.13) if
any non-builtin or non-frozen module (e.g. zlib, base64) is imported then
`importlib._bootstrap_external.PathFinder._path_importer_cache()` attempts to
call os.cwd() without catching PermissionError.

The previous comment about needing an extra .encode() appears to be wrong,
atleast for Python 3.x >= 3.6.

Command size increased by 54 bytes, bootstrap by 804 bytes. Changed from
codecs module to binascii & zlib because they're extensions, and importing
them triggers fewer supporting imports (e.g. encodings module).

Before

```
✗ ./preamble_size.py
SSH command size: 705
Bootstrap (mitogen.core) size: 17078 (16.68KiB)

                              Original          Minimized           Compressed
mitogen.parent            97884 95.6KiB  50515 49.3KiB 51.6%  12727 12.4KiB
13.0%
mitogen.fork               8436  8.2KiB   4130  4.0KiB 49.0%   1648  1.6KiB
19.5%
mitogen.ssh               10892 10.6KiB   6952  6.8KiB 63.8%   2113  2.1KiB
19.4%
mitogen.sudo              12089 11.8KiB   5924  5.8KiB 49.0%   2249  2.2KiB
18.6%
mitogen.select            12325 12.0KiB   2929  2.9KiB 23.8%    964  0.9KiB
7.8%
mitogen.service           41699 40.7KiB  22477 22.0KiB 53.9%   5885  5.7KiB
14.1%
mitogen.fakessh           15577 15.2KiB   7989  7.8KiB 51.3%   2623  2.6KiB
16.8%
mitogen.master            51398 50.2KiB  25715 25.1KiB 50.0%   6886  6.7KiB
13.4%
```

After

```
✗ ./preamble_size.py
SSH command size: 759
Bootstrap (mitogen.core) size: 17882 (17.46KiB)

                              Original          Minimized           Compressed
mitogen.parent            98173 95.9KiB  50571 49.4KiB 51.5%  12747 12.4KiB
13.0%
mitogen.fork               8436  8.2KiB   4130  4.0KiB 49.0%   1648  1.6KiB
19.5%
mitogen.ssh               10892 10.6KiB   6952  6.8KiB 63.8%   2113  2.1KiB
19.4%
mitogen.sudo              12089 11.8KiB   5924  5.8KiB 49.0%   2249  2.2KiB
18.6%
mitogen.select            12325 12.0KiB   2929  2.9KiB 23.8%    964  0.9KiB
7.8%
mitogen.service           41699 40.7KiB  22477 22.0KiB 53.9%   5885  5.7KiB
14.1%
mitogen.fakessh           15577 15.2KiB   7989  7.8KiB 51.3%   2623  2.6KiB
16.8%
mitogen.master            56116 54.8KiB  29427 28.7KiB 52.4%   7627  7.4KiB
13.6%
```

Fixes #885
Refs https://github.com/python/cpython/issues/115911

docs/changelog.rst

@@ -21,6 +21,8 @@ Unreleased
 ----------
 
 * :gh:issue:`987` Support Python 3.11
+* :gh:issue:`885` Fix :py:exc:`PermissionError` in :py:mod:`importlib` when
+  becoming an unprivileged user with Python 3.x
 
 
 v0.3.4 (2023-07-02)

mitogen/core.py

@@ -34,6 +34,34 @@ non-essential code in order to reduce its size, since it is also serves as the
 bootstrap implementation sent to every new slave context.
 """
 
+import sys
+try:
+    import _frozen_importlib_external
+except ImportError:
+    pass
+else:
+    class MonkeyPatchedPathFinder(_frozen_importlib_external.PathFinder):
+        """
+        Meta path finder for sys.path and package __path__ attributes.
+
+        Patched for https://github.com/python/cpython/issues/115911.
+        """
+        @classmethod
+        def _path_importer_cache(cls, path):
+            if path == '':
+                try:
+                    path = _frozen_importlib_external._os.getcwd()
+                except (FileNotFoundError, PermissionError):
+                    return None
+            return super()._path_importer_cache(path)
+
+    if sys.version_info[:2] <= (3, 12):
+        for i, mpf in enumerate(sys.meta_path):
+            if mpf is _frozen_importlib_external.PathFinder:
+                sys.meta_path[i] = MonkeyPatchedPathFinder
+        del i, mpf
+
+
 import binascii
 import collections
 import encodings.latin_1
@@ -49,7 +77,6 @@ import pstats
 import signal
 import socket
 import struct
-import sys
 import syslog
 import threading
 import time

mitogen/parent.py

@@ -34,7 +34,7 @@ sent to any child context that is due to become a parent, due to recursive
 connection.
 """
 
-import codecs
+import binascii
 import errno
 import fcntl
 import getpass
@@ -1405,6 +1405,7 @@ class Connection(object):
     # file descriptor 0 as 100, creates a pipe, then execs a new interpreter
     # with a custom argv.
     #   * Optimized for minimum byte count after minification & compression.
+    #     The script preamble_size.py measures this.
     #   * 'CONTEXT_NAME' and 'PREAMBLE_COMPRESSED_LEN' are substituted with
     #     their respective values.
     #   * CONTEXT_NAME must be prefixed with the name of the Python binary in
@@ -1449,7 +1450,7 @@ class Connection(object):
             os.environ['ARGV0']=sys.executable
             os.execl(sys.executable,sys.executable+'(mitogen:CONTEXT_NAME)')
         os.write(1,'MITO000\n'.encode())
-        C=_(os.fdopen(0,'rb').read(PREAMBLE_COMPRESSED_LEN),'zip')
+        C=zlib.decompress(os.fdopen(0,'rb').read(PREAMBLE_COMPRESSED_LEN))
         fp=os.fdopen(W,'wb',0)
         fp.write(C)
         fp.close()
@@ -1481,16 +1482,16 @@ class Connection(object):
         source = source.replace('PREAMBLE_COMPRESSED_LEN',
                                 str(len(preamble_compressed)))
         compressed = zlib.compress(source.encode(), 9)
-        encoded = codecs.encode(compressed, 'base64').replace(b('\n'), b(''))
-        # We can't use bytes.decode() in 3.x since it was restricted to always
-        # return unicode, so codecs.decode() is used instead. In 3.x
-        # codecs.decode() requires a bytes object. Since we must be compatible
-        # with 2.4 (no bytes literal), an extra .encode() either returns the
-        # same str (2.x) or an equivalent bytes (3.x).
+        encoded = binascii.b2a_base64(compressed).replace(b('\n'), b(''))
+
+        # Just enough to decode, decompress, and exec the first stage.
+        # Priorities: wider compatibility, faster startup, shorter length.
+        # `import os` here, instead of stage 1, to save a few bytes.
+        # `sys.path=...` for https://github.com/python/cpython/issues/115911.
         return self.get_python_argv() + [
             '-c',
-            'import codecs,os,sys;_=codecs.decode;'
-            'exec(_(_("%s".encode(),"base64"),"zip"))' % (encoded.decode(),)
+            'import sys;sys.path=[p for p in sys.path if p];import binascii,os,zlib;'
+            'exec(zlib.decompress(binascii.a2b_base64("%s")))' % (encoded.decode(),),
         ]
 
     def get_econtext_config(self):