fix: Fallback to temp dir if cache does not exist

This fixes a misbehaviour in environments where `XDG_CACHE_DIR` is not
set and `~/.cache` does not exist, e.g. in the docker image. We simply
attempt to create a temporary directory which will be registered with
`atexit` to be deleted on exit using `shutil.rmtree()`. A Python 3
approach for future would be to use a `TemporaryDirectory` instead which
would get cleaned up when the held object gets dropped out of the TLS
stack.

It is still possible for the `context.cache_dir` to be `None` if the
default directory exists but is not writable, which means some code
which uses this property is liable to blow up in such a circumstance.

pwnlib/context/__init__.py

@@ -7,17 +7,22 @@
 from __future__ import absolute_import
 from __future__ import division
 
+import atexit
 import collections
+import errno
 import functools
 import logging
 import os
+import os.path
 import platform
+import shutil
 import six
 import socket
 import stat
 import string
 import subprocess
 import sys
+import tempfile
 import threading
 import time
 
@@ -335,7 +340,7 @@ class ContextType(object):
     # Setting any properties on a ContextType object will throw an
     # exception.
     #
-    __slots__ = '_tls',
+    __slots__ = ('_tls', )
 
     #: Default values for :class:`pwnlib.context.ContextType`
     defaults = {
@@ -346,6 +351,10 @@ class ContextType(object):
         'binary': None,
         'bits': 32,
         'buffer_size': 4096,
+        'cache_dir_base': os.environ.get(
+            'XDG_CACHE_HOME',
+            os.path.join(os.path.expanduser('~'), '.cache')
+        ),
         'cyclic_alphabet': string.ascii_lowercase.encode(),
         'cyclic_size': 4,
         'delete_corefiles': False,
@@ -1213,6 +1222,21 @@ def buffer_size(self, size):
         """
         return int(size)
 
+    @_validator
+    def cache_dir_base(self, new_base):
+        """Base directory to use for caching content.
+
+        Changing this to a different value will clear the `cache_dir` path
+        stored in TLS since a new path will need to be generated to respect the
+        new `cache_dir_base` value.
+        """
+
+        if new_base != self.cache_dir_base:
+            del self._tls["cache_dir"]
+        if os.access(new_base, os.F_OK) and not os.access(new_base, W_OK):
+            raise OSError(errno.EPERM, "Cache base dir is not writable")
+        return new_base
+
     @property
     def cache_dir(self):
         """Directory used for caching data.
@@ -1232,27 +1256,47 @@ def cache_dir(self):
             >>> cache_dir == context.cache_dir
             True
         """
-        xdg_cache_home = os.environ.get('XDG_CACHE_HOME') or \
-                         os.path.join(os.path.expanduser('~'), '.cache')
-
-        if not os.access(xdg_cache_home, os.W_OK):
-            return None
-
-        cache = os.path.join(xdg_cache_home, '.pwntools-cache-%d.%d' % sys.version_info[:2])
-
-        if not os.path.exists(cache):
-            try:
-                os.mkdir(cache)
-            except OSError:
-                return None
+        try:
+            # If the TLS already has a cache directory path, we return it
+            # without any futher checks since it must have been valid when it
+            # was set and if that has changed, hiding the TOCTOU here would be
+            # potentially confusing
+            return self._tls["cache_dir"]
+        except KeyError:
+            pass
 
-        # Some wargames e.g. pwnable.kr have created dummy directories
-        # which cannot be modified by the user account (owned by root).
-        if not os.access(cache, os.W_OK):
+        # Attempt to create a Python version specific cache dir and its parents
+        cache_dirname = '.pwntools-cache-%d.%d' % sys.version_info[:2]
+        cache_dirpath = os.path.join(self.cache_dir_base, cache_dirname)
+        try:
+            os.makedirs(cache_dirpath)
+        except OSError as exc:
+            # If we failed for any reason other than the cache directory
+            # already existing then we'll fall back to a temporary directory
+            # object which doesn't respect the `cache_dir_base`
+            if exc.errno != errno.EEXIST:
+                try:
+                    cache_dirpath = tempfile.mkdtemp(prefix=".pwntools-tmp")
+                except IOError as exc:
+                    # This implies no good candidates for temporary files so we
+                    # have to return `None`
+                    return None
+                else:
+                    # Ensure the temporary cache dir is cleaned up on exit. A
+                    # `TemporaryDirectory` would do this better upon garbage
+                    # collection but this is necessary for Python 2 support.
+                    atexit.register(shutil.rmtree, cache_dirpath)
+        # By this time we have a cache directory which exists but we don't know
+        # if it is actually writable. Some wargames e.g. pwnable.kr have
+        # created dummy directories which cannot be modified by the user
+        # account (owned by root).
+        if os.access(cache_dirpath, os.W_OK):
+            # Stash this in TLS for later reuse
+            self._tls["cache_dir"] = cache_dirpath
+            return cache_dirpath
+        else:
             return None
 
-        return cache
-
     @_validator
     def delete_corefiles(self, v):
         """Whether pwntools automatically deletes corefiles after exiting.