Improve the default code filter's correctness and performance. (#40)

* Improve the default code filter to resolve symlinks

The wrong paths might be used when filtering depending on if the paths
Python is configured to use are actually symlinks on the underlying
system (for example, when /lib64 is a symlink to /lib).

* Significantly improve the default filter's performance with a cache

Resolving symlinks per function call has a large performance penalty.
But there's also no need to constantly recompute the default filter.

monkeytype/config.py

@@ -4,7 +4,9 @@
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
 from contextlib import contextmanager
+import functools
 import os
+import pathlib
 import sys
 import sysconfig
 
@@ -95,14 +97,25 @@ def include_unparsable_defaults(self) -> bool:
     lib_paths.add(
         sysconfig.get_path('stdlib', vars={'installed_base': venv_real_prefix})
     )
-# exclude code objects from frozen importlib, which have a bogus co_filename
-lib_paths.add('<frozen importlib.')
-LIB_PATHS = tuple(p for p in lib_paths if p is not None)
+LIB_PATHS = tuple(pathlib.Path(p).resolve() for p in lib_paths if p is not None)
 
 
+def _startswith(a: pathlib.Path, b: pathlib.Path) -> bool:
+    try:
+        return bool(a.relative_to(b))
+    except ValueError:
+        return False
+
+
+@functools.lru_cache()
 def default_code_filter(code: CodeType) -> bool:
     """A CodeFilter to exclude stdlib and site-packages."""
-    return bool(code.co_filename and not code.co_filename.startswith(LIB_PATHS))
+    # Filter code without a source file
+    if not code.co_filename or code.co_filename[0] == '<':
+        return False
+
+    filename = pathlib.Path(code.co_filename).resolve()
+    return not any(_startswith(filename, lib_path) for lib_path in LIB_PATHS)
 
 
 class DefaultConfig(Config):

tests/test_config.py

@@ -19,7 +19,7 @@ def test_excludes_site_packages(self):
         assert not config.default_code_filter(pytest.skip.__code__)
 
     def test_includes_otherwise(self):
-        assert config.default_code_filter(config.default_code_filter.__code__)
+        assert config.default_code_filter(config.default_code_filter.__wrapped__.__code__)
 
     def test_excludes_frozen_importlib(self):
         assert not config.default_code_filter(_frozen_importlib.spec_from_loader.__code__)