chore(debugging): relax safe dict retrieval (#6024)

The existing safety check around the retrieval of `__dict__` are too
strict and lead to many objects not being captured down to their fields.
This change relaxes the condition on safety by ensuring that we can get
an instance of `__dict__` that is of the _exact_ type `dict`.

## Risks

The example that was found in the FastAPI is probably quite exotic as we
don't expect actual objects to implement a `__dict__` attribute on a
class with potential side-effects.

Author: P403n1x87

ddtrace/debugging/safety.py

@@ -5,10 +5,8 @@
 from typing import Dict
 from typing import Iterator
 from typing import Tuple
-from typing import Type
 
 from ddtrace.internal.safety import get_slots
-from ddtrace.internal.utils.cache import cached
 
 
 GetSetDescriptor = type(type.__dict__["__dict__"])  # type: ignore[index]
@@ -60,20 +58,16 @@ def safe_getitem(obj, index):
     raise TypeError("Type is not indexable collection " + str(type(obj)))
 
 
-@cached()
-def _has_safe_dict(_type):
-    # type: (Type) -> bool
-    try:
-        return type(object.__getattribute__(_type, "__dict__").get("__dict__")) is GetSetDescriptor
-    except AttributeError:
-        return False
-
-
 def _safe_dict(o):
     # type: (Any) -> Dict[str, Any]
-    if _has_safe_dict(type(o)):
-        return object.__getattribute__(o, "__dict__")
-    raise AttributeError("No safe __dict__ attribute")
+    try:
+        __dict__ = object.__getattribute__(o, "__dict__")
+        if type(__dict__) is dict:
+            return __dict__
+    except Exception:
+        pass  # nosec
+
+    raise AttributeError("No safe __dict__")
 
 
 def get_fields(obj):

tests/debugging/test_safety.py

@@ -2,6 +2,8 @@
 
 import inspect
 
+import pytest
+
 from ddtrace.debugging import safety
 
 
@@ -75,3 +77,14 @@ def __init__(self):
 
     assert safety.get_fields(A()) == {"a": "a"}
     assert safety.get_fields(B()) == {"a": "a", "b": "b"}
+
+
+def test_safe_dict():
+    # Found in the FastAPI test suite
+    class Foo(object):
+        @property
+        def __dict__(self):
+            raise NotImplementedError()
+
+    with pytest.raises(AttributeError):
+        safety._safe_dict(Foo())