bpo-25532: Protect against infinite loops in inspect.unwrap() (#1717)

Some objects (like test mocks) auto-generate new objects on
attribute access, which can lead to an infinite loop in
inspect.unwrap().

Ensuring references are retained to otherwise temporary objects
and capping the size of the memo dict turns this case into a
conventional exception instead.

diff --git a/Lib/inspect.py b/Lib/inspect.py
index 9c072eb0747fbd10052383a8f04d5434c4be7727..9a843d6420e2e09133c270296df8fc38bc4cc1e9 100644 (file)
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -505,13 +505,16 @@ def unwrap(func, *, stop=None):
         def _is_wrapper(f):
             return hasattr(f, '__wrapped__') and not stop(f)
     f = func  # remember the original func for error reporting
-    memo = {id(f)} # Memoise by id to tolerate non-hashable objects
+    # Memoise by id to tolerate non-hashable objects, but store objects to
+    # ensure they aren't destroyed, which would allow their IDs to be reused.
+    memo = {id(f): f}
+    recursion_limit = sys.getrecursionlimit()
     while _is_wrapper(func):
         func = func.__wrapped__
         id_func = id(func)
-        if id_func in memo:
+        if (id_func in memo) or (len(memo) >= recursion_limit):
             raise ValueError('wrapper loop when unwrapping {!r}'.format(f))
-        memo.add(id_func)
+        memo[id_func] = func
     return func
 
 # -------------------------------------------------- source code extraction

diff --git a/Lib/test/test_inspect.py b/Lib/test/test_inspect.py
index c55efd69425173f016ab19a21fff605134f7617e..350d5dbd776ac2ff69499cb927ee104bb160402c 100644 (file)
--- a/Lib/test/test_inspect.py
+++ b/Lib/test/test_inspect.py
@@ -3554,6 +3554,19 @@ class TestSignatureDefinitions(unittest.TestCase):
                 self.assertIsNone(obj.__text_signature__)
 
 
+class NTimesUnwrappable:
+    def __init__(self, n):
+        self.n = n
+        self._next = None
+
+    @property
+    def __wrapped__(self):
+        if self.n <= 0:
+            raise Exception("Unwrapped too many times")
+        if self._next is None:
+            self._next = NTimesUnwrappable(self.n - 1)
+        return self._next
+
 class TestUnwrap(unittest.TestCase):
 
     def test_unwrap_one(self):
@@ -3609,6 +3622,11 @@ class TestUnwrap(unittest.TestCase):
             __wrapped__ = func
         self.assertIsNone(inspect.unwrap(C()))
 
+    def test_recursion_limit(self):
+        obj = NTimesUnwrappable(sys.getrecursionlimit() + 1)
+        with self.assertRaisesRegex(ValueError, 'wrapper loop'):
+            inspect.unwrap(obj)
+
 class TestMain(unittest.TestCase):
     def test_only_source(self):
         module = importlib.import_module('unittest')

diff --git a/Misc/NEWS b/Misc/NEWS
index 5b41dcf61dfa58026416c33a13f26ffa9fb3961f..0e32d480b23c9b8cf39d570d9701e8eac229ba7f 100644 (file)
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -995,6 +995,10 @@ Library
 - Issue #29581: ABCMeta.__new__ now accepts ``**kwargs``, allowing abstract base
   classes to use keyword parameters in __init_subclass__. Patch by Nate Soares.
 
+- Issue #25532: inspect.unwrap() will now only try to unwrap an object
+  sys.getrecursionlimit() times, to protect against objects which create a new
+  object on every attribute access.
+
 Windows
 -------