python · ambv · Jul 12, 2021 · Feb 6, 2021 · Feb 6, 2021 · Feb 6, 2021
diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
@@ -92,6 +92,43 @@ def test_base_exception(self):
         lst = traceback.format_exception_only(e.__class__, e)
         self.assertEqual(lst, ['KeyboardInterrupt\n'])
 
+    def test_traceback_context_recursionerror(self):
+        # Test that for long traceback chains traceback does not itself
        # raise a recursion error while printing (Issue43048)
+
+        # Calling f() creates a stack-overflowing __context__ chain.
+        def f():
+            try:
+                raise ValueError('hello')
+            except ValueError:
+                f()
+
+        try:
+            f()
+        except RecursionError:
+            exc_info = sys.exc_info()
+
+        traceback.format_exception(exc_info[0], exc_info[1], exc_info[2])
+
+    def test_traceback_cause_recursionerror(self):
+        # Same as test_traceback_context_recursionerror, but with
+        # a __cause__ chain.
+
+        def f():
+            e = None
+            try:
+                f()
+            except Exception as exc:
+                e = exc
+            raise Exception from e
+
+        try:
+            f()
+        except Exception:
+            exc_info = sys.exc_info()
+
+        traceback.format_exception(exc_info[0], exc_info[1], exc_info[2])
+
     def test_format_exception_only_bad__str__(self):
         class X(Exception):
             def __str__(self):

diff --git a/Lib/traceback.py b/Lib/traceback.py
@@ -476,34 +476,50 @@ def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
         _seen.add(id(exc_value))
         # Gracefully handle (the way Python 2.4 and earlier did) the case of
         # being called with no type or value (None, None, None).
+        self.__suppress_context__ = False
+        self._truncated = False
         if (exc_value and exc_value.__cause__ is not None
             and id(exc_value.__cause__) not in _seen):
-            cause = TracebackException(
-                type(exc_value.__cause__),
-                exc_value.__cause__,
-                exc_value.__cause__.__traceback__,
-                limit=limit,
-                lookup_lines=False,
-                capture_locals=capture_locals,
-                _seen=_seen)
+            try:
+                cause = TracebackException(
+                    type(exc_value.__cause__),
+                    exc_value.__cause__,
+                    exc_value.__cause__.__traceback__,
+                    limit=limit,
+                    lookup_lines=False,
+                    capture_locals=capture_locals,
+                    _seen=_seen)
+            except RecursionError:
+                # The recursive call to the constructor in the try above
+                # may result in a stack
8000
 overflow for long exception chains,
+                # so we must truncate.
+                self._truncated = True
+                cause = None
+                # Suppress the context during printing, so that even when
+                # a cause is truncated we don't print the context.
+                self.__suppress_context__ = True
         else:
             cause = None
         if (exc_value and exc_value.__context__ is not None
             and id(exc_value.__context__) not in _seen):
-            context = TracebackException(
-                type(exc_value.__context__),
-                exc_value.__context__,
-                exc_value.__context__.__traceback__,
-                limit=limit,
-                lookup_lines=False,
-                capture_locals=capture_locals,
-                _seen=_seen)
+            try:
+                context = TracebackException(
+                    type(exc_value.__context__),
+                    exc_value.__context__,
+                    exc_value.__context__.__traceback__,
+                    limit=limit,
+                    lookup_lines=False,
+                    capture_locals=capture_locals,
+                    _seen=_seen)
+            except RecursionError:
+                self._truncated = True
+                context = None
         else:
             context = None
         self.__cause__ = cause
         self.__context__ = context
-        self.__suppress_context__ = \
-            exc_value.__suppress_context__ if exc_value else False
+        self.__suppress_context__ = self.__suppress_context__ or (
+            exc_value.__suppress_context__ if exc_value else False)
         # TODO: locals.
         self.stack = StackSummary.extract(
             walk_tb(exc_traceback), limit=limit, lookup_lines=lookup_lines,
@@ -620,6 +636,10 @@ def format(self, *, chain=True):
                 not self.__suppress_context__):
                 yield from self.__context__.format(chain=chain)
                 yield _context_message
+            if self._truncated:
+                yield (
+                    'Chained exceptions have been truncated to avoid '
+                    'stack overflow in traceback formatting:\n')
         if self.stack:
             yield 'Traceback (most recent call last):\n'
             yield from self.stack.format()

diff --git a/Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst b/Misc/NEWS.d/next/Library/2021-02-06-05-34-01.bpo-43048.yCPUmo.rst
@@ -0,0 +1 @@
+When traceback is formatting a RecursionError with a long __context__ chain it runs the risk of itself raising a RecursionError. This patch catches the corresponding recursive call's RecursionError, truncates the traceback and emits a warning instead.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		When traceback is formatting a RecursionError with a long __context__ chain it runs the risk of itself raising a RecursionError. This patch catches the corresponding recursive call's RecursionError, truncates the traceback and emits a warning instead.
Copy link Member iritkatriel Feb 8, 2021 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. This is ambiguous because there is also traceback formatting code in the interpreter. Also probably too long, and mentions context but not cause. This is probably enough to indicate the impact (and there are always more details in the bpo): Handle `RecursionError`s in :class:`~traceback.TracebackException`'s constructor, so that long exceptions chains are truncated instead of causing traceback formatting to fail. Copy link Author vlad17 Feb 8, 2021 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. done. thanks for the patience!