Issue #27123: When an exception is raised within the context being

managed by a contextlib.ExitStack() and one of the exit stack
generators catches and raises it in a chain, do not re-raise the
original exception when exiting, let the new chained one through.
This avoids the PEP 479 bug described in issue25782.

Lib/contextlib.py

@@ -82,6 +82,9 @@ class _GeneratorContextManager(ContextDecorator):
                 # raised inside the "with" statement from being suppressed.
                 return exc is not value
             except RuntimeError as exc:
+                # Don't re-raise the passed in exception. (issue27112)
+                if exc is value:
+                    return False
                 # Likewise, avoid suppressing if a StopIteration exception
                 # was passed to throw() and later wrapped into a RuntimeError
                 # (see PEP 479).