add incompatibilities to whatsnew

docs/whatsnew-1.9.rst

@@ -46,6 +46,29 @@ Deprecations
 
   See https://github.com/Pylons/pyramid/pull/2854 and https://github.com/Pylons/pyramid/pull/3019
 
+Backward Incompatibilities
+--------------------------
+
+- ``request.exception`` and ``request.exc_info`` will only be set if the
+  response was generated by the EXCVIEW tween. This is to avoid any confusion
+  where a response was generated elsewhere in the pipeline and not in
+  direct relation to the original exception. If anyone upstream wants to
+  catch and render responses for exceptions they should set
+  ``request.exception`` and ``request.exc_info`` themselves to indicate
+  the exception that was squashed when generating the response.
+
+  Similar behavior occurs with
+  :meth:`pyramid.request.Request.invoke_exception_view` in which
+  the exception properties are set to reflect the exception if a response
+  is successfully generated by the method.
+
+  This is a very minor incompatibility. Most tweens right now would give
+  priority to the raised exception and ignore ``request.exception``. This
+  change just improves and clarifies that bookkeeping by trying to be
+  more clear about the relationship between the response and its squashed
+  exception. See https://github.com/Pylons/pyramid/pull/3029 and
+  https://github.com/Pylons/pyramid/pull/3031
+
 Documentation Enhancements
 --------------------------