Changed teardown error handling to be more reliable.

CHANGES

@@ -59,6 +59,9 @@ Release date to be decided.
   strongly discouraged as the interface was flawed.
 - Python requirements changed: requiring Python 2.6 or 2.7 now to prepare
   for Python 3.3 port.
+- Changed how the teardown system is informed about exceptions.  This is now
+  more reliable in case something handles an exception halfway through
+  the error handling process.
 
 Version 0.9
 -----------

flask/app.py

@@ -1510,6 +1510,16 @@ class Flask(_PackageBoundObject):
         rv.allow.update(methods)
         return rv
 
+    def should_ignore_error(self, error):
+        """This is called to figure out if an error should be ignored
+        or not as far as the teardown system is concerned.  If this
+        function returns `True` then the teardown handlers will not be
+        passed the error.
+
+        .. versionadded:: 0.10
+        """
+        return False
+
     def make_response(self, rv):
         """Converts the return value from a view function to a real
         response object that is an instance of :attr:`response_class`.
@@ -1790,12 +1800,20 @@ class Flask(_PackageBoundObject):
                                a list of headers and an optional
                                exception context to start the response
         """
-        with self.request_context(environ):
+        ctx = self.request_context(environ)
+        ctx.push()
+        error = None
+        try:
             try:
                 response = self.full_dispatch_request()
             except Exception as e:
+                error = e
                 response = self.make_response(self.handle_exception(e))
             return response(environ, start_response)
+        finally:
+            if self.should_ignore_error(error):
+                error = None
+            ctx.auto_pop(error)
 
     @property
     def modules(self):

flask/ctx.py

@@ -352,6 +352,13 @@ class RequestContext(object):
         if app_ctx is not None:
             app_ctx.pop(exc)
 
+    def auto_pop(self, exc):
+        if self.request.environ.get('flask._preserve_context') or \
+           (exc is not None and self.app.preserve_context_on_exception):
+            self.preserved = True
+        else:
+            self.pop(exc)
+
     def __enter__(self):
         self.push()
         return self
@@ -362,11 +369,7 @@ class RequestContext(object):
         # access the request object in the interactive shell.  Furthermore
         # the context can be force kept alive for the test client.
         # See flask.testing for how this works.
-        if self.request.environ.get('flask._preserve_context') or \
+        self.auto_pop(exc_value)
-           (tb is not None and self.app.preserve_context_on_exception):
-            self.preserved = True
-        else:
-            self.pop(exc_value)
 
     def __repr__(self):
         return '<%s \'%s\' [%s] of %s>' % (