http-app: use filter predicate to filter out bad backends submitted into the app.

vcsserver/http_main.py

@@ -199,7 +199,8 b' class HTTPApplication(object):'
         self.config.add_view(self.hg_proxy(), route_name='hg_proxy')
         self.config.add_view(self.git_proxy(), route_name='git_proxy')
         self.config.add_view(
-            self.vcs_view, route_name='vcs', renderer='msgpack')
+            self.vcs_view, route_name='vcs', renderer='msgpack',
+            custom_predicates=[self.is_vcs_view])
 
         self.config.add_view(self.hg_stream(), route_name='stream_hg')
         self.config.add_view(self.git_stream(), route_name='stream_git')
@@ -215,7 +216,6 b' class HTTPApplication(object):'
         self.config.add_view(
             self.general_error_handler, context=Exception)
 
-
     def wsgi_app(self):
         return self.config.make_wsgi_app()
 
@@ -351,6 +351,14 b' class HTTPApplication(object):'
                 return app(environ, start_response)
             return _git_stream
 
+    def is_vcs_view(self, context, request):
+        """
+        View predicate that returns true if given backend is supported by
+        defined remotes.
+        """
+        backend = request.matchdict.get('backend')
+        return backend in self._remotes
+
     def is_vcs_exception(self, context, request):
         """
         View predicate that returns true if the context object is a VCS