Defer response headers processing to a finished callback

CHANGES.txt

@@ -1,5 +1,9 @@
 unreleased
 ----------
+- Headers panel defers its processing to a finished callback. This is best
+  effort of displaying actual headers, since they could be modified by
+  a response callback or another finished callback.
+  See https://github.com/Pylons/pyramid_debugtoolbar/pull/310
 
 - Debug squashed exceptions! If you register an exception view for an exception
   it will render a response. The toolbar will see the squashed exception and

CONTRIBUTORS.txt

@@ -121,3 +121,4 @@ Contributors
 - Kashif Iftikhar, 2016-07-01
 - Dan Clark, 2017-05-22
 - Jens Carl, 2017-05-22
+- Andrey Tretyakov, 2017-05-27

pyramid_debugtoolbar/panels/headers.py

@@ -15,16 +15,29 @@ class HeaderDebugPanel(DebugPanel):
     nav_title = title
 
     def __init__(self, request):
+        self.request = request
         self.request_headers = [
             (text_(k), text_(v)) for k, v in sorted(request.headers.items())
         ]
 
     def process_response(self, response):
+        def finished_callback(request):
+            self.process_response_deferred(response)
+
+        def prepare_finished_callback(request):
+            """ Best effort to be the last in case there are other callbacks
+            mutating the response """
+            self.request.add_finished_callback(finished_callback)
+
+        self.request.add_finished_callback(prepare_finished_callback)
+        self.data = {'request_headers': self.request_headers,
+                     'response_headers': []}
+
+    def process_response_deferred(self, response):
         response_headers = [
             (text_(k), text_(v)) for k, v in sorted(response.headerlist)
         ]
-        self.data = {'request_headers': self.request_headers,
-                     'response_headers': response_headers}
+        self.data['response_headers'] = response_headers
 
 def includeme(config):
     config.add_debugtoolbar_panel(HeaderDebugPanel)