1376045 - webRequest.onHeadersReceived emitted twice on 304 Not Modified response

Status: UNCONFIRMED

(WebExtensions :: Request Handling, defect, P3)

Description

[Will Elwood (:Will)]

Attachment: bug_304_not_modified_-_2_webrequest.onheadersreceived_events-1.0.0.zip

User Agent: Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:54.0) Gecko/20100101 Firefox/54.0
Build ID: 20170608105825

Steps to reproduce:

1) Extract attached minimal testing webext.
2) > web-ext run --browser-console --start-url https://ftp.mozilla.org/favicon.ico --firefox nightly
3) Ctrl-Shift-E
4) F5


Actual results:

The tab's network monitor confirms the 304 Not Modified response code.
The browser console shows 2 consecutive onHeadersReceived events for the same requestId, 1st with a 304 status code and 2nd with a 200 status code.

Encountered in release channel 54.0 (64-bit) and confirmed in nightly 56.0a1 (2017-06-22) (64-bit), both on Windows 8.1 x64.


Expected results:

The tab's network monitor confirms the 304 Not Modified response code.
The browser console shows only a single onHeadersReceived event, consistent with the webRequest event flow chart shown on MDN.

The same/equivalent steps run in Chrome 59 result in only a single onHeadersReceived event, with status code 200 (again with the tab's network monitor confirming actually receiving a 304 code).