Bug 94490 – Web Inspector: After overriding user-agent via dev-tools, some resources still communicate using the default one

• Component: Web Inspector (Deprecated)
• Status: RESOLVED INVALID

[Ohad Kravchick]

Created attachment 159456 [details]
An example of the bug, using node and npm

When overriding the default user-agent via developer tools, all resources being requested from the HTML page are using the specified user-agent.
However, if the page uses the offline web application feature (aka cache.manifest - http://www.whatwg.org/specs/web-apps/current-work/multipage/offline.html), then cache updates, which are triggered automatically with every launch of the page, use the default user-agent, both for retrieving the cache.manifest file and the other resources (HTML, CSS, JS, Fonts, etc.). Also, the favicon.ico is being fetched using the default user-agent.
This is important to fix for when debugging mobile sites: you change your user-agent to get to a mobile site, but then the cache.manifest check and all related resources are using the desktop user-agent for fetching updates, which might get a different result (302 rather than 200, or simply a desktop site's resource as oppose to a mobile site's one). This might cause a cache update for a cached app to fail in scenarios where 302 is returned or when different resources exist for the desktop site.
When overriding the user-agent, all resources should use it.

A work around for upgrading a cached app is to simply clear the cache, although this inhibit from debugging the upgrade sequence.

The attachment is a node project designed to help troubleshoot this issue. You'll need to have node and npm installed. Then, run "npm start" to kick-off the server on port 8088. Then browse to either http://localhost:8088/cache.html to get a cache.manifest driven HTML file or http://localhost:8088/no_cache.html to get a normal page. For each hit, the server spits out the user-agent used in the request. You can try hitting both URLs with and without DevTools' user-agent override and see the behavior. If you want to "push an update" to the cached app, run "./update_files.sh".