Error loading from the disk cache pages obtained by blocking remote fonts.

[SCmkQrK1NhwcWoPJF3FImaqHnG2PqPLWBt2GtSU]

Prerequisites

• [ 🗸 ] I verified that this is not a filter issue
  • Filter issues MUST be reported at filter issue tracker
• [ 🗸 ] This is not a support issue or a question
  • Support issues and questions are handled at /r/uBlockOrigin
• [ 🗸 ] I performed a cursory search of the issue tracker to avoid opening a duplicate issue
  • Your issue may already be reported.
• I tried to reproduce the issue when...
  • [ 🗸 ] uBlock Origin is the only extension
  • [ 🗸 ] uBlock Origin with default lists/settings
  • [ 🗸 ] using a new, unmodified browser profile
• [ 🗸 ] I am running the latest version of uBlock Origin
• [ 🗸 ] I checked the documentation to understand that the issue I report is not a normal behavior

Description

When uBlock blocks remote fonts, the downloaded pages (except images) in the disk cache are not stored correctly: they are not then loaded offline, but should be loaded.

A specific URL where the issue occurs

https://example.com/

Steps to Reproduce

1. Create a new clean Firefox profile.
2. Install uBlock Origin (https://addons.mozilla.org/firefox/addon/ublock-origin/).
3. In uBlock Origin, enable "no remote fonts".
4. Go online mode and download a page (not an image), for example, https://example.com/ (checked). (I'm using localhost.)
5. Go offline mode and hold the browser in it.
6. The option "no remote fonts" can be turned off or not to change – it does not matter.
7. Reload the page opened on step 4 from the disk cache. 2 options: a and b:
   7.a. Re-open the tab with this page.
   7.b.1. "Reload the page bypassing the cache" (Ctrl+F5). (The result is not important: in fact, in offline mode, this will reload the page from the disk cache without checking it.)
   7.b.2. "Reload this Page with cache" (F5).
   The page will not load from the cache, but will report a "browser is offline mode" error. But the page should load.
   Disregarding the "no remote fonts" is changing the case, but I think that the blocking is needed. Cache is also needed.

Expected behavior:

The page will load from the cache.

Actual behavior:

The page will not load from the cache.

Your environment

• uBlock Origin version: 1.16.20.
• Browser Name and version: Firefox 56.
• Operating System and version: Linux Ubuntu 18.

[gorhill]

This is unfortunately a side effect of the fix to #42 -- in which the workaround suggested by Firefox devs in https://bugzilla.mozilla.org/show_bug.cgi?id=1376932 was used.

[gorhill]

I am experimenting and it seems if I just use no-cache instead of no-cache, no-store, must-revalidate, I still cannot reproduce #42, and that also seems to fix the offline issue reported here.

[gorhill]

I am going to revert the fix here -- the side effects are undesirable for whoever does not care to work in offline mode.

Since either ways cause side effects which may not be acceptable to group of users, I will add an advanced setting to let users configure how uBO should affect caching for the purpose of dealing with https://bugzilla.mozilla.org/show_bug.cgi?id=1376932.

The default value will be the one used before, i.e. the one causing the issue here.

New advanced setting: cacheControlForFirefox1376932.

Default value: no-cache, no-store, must-revalidate.

If using no-cache, no-store, must-revalidate:

• Undesirable side effect: documents themselves for which uBO has to inject CSP directives as a result of filters/ruleset won't be available offline.

If using no-cache:

• Undesirable side effect: One will need to explicitly cache-bypass reload a page each time uBO has to inject CSP directives as a result of filters/ruleset. Note that such cache-bypass reload does not affect only the document itself, but also all secondary resources inside that document.

This new advanced setting will be removed once https://bugzilla.mozilla.org/show_bug.cgi?id=1376932 is fixed.

Firefox bug has been closed. I will like to test, but don't see a way to configure cacheControlForFirefox1376932 to not be applied.