Performance regression in 3.9

Environment

• Chrome 83, Firefox 78 (Windows 10) and Edge (see forum post).
• Adblock Plus 3.9 and Adblock (see forum post and forum post).

How to reproduce

• Add the custom filter @@||*$document

Observed behavior

• The following error message is shown:
  • Chrome: "Waiting for extension Adblock Plus"
  • Firefox: "A script in the extension 'Adblock Plus - free ad blocker' is causing Firefox to slow down."
• Pages aren't loading.

Expected behavior

• No error should be shown.
• Pages should be loading without delay.

Further information

Since the pattern of this filter is "", the condition index != -1 in the while-statement in adblockpluscore/lib/filterClasses.js is never true, causing it to loop indefinitely.

In addition to fixing the handling of the empty string, we should probably also add a second exit condition, to avoid getting stuck, if the index is greater than expected (e.g. index >= location.length).

Notes

• Unable to open settings page to remove filter again (see forum post and forum post).
• See also related forum threads:
  • https://adblockplus.org/forum/viewtopic.php?f=1&t=79646
  • https://adblockplus.org/forum/viewtopic.php?f=10&t=79648
• The regression was introduced with this commit: https://gitlab.com/eyeo/adblockplus/adblockpluscore/-/merge_requests/345/diffs