Simplify logic for domains property

[mjethani]

Background

See ​patch #1 and ​patch #2.

With the changes for #6815, #7046, and #7265, the implementation of the domains property getter in the ActiveFilter class has become somewhat complex. Now it can be simplified in the following manner:

1. Always cache the Map object for the domains property using a Cache object (#7285) with some reasonable capacity (1,000 works)
2. Keep the Map object in the ActiveFilter object itself only if domainSource is non-null

In my tests with the above patches, I got the following results:

1. Loading of the subscriptions speeds up slightly
2. Peak memory usage (heap allocated) on startup goes from ~51.5 MB to ~45 MB
3. Initial memory usage (heap allocated) after initial GC goes from ~39 MB to ~36.2 MB
4. The performance of request blocking and element hiding (style sheet generation) is not significantly affected

This change also simplifies the implementation logic and the tests.

What to change

As described above.

Hints for testers

Follow the hints for #6815 and #7265.

[abpbot]

A commit referencing this issue has landed:
​Issue 7324 - Simplify logic for domains property

[mjethani]

I have reopened this because the change here has in fact caused a serious performance regression in element hiding. I had tested with EasyList only. When I tested with EasyList+AA, the code spent a significant amount of time in the domains getter, called from getException in lib/elemHideExceptions.js.

Now we have these options:

1. Revert the change: I do not want to do this, because the idea was to simplify the logic and make it less hacky
2. Revert part of the change by bringing back the _cacheDomains property: I don't like this because I am trying to move away from the expectation that filter objects will be persistent in memory
3. Organize exceptions by domain
4. Some other way of avoiding repeated calls to the domains getter

I'll try to work this out and leave this ticket open until then. In the worst case we go with option 2 above.

[abpbot]

A commit referencing this issue has landed:
​Issue 7324 - Set _domains property for non-empty domain maps

[Ross]

Done. Domain matching still works as expected.

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