Provide way of handling unsupported DNR rules

[oliverdunk]

In our TPAC meeting on Tuesday (#448), it was brought up that there is currently no easy way to support new types of DNR rules while being compatible with browsers that do not recognise them. This is primarily an issue for static rulesets.

It was mentioned that Chrome has a hard-error when any DNR rule in a ruleset fails to load, and Firefox has a soft failure where the rules are ignored. The behaviour in Safari needs to be determined.

There were suggestions to add API methods like getFailedRules() or isRuleSupported().

[oliverdunk]

@sammacbeth, feel free to add anything you think I missed.

@Rob--W @xeenon Could you add respective labels here?

[sammacbeth]

There are two aspects to this:

1. How agents should handle individual DNR rules they do not support.
2. How the extension gets feedback about an invalid rule.

This, of course, can differ between static and dynamic/session rules. Here's a basic compat table from what I've been able to test and what I heard from the TPAC discussion:

What is an invalid rule?

• A rule that the platform does not recognize nor support.
• A rule/ruleset that cannot be loaded due to constraint failures (rule limit reached, duplicate ruleID)

	Chrome	Firefox	Safari
Invalid rule in static ruleset: duplicate rule ID	Extension will not load.	Invalid rules are ignored, the rest are loaded. Warning is shown in about:debugging	No errors or warnings thrown. Rules are loaded and active.
Invalid rule in static ruleset: unrecognized action type	Unrecognized rules are ignored, rest are loaded. Warning shown on the extension errors page.	Unrecognized rules are ignored, the rest are loaded. Warning is shown in about:debugging	Unrecognized rules are ignored, the rest are loaded. Warning is shown on extension settings page.
Feedback for invalid static rule	Modal error when loading extension or warning on the extension errors page	Warning in about:debugging	Error on extension settings page ('non-fatal issue')
Invalid rule in updateDynamicRules call	Exception thrown. None of the rules are registered.	Exception thrown. None of the rules are registered.	Exception thrown. None of the rules are registered.
Feedback about which dynamic rule is not supported	In text of exception.	In text of exception.	In text of exception.

Screenshots of feedback given for static rule errors:

Browser	Error type
Chrome	Duplicate rule ID
Chrome	Unrecognized rule
Firefox	Duplicate rule ID
Firefox	Unrecognized rule
Safari	Unrecognized rule

[sammacbeth]

I've updated my comment above to complete the current state of Chrome, Firefox and Safari w.r.t. handling of invalid static and dynamic rules. The method I used to test these can be seen from the tests I wrote: duckduckgo/mv3-compat-tests@c24af43.

Some conclusions:

• updateDynamicRules seems to work consistently across platforms - i.e. it throws if there any any issues with the provided rules, and no rules from the batch get added.
• Safari seems to ignore duplicate rule IDs in static rules.
• For static rules, Chrome behavior varies depending on the kind of exception. Duplicate rule IDs prevent the extension loading entirely, while unsupported rules are just ignored with a warning.
• Issues with static rules are reported on extension debugging pages, but are not available programmatically. I've attached screenshots of how these errors are reported on each platform.

[oliverdunk]

We are supportive of changing the behavior in Chrome to avoid a hard error when loading an extension with duplicate rule IDs. Instead, we will ignore rules with duplicate IDs.

We are also interested in exploring the proposed getFailedRules() or isRuleSupported() APIs longer term.

Chromium bug: https://issues.chromium.org/issues/330579934

[danielhjacobs]

Opened https://issues.chromium.org/issues/347186592

[Rob--W]

The responseHeaders condition is a special case. Ordinarily Chrome does already throw for unsupported conditions. I explained what is going on in Chromium plus added a code snippet for feature detection at #638 (comment)