Consider using Referrer Policy

[ameshkov]

https://www.w3.org/TR/referrer-policy/

Relevant discussion:
https://forum.adguard.com/index.php?threads/sources-of-some-requests-were-not-identified.13260/

[sfionov]

I think here is an important thing: if policy is changed by us, it must be applied on AdGuard's side because site authors designed it to not to leak referrers.

[sfionov]

Here's some sites that may be affected by Referrer-Policy:

AdguardTeam/Scriptlets#196

[ameshkov]

This one to be precise:
AdguardTeam/AdguardFilters#109135

It seems that we should provide a core-libs specific way to deal with referrer-policy.
Something like ||example.org^$referrer-policy=blahblah

[ngorskikh]

@ameshkov

What if we added a new basic rule modifier like this:

||example.org^$referrerpolicy=one|two|three

Which, in this case, would replace all Referrer-Policy headers (or add one, if none are present), and replace the content of <meta name="referrer"> tag with the policy one,two,three.

Exceptions could work like this:

||example.org^$referrerpolicy=one|two|three

! Remove the policies "one" and "two" from the patched value
@@||example.org^$referrerpolicy=one|two

! Completely disable referrerpolicy rules on matching requests
@@||example.org^$referrerpolicy

! Completely disable the $referrerpolicy=one|two|three rule
@@||example.org^$referrerpolicy=one|two|three 

Since the referrer policy is sensitive to order (the latest policy understood by the browser is applied), and we can't preserve the order of rules when parsing them, we would have to introduce a gotcha such as "if multiple rules match the same request, only one of them is applied, unspecified which one".

[sfionov]

@ngorskikh Support of multi-values is really redundant because syntax of Referrer-Policy header allows multiple values, separated by commas with optional spaces.

Multi-values are needed for matches to conveniently turning off some of them. In this case, this is too redundant, and also alters original header syntax.

[ngorskikh]

@sfionov
Then I propose that simply

||example.org^$referrerpolicy=one\,   two\,  three

should replace the policy with the unescaped value of the modifier, and exceptions should work like this:

! Disable the rule with the modifier value equal to `one\,   two\,  three`
@@||example.org^$referrerpolicy=one\,   two\,  three

! Disable all rules for matching requests
@@||example.org^$referrerpolicy

And, again, if multiple rules match the same request, only one of them (unspecified, which one) is applied.

[sfionov]

Looks good for me