[auth] smarter backend lookup aborts

[miodvallat]

Short description

In a PR sometime ago, I expressed the wish for a way to abort a record lookup once we have enough information for our current needs and do not need to process the remaining results.

It turns out that such a function exists, it's Ueberbackend::lookupEnd(), and it works by reading the remaining results in a vacuum.

This PR performs three things:

• it moves part of the lookupEnd logic down one stair to the DNSBackend level, and allows that part to be overridden by backends, should they have faster/better logic for that operation.
• it adds a few more uses of lookupEnd in pdns_server and pdnsutil.
• it adds better lookupEnd logic for a few backends. (it might be possible to also do this for the SQL-based backends, at least some of them, but that's probably for a later PR)

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• forgotten that it is generally frowned upon to submit evil PRs on friday

[coveralls]

Pull Request Test Coverage Report for Build 16339109770

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 32 of 34 (94.12%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on abort_retry_ignore at 65.745%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
modules/lmdbbackend/lmdbbackend.cc	4	6	66.67%

Totals
Change from base Build 16339020776:	65.7%
Covered Lines:	127512
Relevant Lines:	165360

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💛 - Coveralls

[mind04]

I wonder how big the performance hit of this pull is. Stopping half way means no caching in the query cache.

[miodvallat]

I wonder how big the performance hit of this pull is. Stopping half way means no caching in the query cache.

Allow me to disagree. This is "below" the query cache, and only affects communication with the backend. None of these individual records, which are now skipped, would have ended up in a cache.