Amcheck: do rightlink verification with lock coupling

Hi!

This is a thread to discuss amcheck feature started in other thread[0].

Currently amcheck is scanning every B-tree level. If verification is done with ShareLock - amcheck will test that each page leftlink is pointing to page with rightlink backwards.
This is important invariant, in our experience it proved to be good at detecting various corruptions.
But this invariant can be detected only if both pages are not modified (e.g. split concurrently).

PFA patch, that in case of suspicion locks two pages and retests that invariant. This allows detection of several corruptions on standby.

This patch violates one of amcheck design principles: current code does not ever take more than one page lock. I do not know: should we hold this rule or should we use more deep check?

Thanks!

Best regards, Andrey Borodin.

[0] https://www.postgresql.org/message-id/flat/DA9B33AC-53CB-4643-96D4-7A0BBC037FA1(at)yandex-team(dot)ru