[BTS-1644] ResignLeadership + Wait

[maierlars]

Scope & Purpose

Added waitForInSync and waitForInSyncTimeout parameters to the resign leadership job. This allows the user to wait for a certain amount of time to make sure that there exist common in sync followers. Previously they were just ignored and could cause downtime.

Design Doc: https://github.com/arangodb/documents/pull/136

[jvolmer]

I've got two type-improvement suggestions, otherwise LGTM.

[jvolmer]

Do I understand it correctly that if _waitForInSyncTimeout == 0, there is no timeout and we can wait indefinitely? This is a bit confusing, because 0 timeout normally just means instantly.
Possible fix: We could have an optional instead of just a number and if the the snapshot-velocypack does not include waitForInSyncTimeout, the option is std::nullopt.

[jvolmer]

30 minutes sound a lot to me, but perhaps this is correct.
This value is actually set per default to 0 in the ResignLeadership constructor, so I guess it is more readable to set it to zero here as well.

[jvolmer]

Another thing: I would be nice to make it visible that they both belong together - _waitForInSyncTimeout does not make any sense if _waitForInSync is false - as far as I understood.
E.g. via an std::variant<bool, std::tuple<bool, TimeoutInSec>> or with the comment above: std::variant<bool, std::tuple<bool, std::optional<TimeoutInSec>>>

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Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Lars Maier.
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[neunhoef]

This is still relevant and I will finish it eventually, when no more pressing issues are there.

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