Exclude non_voter server from quorum calculation

[sile]

Proposed Changes

In my understanding, non_voter servers (introduced in #375) have to be handled as same as promotable servers except for the latter could promote to voter.
However, the current implementation seems to include non_voter servers in the quorum (while promotable servers are excluded from that).
This PR fixes the issue by excluding non_voter server from quorum calculation.
Feel free to close this PR if I misunderstand something.

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[illotum]

You got it right.

For other reviewers context, non_voter are a plug for future passive witness API, but implementation was cut down to simplify the original review. There is no ra API to add non_voters atm, it is a "hidden" feature.

In this light, @sile would you be able to re-work counting functions to include all possible states? Perhaps invert it to only include voter.

-type ra_membership() :: voter | promotable | non_voter | unknown.

[michaelklishin]

@illotum thank you for chiming in!

[sile]

@illotum Sounds good! (fixed in 924c024)

[sile]

This PR has received one approval (thanks, @michaelklishin!). What is holding it back from being merged?

[lukebakken]

What is holding it back from being merged?

@sile, please be patient. Team RabbitMQ and @kjnilsson in particular are busy working on higher-priority work, some of which is for customers who pay for RabbitMQ.

We will merge this PR when time allows.

[sile]

@lukebakken I see. Thank you for sharing the reason!

[michaelklishin]

I don't know if the next release will be 2.11.0 or 2.10.1 but let's stick to 2.11.0 as the milestone for now.