PD cannot satisfy 3 Voter and 1 learner rules

[nolouch]

Bug Report

What did you do?

the region

peer 2 in zone2, peer 3  in zone3, peer 4 in zone3

add the rule

[]string{"1/voter/zone=zone1//", "1/voter/zone=zone2/", "1/voter/zone=zone3/", "1/learner/zone=zone4/"

What did you expect to see?

the region became 3voter + 1 learner

What did you see instead?

the operator adds peer 5 in zone1, and blocking on 4 voters.

What version of PD are you using (pd-server -V)?

master

[nolouch]

the reason is only 3 rules can find the fit peer, the learner rule is not satisfied. but fixOrphanPeers only works on all rules are satisfied. and there are only 4 tikv, cannot add learners and remove extra voters. we need to support demote voter to fix this situation.

[mayjiang0203]

/severity Major