[v23.2.x] raft: don't promote to voter if previous config is not committed #17797
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Otherwise we may add several new voters in quick succession, that the old voters will not know of, resulting in a possibility of non-intersecting quorums. Example scenario: 1. we start with leader id 1, committed configuration: voters:1,2,3; learners:4,5 2. 1,4,5 are partitioned from 2,3 3. 1 finishes recovery of 4 and 5 and adds them as voters 4. now 1 can remain the leader getting responses from 1,4,5 (that form a quorum in the new voter set 1,2,3,4,5) and 2,3 can elect a leader among themselves, say 2 (because 2,3 is a quorum in the old voter set 1,2,3) 5. now 1 and 2 both think that they are legitimate leaders and can commit new entries, resulting in divergent logs. To prevent this, in step 3 we don't add 5 as voter until the configuration with voter 4 is committed (or vice versa). (cherry picked from commit b1e1e02)
mmaslankaprv
approved these changes
Apr 12, 2024
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Backport of PR #17675
Fixes #17790