learner can became leader after (request_leadership/yield_leadership)

[UnamedRus]

We observe behavior, that if we run request_leadership on learner, it can actually became leader without change of it's "learner" status

peer 1, DC ID 0, 172.25.0.2:9234, voting member, 1
peer 2, DC ID 0, 172.25.0.3:9234, voting member, 1
peer 3, DC ID 0, 172.25.0.4:9234, voting member, 1
peer 4, DC ID 0, 172.26.0.2:9234, learner, 1
peer 5, DC ID 0, 172.26.0.3:9234, learner, 1
peer 6, DC ID 0, 172.26.0.4:9234, learner, 1
my id: 5, leader: 5, term: 2

From code comments:

* Learner will not initiate or participate in leader election.

    /**
     * (Read-only)
     * `true` if this server is a learner. Will not participate
     * leader election.
     */  

From readonly(learner) documentation
https://github.com/eBay/NuRaft/blob/master/docs/readonly_member.md

Both read-only members and zero-priority members do not initiate leader election, and never be a leader.

1. Does it mean that learner, only affect behavior around elections, and not straight nomination of node as leader?
2. If so, should documentation for readonly_member, corrected that in some cases, it can became leader?

[greensky00]

Leaners should never be a leader in any circumstance. They neither initiate an election nor vote for others. Can you please share the full log of peer 5 so that I can see what happened during the leader election?

[greensky00]

Ah, you executed request_leadership on learner. If it worked, then it is a bug; request_leadership should be denied on learners. Let me check if that's the case.

[UnamedRus]

2024.11.14 15:44:47.624300 [ 34 ] {} <Debug> KeeperTCPHandler: Receive four letter command rqld
2024.11.14 15:44:47.624451 [ 34 ] {} <Information> RaftInstance: sent leadership request to leader 1
2024.11.14 15:44:47.624903 [ 55 ] {} <Information> RaftInstance: 0x7f55f8e87898 connected to 172.25.0.2:9234 (as a client)
2024.11.14 15:44:47.625424 [ 48 ] {} <Debug> RaftInstance: Receive a custom_notification_response message from peer 1 with Result=1, Term=1, NextIndex=39
2024.11.14 15:44:47.625833 [ 36 ] {} <Trace> KeeperDispatcher: Processing requests batch, size: 1, bytes: 0
2024.11.14 15:44:47.625837 [ 31 ] {} <Debug> KeeperTCPHandler: Received close event with xid 2147483647 for session id #19
2024.11.14 15:44:47.626203 [ 48 ] {} <Debug> RaftInstance: Receive a append_entries_request message from 1 with LastLogIndex=38, LastLogTerm 1, EntriesLength=0, CommitIndex=38 and Term=1
2024.11.14 15:44:47.626248 [ 48 ] {} <Debug> RaftInstance: Response back a append_entries_response message to 1 with Accepted=1, Term=1, NextIndex=39
2024.11.14 15:44:47.633617 [ 54 ] {} <Debug> RaftInstance: Receive a custom_notification_request message from 1 with LastLogIndex=38, LastLogTerm 1, EntriesLength=1, CommitIndex=38 and Term=1
2024.11.14 15:44:47.633642 [ 54 ] {} <Information> RaftInstance: [LEADERSHIP TAKEOVER] got request
2024.11.14 15:44:47.633768 [ 54 ] {} <Debug> IDisk: Copying from LocalStateFileDisk (path: /var/lib/clickhouse/coordination) state to LocalStateFileDisk (path: /var/lib/clickhouse/coordination) state-OLD.
2024.11.14 15:44:47.638704 [ 54 ] {} <Information> RaftInstance: [VOTE INIT] my id 5, my role candidate, term 2, log idx 38, log term 1, priority (target 1 / mine 1)
2024.11.14 15:44:47.638804 [ 54 ] {} <Debug> RaftInstance: send request_vote_request to server 3 with term 2
2024.11.14 15:44:47.638846 [ 54 ] {} <Debug> RaftInstance: send request_vote_request to server 2 with term 2
2024.11.14 15:44:47.638855 [ 54 ] {} <Debug> RaftInstance: send request_vote_request to server 1 with term 2
2024.11.14 15:44:47.638970 [ 54 ] {} <Debug> RaftInstance: Response back a custom_notification_response message to 1 with Accepted=1, Term=1, NextIndex=39
2024.11.14 15:44:47.639454 [ 54 ] {} <Information> RaftInstance: 0x7f55f8e87c18 connected to 172.25.0.3:9234 (as a client)
2024.11.14 15:44:47.639524 [ 55 ] {} <Information> RaftInstance: 0x7f55f8e87f98 connected to 172.25.0.4:9234 (as a client)
2024.11.14 15:44:47.646386 [ 56 ] {} <Debug> RaftInstance: Receive a request_vote_response message from peer 1 with Result=1, Term=2, NextIndex=39
2024.11.14 15:44:47.646413 [ 56 ] {} <Information> RaftInstance: [VOTE RESP] peer 1 (O), resp term 2, my role candidate, granted 2, responded 2, num voting members 3, quorum 2
2024.11.14 15:44:47.646417 [ 56 ] {} <Information> RaftInstance: Server is elected as leader for term 2
2024.11.14 15:44:47.646440 [ 56 ] {} <Information> RaftInstance: number of pending commit elements: 0
2024.11.14 15:44:47.646447 [ 56 ] {} <Information> RaftInstance: state machine commit index 38, precommit index 38, last log index 38      
2024.11.14 15:44:47.646455 [ 56 ] {} <Trace> RaftInstance: peer 4, interval: 500
2024.11.14 15:44:47.646465 [ 56 ] {} <Trace> RaftInstance: peer 3, interval: 500
2024.11.14 15:44:47.646472 [ 56 ] {} <Trace> RaftInstance: peer 2, interval: 500
2024.11.14 15:44:47.646476 [ 56 ] {} <Trace> RaftInstance: peer 6, interval: 500
2024.11.14 15:44:47.646481 [ 56 ] {} <Trace> RaftInstance: peer 1, interval: 500
2024.11.14 15:44:47.646672 [ 56 ] {} <Information> RaftInstance: [BECOME LEADER] appended new config at 39
2024.11.14 15:44:47.646702 [ 56 ] {} <Warning> RaftInstance: connection to peer 4 is not active long time: 152728 ms, force re-connect     
2024.11.14 15:44:47.646717 [ 56 ] {} <Debug> RaftInstance: reset RPC client for peer 4
2024.11.14 15:44:47.646931 [ 56 ] {} <Debug> RaftInstance: append_entries for 4 with LastLogIndex=39, LastLogTerm=2, EntriesLength=0, CommitIndex=38, Term=2, peer_last_sent_idx 0
2024.11.14 15:44:47.646977 [ 56 ] {} <Warning> RaftInstance: connection to peer 3 is not active long time: 152728 ms, force re-connect     
2024.11.14 15:44:47.646989 [ 56 ] {} <Debug> RaftInstance: reset RPC client for peer 3
2024.11.14 15:44:47.647011 [ 56 ] {} <Debug> RaftInstance: append_entries for 3 with LastLogIndex=39, LastLogTerm=2, EntriesLength=0, CommitIndex=38, Term=2, peer_last_sent_idx 0
2024.11.14 15:44:47.647068 [ 56 ] {} <Warning> RaftInstance: connection to peer 2 is not active long time: 152728 ms, force re-connect     
2024.11.14 15:44:47.647078 [ 56 ] {} <Debug> RaftInstance: reset RPC client for peer 2
2024.11.14 15:44:47.647121 [ 56 ] {} <Debug> RaftInstance: append_entries for 2 with LastLogIndex=39, LastLogTerm=2, EntriesLength=0, CommitIndex=38, Term=2, peer_last_sent_idx 0
2024.11.14 15:44:47.647148 [ 56 ] {} <Warning> RaftInstance: connection to peer 6 is not active long time: 152728 ms, force re-connect     
2024.11.14 15:44:47.647158 [ 56 ] {} <Debug> RaftInstance: reset RPC client for peer 6
2024.11.14 15:44:47.647274 [ 56 ] {} <Debug> RaftInstance: append_entries for 6 with LastLogIndex=39, LastLogTerm=2, EntriesLength=0, CommitIndex=38, Term=2, peer_last_sent_idx 0
2024.11.14 15:44:47.647332 [ 56 ] {} <Debug> RaftInstance: append_entries for 1 with LastLogIndex=38, LastLogTerm=1, EntriesLength=1, CommitIndex=38, Term=2, peer_last_sent_idx 0
2024.11.14 15:44:47.647424 [ 56 ] {} <Information> RaftInstance:   === LEADER (term 2) ===
2024.11.14 15:44:47.649645 [ 59 ] {} <Information> RaftInstance: 0x7f55f8e88318 connected to 172.25.0.4:9234 (as a client)
2024.11.14 15:44:47.649664 [ 54 ] {} <Warning> RaftInstance: [EDGE CASE] got stale RPC response from 3: current 0x7f55f8e88318 (8), from parameter 0x7f55f8e87f98 (3). will ignore this response
2024.11.14 15:44:47.649650 [ 46 ] {} <Information> RaftInstance: 0x7f55ce0a0018 connected to 172.26.0.2:9234 (as a client)
2024.11.14 15:44:47.649759 [ 59 ] {} <Debug> RaftInstance: Receive a append_entries_response message from peer 1 with Result=1, Term=2, NextIndex=40
2024.11.14 15:44:47.649773 [ 59 ] {} <Trace> RaftInstance: peer 1, prev matched idx: 0, new matched idx: 39
2024.11.14 15:44:47.649787 [ 59 ] {} <Debug> RaftInstance: Server 4 is busy, skip the request
2024.11.14 15:44:47.649793 [ 59 ] {} <Debug> RaftInstance: Server 3 is busy, skip the request
2024.11.14 15:44:47.649797 [ 59 ] {} <Debug> RaftInstance: Server 2 is busy, skip the request
2024.11.14 15:44:47.649801 [ 59 ] {} <Debug> RaftInstance: Server 6 is busy, skip the request
2024.11.14 15:44:47.649808 [ 59 ] {} <Debug> RaftInstance: append_entries for 1 with LastLogIndex=39, LastLogTerm=2, EntriesLength=0, CommitIndex=39, Term=2, peer_last_sent_idx 39
2024.11.14 15:44:47.649861 [ 59 ] {} <Trace> RaftInstance: leadership transfer is disabled
2024.11.14 15:44:47.649887 [ 60 ] {} <Debug> RaftInstance: commit upto 39, current idx 38
2024.11.14 15:44:47.649894 [ 60 ] {} <Trace> RaftInstance: commit upto 39, current idx 39
2024.11.14 15:44:47.649904 [ 60 ] {} <Information> RaftInstance: config at index 39 is committed, prev config log idx 1
2024.11.14 15:44:47.649945 [ 60 ] {} <Information> RaftInstance: new config log idx 39, prev log idx 1, cur config log idx 1, prev log idx 0
2024.11.14 15:44:47.649953 [ 60 ] {} <Debug> RaftInstance: system is reconfigured to have 6 servers, last config index: 1, this config index: 39
2024.11.14 15:44:47.650001 [ 60 ] {} <Information> RaftInstance: new configuration: log idx 39, prev log idx 1
peer 1, DC ID 0, 172.25.0.2:9234, voting member, 1
peer 2, DC ID 0, 172.25.0.3:9234, voting member, 1
peer 3, DC ID 0, 172.25.0.4:9234, voting member, 1
peer 4, DC ID 0, 172.26.0.2:9234, learner, 1
peer 5, DC ID 0, 172.26.0.3:9234, learner, 1
peer 6, DC ID 0, 172.26.0.4:9234, learner, 1
my id: 5, leader: 5, term: 2
2024.11.14 15:44:47.650072 [ 60 ] {} <Debug> RaftInstance: DONE: commit upto 39, current idx 39