[Tarantool-patches] [PATCH] raft: fix an assertion failure on transition to voter

[Serge Petrenko]

22.10.2020 13:44, Serge Petrenko пишет:
> When an instance is configured as candidate, it has a leader death timer
> ticking constantly to schedule an election as soon as leader disappears.
> When the instance receives the leader's heartbeat, it resets the timer
> to its initial value.
>
> When being a voter, the instance ignores heartbeats, since it has
> nothing to wait for. So its timer must be stopped. Otherwise it'll try
> to schedule a new election and fail.
>
> Stop the timer on transition from candidate to voter.


I've added a small test. It crashes on master and passes on this patch.

The diff's below.

diff --git a/test/replication/gh-5426-election-on-off.result 
b/test/replication/gh-5426-election-on-off.result
index 1abfb9154..7444ef7f2 100644
--- a/test/replication/gh-5426-election-on-off.result
+++ b/test/replication/gh-5426-election-on-off.result
@@ -111,6 +111,45 @@ test_run:wait_cond(function() return 
box.info.election.leader == 0 end)
   | - true
   | ...

+-- A crash when follower transitions from candidate to voter.
+test_run:switch('default')
+ | ---
+ | - true
+ | ...
+box.cfg{election_mode='candidate'}
+ | ---
+ | ...
+test_run:wait_cond(function() return box.info.election.state == 
'leader' end)
+ | ---
+ | - true
+ | ...
+box.cfg{replication_timeout=0.01}
+ | ---
+ | ...
+
+test_run:switch('replica')
+ | ---
+ | - true
+ | ...
+-- A small timeout so that the timer goes off faster and the crash happens.
+box.cfg{replication_timeout=0.01}
+ | ---
+ | ...
+test_run:wait_cond(function() return box.info.election.leader ~= 0 end)
+ | ---
+ | - true
+ | ...
+box.cfg{election_mode='candidate'}
+ | ---
+ | ...
+box.cfg{election_mode='voter'}
+ | ---
+ | ...
+-- Wait for the timer to go off.
+require('fiber').sleep(4 * box.cfg.replication_timeout)
+ | ---
+ | ...
+
  test_run:switch('default')
   | ---
   | - true
diff --git a/test/replication/gh-5426-election-on-off.test.lua 
b/test/replication/gh-5426-election-on-off.test.lua
index d6b980d0a..bdf06903b 100644
--- a/test/replication/gh-5426-election-on-off.test.lua
+++ b/test/replication/gh-5426-election-on-off.test.lua
@@ -47,6 +47,21 @@ box.cfg{election_mode = 'off'}
  test_run:switch('replica')
  test_run:wait_cond(function() return box.info.election.leader == 0 end)

+-- A crash when follower transitions from candidate to voter.
+test_run:switch('default')
+box.cfg{election_mode='candidate'}
+test_run:wait_cond(function() return box.info.election.state == 
'leader' end)
+box.cfg{replication_timeout=0.01}
+
+test_run:switch('replica')
+-- A small timeout so that the timer goes off faster and the crash happens.
+box.cfg{replication_timeout=0.01}
+test_run:wait_cond(function() return box.info.election.leader ~= 0 end)
+box.cfg{election_mode='candidate'}
+box.cfg{election_mode='voter'}
+-- Wait for the timer to go off.
+require('fiber').sleep(4 * box.cfg.replication_timeout)
+
  test_run:switch('default')
  test_run:cmd('stop server replica')
  test_run:cmd('delete server replica')

> ---
> https://github.com/tarantool/tarantool/tree/sp/raft-crash-on-voter
>
>   src/box/raft.c | 18 ++++++++++++------
>   1 file changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/src/box/raft.c b/src/box/raft.c
> index b70f47006..4a8e54cac 100644
> --- a/src/box/raft.c
> +++ b/src/box/raft.c
> @@ -952,12 +952,18 @@ raft_cfg_is_candidate(bool is_candidate)
>   			 * until the new state is fully persisted.
>   			 */
>   		}
> -	} else if (raft.state != RAFT_STATE_FOLLOWER) {
> -		if (raft.state == RAFT_STATE_LEADER)
> -			raft.leader = 0;
> -		raft.state = RAFT_STATE_FOLLOWER;
> -		/* State is visible and changed - broadcast. */
> -		raft_schedule_broadcast();
> +	} else {
> +		if (raft.state != RAFT_STATE_LEADER) {
> +			/* Do not wait for anything while being a voter. */
> +			ev_timer_stop(loop(), &raft.timer);
> +		}
> +		if (raft.state != RAFT_STATE_FOLLOWER) {
> +			if (raft.state == RAFT_STATE_LEADER)
> +				raft.leader = 0;
> +			raft.state = RAFT_STATE_FOLLOWER;
> +			/* State is visible and changed - broadcast. */
> +			raft_schedule_broadcast();
> +		}
>   	}
>   	box_update_ro_summary();
>   }

-- 
Serge Petrenko