adoroszlai opened a new pull request #581:
URL: https://github.com/apache/ratis/pull/581
## What changes were proposed in this pull request?
`RaftServerImpl` creates a new `FollowerState` for each vote request
received. The old `FollowerState` may trigger change to candidate role even
after it was shutdown by the server. Both `RaftServerImpl#requestVote` and the
change to candidate by `FollowerState` requires lock on `server`
(`RaftServerImpl` object). The order of events which triggers the problem:
* `RaftServerImpl` executing `requestVote` [with lock on
itself](https://github.com/apache/ratis/blob/b57be797fe97c00abc0e3d7353d30e1b2961714d/ratis-server/src/main/java/org/apache/ratis/server/impl/RaftServerImpl.java#L1099)
* `FollowerState` [checks conditions, then waits for lock on
`server`](https://github.com/apache/ratis/blob/b57be797fe97c00abc0e3d7353d30e1b2961714d/ratis-server/src/main/java/org/apache/ratis/server/impl/FollowerState.java#L126-L130)
* `RaftServerImpl` [shuts down old `FollowerState`, starts new
one](https://github.com/apache/ratis/blob/b57be797fe97c00abc0e3d7353d30e1b2961714d/ratis-server/src/main/java/org/apache/ratis/server/impl/RaftServerImpl.java#L475-L477)
* `RaftServerImpl` [sets last RPC time on new
`FollowerState`](https://github.com/apache/ratis/blob/b57be797fe97c00abc0e3d7353d30e1b2961714d/ratis-server/src/main/java/org/apache/ratis/server/impl/RaftServerImpl.java#L1116-L1117)
* `RaftServerImpl` [releases
lock](https://github.com/apache/ratis/blob/b57be797fe97c00abc0e3d7353d30e1b2961714d/ratis-server/src/main/java/org/apache/ratis/server/impl/RaftServerImpl.java#L1127)
* *old* `FollowerState` [enters block with lock on `server`, triggers
change to
candidate](https://github.com/apache/ratis/blob/b57be797fe97c00abc0e3d7353d30e1b2961714d/ratis-server/src/main/java/org/apache/ratis/server/impl/FollowerState.java#L130-L138)
Let `FollowerState` check if it is still running after entering the
`synchronized` block.
https://issues.apache.org/jira/browse/RATIS-1490
## How was this patch tested?
Tweaked `FollowerState`, `RaftServerImpl` and `LeaderElectionTests` with
code injection. It required too much test-specific code, even in
`synchronized` blocks, so I think it should not be part of this patch. Ideas
for unit tests welcome.
Regular CI:
https://github.com/adoroszlai/incubator-ratis/runs/4845475132
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