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6e4f304f88
Today we use a ConcurrentHashSet to track the in-flight outgoing joins in the `JoinHelper`. This is fine for adding and removing elements but not for the emptiness test in `isJoinPending()` which might return false if one join finishes just after another one starts, even though joins were pending throughout. As used today this is ok: it means the node was trying to join a master but this join attempt just finished unsuccessfully, and causes it to (rightfully) reject a `FollowerCheck` from the failed master. However this kind of API inconsistency is trappy and there is no need to be clever here, so this change replaces the set with a `synchronizedSet()`.