Node goes from follower to leader state

[danluu]

If we look at a trace of node state transitions, we see that a node goes from candidate to follower to leader. It should probably not become a follower in between the candidate and leader states:

src/world_broker.py execute_step
timer_trip 2
change_type 2: Follower -> Candidate
change_type 2: Candidate -> Follower
change_type 4: Follower -> Follower
change_type 3: Follower -> Follower
change_type 2: Follower -> Leader
change_type 1: Follower -> Follower
change_type 2: Leader -> Leader
change_type 0: Follower -> Follower
change_type 2: Leader -> Leader

[danluu]

I believe the issue is:

1. In timer_trip, a node becoming a candidate increases its term:

        if not self.is_leader():
            self.change_type('Candidate')
            self.update_term(self.term+1)

2. This means that we compare self.term to self.term + 1. self.term < self.term + 1, therefore we become a follower.

3. Due to perhaps another bug, we don't block followers from becoming leaders. If we get a response that's "successful", the node becomes a leader regardless of its current state.

[danluu]

Jinny and I pushed a fix for this in the test branch. However, this involves adding a boolean flag to update_term, which is sort of ugly. Does anyone have a better idea on how to fix this?