Health watcher majority formula

[lynshi]

The health_watcher.py script uses the following logic to determine the majority:

majority = (len(nodes) + 1) // 2

This logic isn't accurate for networks with an even number of nodes, right (disregarding how common such network configurations are)? 2f + 1 nodes are required to tolerate f failures, so a 5 node network is required to tolerate 2 failures. Then, majority in a 4 node network should be 3 as only 1 failure is tolerated, but this formula gives majority = (4 + 1) // 2 = 2.

If I'm not mistaken, majority = math.ceil((len(nodes) + 1) / 2) is more accurate.

[jumaffre]

Well spotted! As heath_watcher.py mostly is a sample script, we don't test in a wide range of scenarios and network configuration, so thank you very much for raising this.

I've fixed this in #3820.