KAFKA-3226: Replicas collections should use List instead of Set in or…

[granthenke]

…der to maintain order

[granthenke]

@ijuma This is the jira/pr related to #896

[gwenshap]

I thought PartitionState contains a leader value specifically so we won't need to make assumptions on order of replicas?

[granthenke]

@gwenshap Good point...let me look what going on there and report back

[ijuma]

@gwenshap I thought that too, but @granthenke said there were test failures in #896 when the server started using the Java classes. It could be that the tests are wrong or that it was working accidentally because small immutable sets (up to 4 elements) in Scala maintain ordering.

[granthenke]

@gwenshap @ijuma It looks like the actual required behavior is correct. The test failure was due to replicas that did not match the same order. The change in #896 can definitely impact the order from what it was, but its being handled correctly by Kafka. I will close this and update the test in my original PR and leave replicas as a Set.

[ijuma]

Awesome, thanks for checking this Grant.