Protocol changes can result in a oneof not being cleared

[joshgarnett]

At the moment we currently only have a single event for setting a oneof. This can cause issues if an entry in a oneof is removed.

For example:

message Foo {
  oneof bar {
    int32 a = 1;
    int32 b = 2;
  }
}

Assuming the following actions:

var foo = new Foo();
foo.A = 1;
foo.B = 2;

When you apply the events you would expect that A would no longer be set and B would have the value 2.

Now assume we make the following protocol change:

message Foo {
  oneof bar {
    int32 a = 1;
    // remove b
  }
}

If we apply those same events from before, we actually end up in a case where A is set to 1 still as the B set event is no longer valid and was just dropped in the ApplyEvent code.

We have a few options here. We can just document the edge case or we can look at generating two events for each oneof set. One that clears the old value and one that sets the new value.