In TriggerRunner, avoid reading the set of finished subtriggers when no subtrigger ever finishes.

[kennknowles]

For a trigger expression, TriggerRunner tracks which of the subexpressions have finished to track which subexpression is the currently active trigger that determines when output is permitted. For some triggers (such as the default trigger) this information is not needed. The reads and writes of this data can be elided.

Imported from Jira BEAM-30. Original Jira may contain additional context.
Reported by: kenn.

[Moemenmohamed24]

Hi ! @kennknowles Is anyone working on this?

[kennknowles]

No I don't think anyone is working on it. It is a pretty niche issue that has been sitting here for a very very long time. But it probably still applies! It may not make any difference to a real-world pipeline, actually... but I must have filed it for some reason 😅 . Feel free to take a look if you like.