Refactoring event codes with respect to recent changes

[alexcrichton]

Currently we have:

class EventCode(IntEnum):
  NONE = 0
  CALL_STARTING = CallState.STARTING
  CALL_STARTED = CallState.STARTED
  CALL_RETURNED = CallState.RETURNED
  STREAM_READ = 5
  STREAM_WRITE = 6
  FUTURE_READ = 7
  FUTURE_WRITE = 8

Additionally the callback for async lifted exports receives three 32-bit integers at this time: an EventCode, an optional waitable index, and an optional code for the event.

With futures/streams there's currently a nice property that for any waitable there's a single EventCode that corresponds to it, for example the two reads of a stream/future have a read/write event. Additionally the optional code to the callback exactly corresponds to what the read/write/cancel intrinsics all return.

For subtasks, however, the three events a subtask can receive are split across different EventCodes where the optional code for each event is always zero.

How about refactoring these codes/states with respect to the upcoming change of two-handle streams/futures? The idea would be to fold CALL_* into a single EventCode.SUBTASK. The waitable index to the callback would be the same and the optional code would be the CallState.

[alexcrichton]

Also, to confirm, CALL_STARTING isn't possible, right? That's implicit in the creation of a subtask?

[lukewagner]

That's a good idea; makes sense to me. There's already a pending breaking ABI tweak for #490, so we could include this change to minimize the number of such breaks, if that makes sense? And you're right, CALL_STARTING isn't possible.

[alexcrichton]

Sounds good to me!