CABI: Remove the zero-index special case for callback return value

design/mvp/Async.md

@@ -906,7 +906,11 @@ not externally-visible behavior.
     (func (export "cb") (param $event i32) (param $p1 i32) (param $p2 i32)
       ...
       if (result i32)         ;; if subtasks remain:
-        i32.const 2           ;; return WAIT
+        (i32.or               ;; return (WAIT | ($wsi << 4))
+          (i32.const 2)
+          (i32.shl
+            (global.get $wsi)
+            (i32.const 4)))
       else                    ;; if no subtasks remain:
         ...
         call $task_return     ;; return the string result (pointer,length)

design/mvp/CanonicalABI.md

@@ -3004,31 +3004,25 @@ returning to an event loop in `canon_lift`, with the `callback` called
 repeatedly until the `EXIT` code is returned:
 ```python
   [packed] = await call_and_trap_on_throw(callee, task, flat_args)
-  s = None
   while True:
     code,si = unpack_callback_result(packed)
-    if si != 0:
-      s = task.inst.table.get(si)
-      trap_if(not isinstance(s, WaitableSet))
     match code:
       case CallbackCode.EXIT:
         task.exit()
         return
       case CallbackCode.YIELD:
         e = await task.yield_(sync = False)
       case CallbackCode.WAIT:
-        trap_if(not s)
+        s = task.inst.table.get(si)
+        trap_if(not isinstance(s, WaitableSet))
         e = await task.wait_for_event(s, sync = False)
       case CallbackCode.POLL:
-        trap_if(not s)
+        s = task.inst.table.get(si)
+        trap_if(not isinstance(s, WaitableSet))
         e = await task.poll_for_event(s, sync = False)
     event_code, p1, p2 = e
     [packed] = await call_and_trap_on_throw(opts.callback, task, [event_code, p1, p2])
 ```
-One detail worth noting here is that the index of the waitable set does not need
-to be returned every time; as an optimization to avoid a table access on every
-turn of the event loop, if the returned waitable set index is `0` (which is an
-invalid table index), the previous waitable set will be used.
 
 The bit-packing scheme used for the `i32` `packed` return value is defined as
 follows:

design/mvp/canonical-abi/definitions.py

@@ -1866,23 +1866,21 @@ async def canon_lift(opts, inst, ft, callee, caller, on_start, on_resolve, on_bl
     return
 
   [packed] = await call_and_trap_on_throw(callee, task, flat_args)
-  s = None
   while True:
     code,si = unpack_callback_result(packed)
-    if si != 0:
-      s = task.inst.table.get(si)
-      trap_if(not isinstance(s, WaitableSet))
     match code:
       case CallbackCode.EXIT:
         task.exit()
         return
       case CallbackCode.YIELD:
         e = await task.yield_(sync = False)
       case CallbackCode.WAIT:
-        trap_if(not s)
+        s = task.inst.table.get(si)
+        trap_if(not isinstance(s, WaitableSet))
         e = await task.wait_for_event(s, sync = False)
       case CallbackCode.POLL:
-        trap_if(not s)
+        s = task.inst.table.get(si)
+        trap_if(not isinstance(s, WaitableSet))
         e = await task.poll_for_event(s, sync = False)
     event_code, p1, p2 = e
     [packed] = await call_and_trap_on_throw(opts.callback, task, [event_code, p1, p2])

design/mvp/canonical-abi/run_tests.py

@@ -683,6 +683,7 @@ async def core_producer_pre(fut, task, args):
   producer2 = partial(canon_lift, producer_opts, producer_inst, producer_ft, core_producer2)
 
   consumer_ft = FuncType([],[U32Type()])
+  seti = 0
   async def consumer(task, args):
     assert(len(args) == 0)
 
@@ -696,6 +697,7 @@ async def consumer(task, args):
     assert(subi2 == 2)
     assert(state == Subtask.State.STARTED)
 
+    nonlocal seti
     [seti] = await canon_waitable_set_new(task)
     assert(seti == 3)
     [] = await canon_waitable_join(task, subi1, seti)
@@ -707,7 +709,6 @@ async def consumer(task, args):
 
   async def callback(task, args):
     assert(len(args) == 3)
-    seti = 1
     [ctx] = await canon_context_get('i32', 0, task)
     match ctx:
       case 42:
@@ -723,7 +724,7 @@ async def callback(task, args):
         assert(args[2] == 0)
         fut2.set_result(None)
         [] = await canon_context_set('i32', 0, task, 62)
-        return [definitions.CallbackCode.WAIT]
+        return [definitions.CallbackCode.WAIT | (seti << 4)]
       case 62:
         assert(args[0] == EventCode.SUBTASK)
         assert(args[1] == 2)