Switch from implicit to explicit drop of async subtasks

design/mvp/Async.md

@@ -451,8 +451,8 @@ features will be added in future chunks to complete "async" in Preview 3:
   concurrency
 * `subtask.cancel`: allow a supertask to signal to a subtask that its result is
   no longer wanted and to please wrap it up promptly
-* `subtask.drop`: allow tail-calling a subtask so that the current wasm
-  instance can be torn down eagerly
+* allow "tail-calling" a subtask so that the current wasm instance can be torn
+  down eagerly
 * `task.index`+`task.wake`: allow tasks in the same instance to wait on and
   wake each other (async condvar-style)
 * `nonblocking` function type attribute: allow a function to declare in its

design/mvp/Binary.md

@@ -282,6 +282,7 @@ canon    ::= 0x00 0x00 f:<core:funcidx> opts:<opts> ft:<typeidx> => (canon lift
            | 0x0b                                                => (canon task.wait (core func))
            | 0x0c                                                => (canon task.poll (core func))
            | 0x0d                                                => (canon task.yield (core func))
+           | 0x0e                                                => (canon subtask.drop (core func))
 opts     ::= opt*:vec(<canonopt>)                                => opt*
 canonopt ::= 0x00                                                => string-encoding=utf8
            | 0x01                                                => string-encoding=utf16

design/mvp/CanonicalABI.md

@@ -619,9 +619,6 @@ once).
   def process_event(self, subtask):
     assert(subtask.supertask is self)
     subtask.enqueued = False
-    if subtask.state == AsyncCallState.DONE:
-      self.inst.async_subtasks.remove(subtask.index)
-      self.num_async_subtasks -= 1
     return (EventCode(subtask.state), subtask.index)
 ```
 The `pending_tasks` queue (appended to by `enter` above) is emptied by `wait`
@@ -2399,6 +2396,29 @@ async def canon_task_yield(task):
   return []
 ```
 
+### 🔀 `canon subtask.drop`
+
+For a canonical definition:
+```wasm
+(canon subtask.drop (core func $f))
+```
+validation specifies:
+* `$f` is given type `(func (param i32))`
+
+Calling `$f` removes the indicated subtask from the instance's table, trapping
+if the subtask isn't done or isn't a subtask of the current task. The guard
+on `enqueued` ensures that supertasks can only drop subtasks once they've been
+officially notified of their completion (via `task.wait` or callback).
+```python
+async def canon_subtask_drop(task, i):
+  subtask = task.inst.async_subtasks.remove(i)
+  trap_if(subtask.enqueued)
+  trap_if(subtask.state != AsyncCallState.DONE)
+  trap_if(subtask.supertask is not task)
+  task.num_async_subtasks -= 1
+  return []
+```
+
 ### 🧵 `canon thread.spawn`
 
 For a canonical definition:

design/mvp/Explainer.md

@@ -1311,6 +1311,7 @@ canon ::= ...
         | (canon task.wait (core func <id>?)) 🔀
         | (canon task.poll (core func <id>?)) 🔀
         | (canon task.yield (core func <id>?)) 🔀
+        | (canon subtask.drop (core func <id>?)) 🔀
         | (canon thread.spawn <typeidx> (core func <id>?)) 🧵
         | (canon thread.hw_concurrency (core func <id>?)) 🧵
 ```
@@ -1409,6 +1410,10 @@ switch to another task, allowing a long-running computation to cooperatively
 interleave with other tasks. (See also [`canon_task_yield`] in the Canonical
 ABI explainer.)
 
+The `subtask.drop` built-in has type `[i32] -> []` and removes the indicated
+[subtask](Async.md#subtask-and-supertask) from the current instance's subtask
+table, trapping if the subtask isn't done.
+
 ##### 🧵 Threading built-ins
 
 The [shared-everything-threads] proposal adds component model built-ins for

design/mvp/canonical-abi/definitions.py

@@ -461,9 +461,6 @@ def maybe_start_pending_task(self):
   def process_event(self, subtask):
     assert(subtask.supertask is self)
     subtask.enqueued = False
-    if subtask.state == AsyncCallState.DONE:
-      self.inst.async_subtasks.remove(subtask.index)
-      self.num_async_subtasks -= 1
     return (EventCode(subtask.state), subtask.index)
 
   def poll(self):
@@ -1523,3 +1520,13 @@ async def canon_task_yield(task):
   trap_if(task.opts.callback is not None)
   await task.yield_()
   return []
+
+### 🔀 `canon subtask.drop`
+
+async def canon_subtask_drop(task, i):
+  subtask = task.inst.async_subtasks.remove(i)
+  trap_if(subtask.enqueued)
+  trap_if(subtask.state != AsyncCallState.DONE)
+  trap_if(subtask.supertask is not task)
+  task.num_async_subtasks -= 1
+  return []

design/mvp/canonical-abi/run_tests.py

@@ -549,6 +549,8 @@ async def consumer(task, args):
     event, callidx = await task.wait()
     assert(event == EventCode.CALL_DONE)
     assert(callidx == 1)
+    assert(task.num_async_subtasks == 1)
+    await canon_subtask_drop(task, callidx)
     assert(task.num_async_subtasks == 0)
 
     dtor_fut = asyncio.Future()
@@ -571,6 +573,7 @@ async def dtor(task, args):
     event, callidx = await task.wait()
     assert(event == AsyncCallState.DONE)
     assert(callidx == 1)
+    await canon_subtask_drop(task, callidx)
     assert(task.num_async_subtasks == 0)
 
     [] = await canon_task_return(task, CoreFuncType(['i32'],[]), [42])
@@ -632,6 +635,7 @@ async def callback(task, args):
     if args[0] == 42:
       assert(args[1] == EventCode.CALL_DONE)
       assert(args[2] == 1)
+      await canon_subtask_drop(task, 1)
       return [53]
     elif args[0] == 52:
       assert(args[1] == EventCode.YIELDED)
@@ -642,6 +646,7 @@ async def callback(task, args):
       assert(args[0] == 62)
       assert(args[1] == EventCode.CALL_DONE)
       assert(args[2] == 2)
+      await canon_subtask_drop(task, 2)
       [] = await canon_task_return(task, CoreFuncType(['i32'],[]), [83])
       return [0]
 
@@ -708,6 +713,7 @@ async def consumer(task, args):
     event, callidx = await task.wait()
     assert(event == EventCode.CALL_DONE)
     assert(callidx == 1)
+    await canon_subtask_drop(task, callidx)
     assert(producer1_done == True)
 
     assert(producer2_done == False)
@@ -716,6 +722,7 @@ async def consumer(task, args):
     event, callidx = task.poll()
     assert(event == EventCode.CALL_DONE)
     assert(callidx == 2)
+    await canon_subtask_drop(task, callidx)
     assert(producer2_done == True)
 
     assert(task.poll() is None)
@@ -796,6 +803,9 @@ async def consumer(task, args):
     assert(callidx == 2)
     assert(producer2_done == True)
 
+    await canon_subtask_drop(task, 1)
+    await canon_subtask_drop(task, 2)
+
     assert(task.poll() is None)
 
     await canon_task_start(task, CoreFuncType([],[]), [])
@@ -851,6 +861,10 @@ async def core_func(task, args):
     event, callidx = await task.wait()
     assert(event == EventCode.CALL_DONE)
     assert(callidx == 2)
+
+    await canon_subtask_drop(task, 1)
+    await canon_subtask_drop(task, 2)
+
     return []
 
   inst = ComponentInstance()