Change task.start/return to take a core function type and use flat params/results

Author: lukewagner

design/mvp/Binary.md

@@ -272,8 +272,8 @@ canon    ::= 0x00 0x00 f:<core:funcidx> opts:<opts> ft:<typeidx> => (canon lift
            | 0x04 rt:<typeidx>                                   => (canon resource.rep rt (core func))
            | 0x05 ft:<typeidx>                                   => (canon thread.spawn ft (core func))
            | 0x06                                                => (canon thread.hw_concurrency (core func))
-           | 0x08                                                => (canon task.start (core func))
-           | 0x09                                                => (canon task.return (core func))
+           | 0x08 ft:<core:typeidx>                              => (canon task.start ft (core func))
+           | 0x09 ft:<core:typeidx>                              => (canon task.return ft (core func))
            | 0x0a                                                => (canon task.wait (core func))
            | 0x0b                                                => (canon task.poll (core func))
            | 0x0c                                                => (canon task.yield (core func))

design/mvp/CanonicalABI.md

@@ -1866,12 +1866,14 @@ def lower_heap_values(cx, vs, ts, out_param):
   tuple_value = {str(i): v for i,v in enumerate(vs)}
   if out_param is None:
     ptr = cx.opts.realloc(0, 0, alignment(tuple_type), elem_size(tuple_type))
+    flat_vals = [ptr]
   else:
     ptr = out_param.next('i32')
+    flat_vals = []
   trap_if(ptr != align_to(ptr, alignment(tuple_type)))
   trap_if(ptr + elem_size(tuple_type) > len(cx.opts.memory))
   store(cx, tuple_value, tuple_type, ptr)
-  return [ptr]
+  return flat_vals
 ```
 The `may_leave` flag is guarded by `canon_lower` below to prevent a component
 from calling out of the component while in the middle of lowering, ensuring
@@ -2205,43 +2207,60 @@ component instance defining a resource can access its representation.
 
 For a canonical definition:
 ```wasm
-(canon task.start (core func $f))
+(canon task.start $ft (core func $f))
 ```
 validation specifies:
-* `$f` is given type `(func (param i32))`
+* `$f` is given type `$ft`, which validation requires to be a (core) function type
 
 Calling `$f` invokes the following function which extracts the arguments from the
 caller and lowers them into the current instance:
 ```python
-async def canon_task_start(task, i):
+async def canon_task_start(task, core_ft, flat_args):
+  assert(len(core_ft.params) == len(flat_args))
   trap_if(task.opts.sync)
+  trap_if(core_ft != flatten_functype(CanonicalOptions(), FuncType([], task.ft.params), 'lower'))
   task.start()
-  lower_async_values(task, task.start_thunk(), task.ft.param_types(), CoreValueIter([i]))
-  return []
+  args = task.start_thunk()
+  flat_results = lower_sync_values(task, MAX_FLAT_RESULTS, args, task.ft.param_types(), CoreValueIter(flat_args))
+  assert(len(core_ft.results) == len(flat_results))
+  return flat_results
 ```
-The call to the `Task.start` (defined above) ensures that `canon task.start` is
-called exactly once, before `canon task.return`, before an async call finishes.
+An expected implementation of `task.start` would generate a core wasm function
+for each `async`-lifted export that performs the fused copy of the arguments
+into the caller, storing the index of this function in the `Task` structure and
+using `call_indirect` to perform the function-type-equality check required
+here. The call to `Task.start` (defined above) ensures that `canon task.start`
+is called exactly once, before `canon task.return`, before an async call
+finishes.
 
 ### 🔀 `canon task.return`
 
 For a canonical definition:
 ```wasm
-(canon task.return (core func $f))
+(canon task.return $ft (core func $f))
 ```
 validation specifies:
-* `$f` is given type `(func (param i32))`
+* `$f` is given type `$ft`, which validation requires to be a (core) function type
 
 Calling `$f` invokes the following function which lifts the results from the
 current instance and passes them to the caller:
 ```python
-async def canon_task_return(task, i):
+async def canon_task_return(task, core_ft, flat_args):
+  assert(len(core_ft.params) == len(flat_args))
   trap_if(task.opts.sync)
+  trap_if(core_ft != flatten_functype(CanonicalOptions(), FuncType(task.ft.results, []), 'lower'))
   task.return_()
-  task.return_thunk(lift_async_values(task, CoreValueIter([i]), task.ft.result_types()))
+  results = lift_sync_values(task, MAX_FLAT_PARAMS, CoreValueIter(flat_args), task.ft.result_types())
+  task.return_thunk(results)
+  assert(len(core_ft.results) == 0)
   return []
 ```
-The call to `Task.return_` (defined above) ensures that `canon task.return` is
-called exactly once, after `canon task.start`, before an async call finishes.
+An expected implementation of `task.return` would generate a core wasm function
+for each `async`-lifted export that performs the fused copy of the results into
+the caller, storing the index of this function in the `Task` structure and
+using `call_indirect` to perform the function-type-equality check required
+here. The call to `Task.return_` (defined above) ensures that `canon task.return`
+is called exactly once, after `canon task.start`, before an async call finishes.
 
 ### 🔀 `canon task.wait`
 

design/mvp/Explainer.md

@@ -1306,8 +1306,8 @@ canon ::= ...
         | (canon resource.new <typeidx> (core func <id>?))
         | (canon resource.drop <typeidx> async? (core func <id>?))
         | (canon resource.rep <typeidx> (core func <id>?))
-        | (canon task.start (core func <id>?)) 🔀
-        | (canon task.return (core func <id>?)) 🔀
+        | (canon task.start <core:typeidx> (core func <id>?)) 🔀
+        | (canon task.return <core:typeidx> (core func <id>?)) 🔀
         | (canon task.wait (core func <id>?)) 🔀
         | (canon task.poll (core func <id>?)) 🔀
         | (canon task.yield (core func <id>?)) 🔀
@@ -1367,21 +1367,26 @@ transferring ownership of the newly-created resource to the export's caller.
 See the [async explainer](Async.md) for high-level context and terminology
 and the [Canonical ABI explainer] for detailed runtime semantics.
 
-The `task.start` built-in has type `[i32] -> []` where the `i32` is a pointer
-into a linear memory buffer that will receive the arguments of the call to
-the current task. This built-in must be called from an `async`-lifted export
-exactly once per export activation. Delaying the call to `task.start` allows
-the async callee to exert *backpressure* on the caller. (See also
-[Starting](Async.md#starting) in the async explainer and [`canon_task_start`]
-in the Canonical ABI explainer.)
-
-The `task.return` built-in has type `[i32] -> []` where the `i32` is a pointer
-to a linear memory buffer containing the value to be returned from the current
+The `task.start` built-in returns the arguments to the currently-executing
 task. This built-in must be called from an `async`-lifted export exactly once
-per export activation after `task.start`. After calling `task.return`, the
-callee can continue executing for an arbitrary amount of time before returning
-to the caller. (See also [Returning](Async.md#returning) in the async explainer
-and [`canon_task_return`] in the Canonical ABI explainer.)
+per export activation. Delaying the call to `task.start` allows the async
+callee to exert *backpressure* on the caller. The `canon task.start` definition
+takes the type index of a core function type and produces a core function with
+exactly that type. When called, the declared core function type is checked
+to match the lowered function type of a component-level function returning the
+parameter types of the current task. (See also [Starting](Async.md#starting) in
+the async explainer and [`canon_task_start`] in the Canonical ABI explainer.)
+
+The `task.return` built-in takes as parameters the result values of the
+currently-executing task. This built-in must be called from an `async`-lifted
+export exactly once per export activation after `task.start`. After calling
+`task.return`, the callee can continue executing for an arbitrary amount of
+time before returning to the caller. The `canon task.return` definition takes
+the type index of a core function type and produces a core function with
+exactly that type. When called, the declared core function type is checked
+to match the lowered function type of a component-level function taking the
+result types of the current task. (See also [Returning](Async.md#returning) in
+the async explainer and [`canon_task_return`] in the Canonical ABI explainer.)
 
 The `task.wait` built-in has type `[i32] -> [i32]`, returning an event and
 storing the 4-byte payload of the event at the address passed as parameter.

design/mvp/canonical-abi/definitions.py

@@ -49,6 +49,8 @@ class ModuleType(ExternType):
 class CoreFuncType(CoreExternType):
   params: list[str]
   results: list[str]
+  def __eq__(self, other):
+    return self.params == other.params and self.results == other.results
 
 @dataclass
 class CoreMemoryType(CoreExternType):
@@ -1332,12 +1334,14 @@ def lower_heap_values(cx, vs, ts, out_param):
   tuple_value = {str(i): v for i,v in enumerate(vs)}
   if out_param is None:
     ptr = cx.opts.realloc(0, 0, alignment(tuple_type), elem_size(tuple_type))
+    flat_vals = [ptr]
   else:
     ptr = out_param.next('i32')
+    flat_vals = []
   trap_if(ptr != align_to(ptr, alignment(tuple_type)))
   trap_if(ptr + elem_size(tuple_type) > len(cx.opts.memory))
   store(cx, tuple_value, tuple_type, ptr)
-  return [ptr]
+  return flat_vals
 
 ### `canon lift`
 
@@ -1464,18 +1468,26 @@ async def canon_resource_rep(rt, task, i):
 
 ### `canon task.start`
 
-async def canon_task_start(task, i):
+async def canon_task_start(task, core_ft, flat_args):
+  assert(len(core_ft.params) == len(flat_args))
   trap_if(task.opts.sync)
+  trap_if(core_ft != flatten_functype(CanonicalOptions(), FuncType([], task.ft.params), 'lower'))
   task.start()
-  lower_async_values(task, task.start_thunk(), task.ft.param_types(), CoreValueIter([i]))
-  return []
+  args = task.start_thunk()
+  flat_results = lower_sync_values(task, MAX_FLAT_RESULTS, args, task.ft.param_types(), CoreValueIter(flat_args))
+  assert(len(core_ft.results) == len(flat_results))
+  return flat_results
 
 ### `canon task.return`
 
-async def canon_task_return(task, i):
+async def canon_task_return(task, core_ft, flat_args):
+  assert(len(core_ft.params) == len(flat_args))
   trap_if(task.opts.sync)
+  trap_if(core_ft != flatten_functype(CanonicalOptions(), FuncType(task.ft.results, []), 'lower'))
   task.return_()
-  task.return_thunk(lift_async_values(task, CoreValueIter([i]), task.ft.result_types()))
+  results = lift_sync_values(task, MAX_FLAT_PARAMS, CoreValueIter(flat_args), task.ft.result_types())
+  task.return_thunk(results)
+  assert(len(core_ft.results) == 0)
   return []
 
 ### `canon task.wait`

design/mvp/canonical-abi/run_tests.py

@@ -482,8 +482,8 @@ async def test_async_to_async():
   eager_ft = FuncType([], [U8()])
   async def core_eager_producer(task, args):
     assert(len(args) == 0)
-    [] = await canon_task_start(task, 1000)
-    [] = await canon_task_return(task, 43)
+    [] = await canon_task_start(task, CoreFuncType([],[]), [])
+    [] = await canon_task_return(task, CoreFuncType(['i32'],[]), [43])
     return []
   eager_callee = partial(canon_lift, producer_opts, producer_inst, core_eager_producer, eager_ft)
 
@@ -505,11 +505,9 @@ async def blocking_callee(entered, start_thunk, return_thunk):
   async def consumer(task, args):
     assert(len(args) == 0)
 
-    ptr = consumer_heap.realloc(0, 0, 1, 1)
-    [] = await canon_task_start(task, ptr)
-    b = consumer_heap.memory[ptr]
-    assert(b == True)
+    [b] = await canon_task_start(task, CoreFuncType([],['i32']), [])
 
+    ptr = consumer_heap.realloc(0, 0, 1, 1)
     [ret] = await canon_lower(consumer_opts, eager_callee, eager_ft, task, [0, ptr])
     assert(ret == 0)
     u8 = consumer_heap.memory[ptr]
@@ -565,7 +563,7 @@ async def dtor(task, args):
     assert(callidx == 1)
     assert(task.num_async_subtasks == 0)
 
-    [] = await canon_task_return(task, 42)
+    [] = await canon_task_return(task, CoreFuncType(['i32'],[]), [42])
     return []
 
   ft = FuncType([Bool()],[U8()])
@@ -602,7 +600,7 @@ async def producer(fut, entered, start_thunk, return_thunk):
   consumer_ft = FuncType([],[U32()])
   async def consumer(task, args):
     assert(len(args) == 0)
-    [] = await canon_task_start(task, 0)
+    [] = await canon_task_start(task, CoreFuncType([],[]), [])
 
     [ret] = await canon_lower(opts, producer1, producer_ft, task, [0, 0])
     assert(ret == (1 | (AsyncCallState.STARTED << 30)))
@@ -624,7 +622,7 @@ async def callback(task, args):
       assert(args[0] == 43)
       assert(args[1] == AsyncCallState.DONE)
       assert(args[2] == 2)
-      [] = await canon_task_return(task, 83)
+      [] = await canon_task_return(task, CoreFuncType(['i32'],[]), [83])
       return [0]
 
   consumer_inst = ComponentInstance()
@@ -701,8 +699,8 @@ async def consumer(task, args):
 
     assert(task.poll() is None)
 
-    await canon_task_start(task, 0)
-    await canon_task_return(task, 83)
+    await canon_task_start(task, CoreFuncType([],[]), [])
+    await canon_task_return(task, CoreFuncType(['i32'],[]), [83])
     return []
 
   consumer_inst = ComponentInstance()