Remove restriction on write-before-lift, remove invalid assert, add test

Author: lukewagner

design/mvp/CanonicalABI.md

@@ -1655,7 +1655,6 @@ def lift_async_value(ReadableHandleT, WritableHandleT, cx, i, t):
       cx.inst.waitables.remove(i)
     case WritableHandleT():
       trap_if(h.paired)
-      assert(not h.copying_buffer)
       h.paired = True
       if contains_borrow(t):
         h.borrow_scope = cx.borrow_scope
@@ -3118,14 +3117,14 @@ async def async_copy(HandleT, BufferT, t, opts, event_code, task, i, ptr, n):
   h = task.inst.waitables.get(i)
   trap_if(not isinstance(h, HandleT))
   trap_if(h.t != t)
-  trap_if(not h.paired)
   trap_if(h.copying_buffer)
   cx = LiftLowerContext(opts, task.inst, h.borrow_scope)
   buffer = BufferT(cx, t, ptr, n)
   if h.stream.closed():
     flat_results = [pack_async_copy_result(task, buffer, h)]
   else:
     if opts.sync:
+      trap_if(not h.paired)
       await task.call_sync(h.copy, buffer)
       flat_results = [pack_async_copy_result(task, buffer, h)]
     else:
@@ -3147,12 +3146,11 @@ async def async_copy(HandleT, BufferT, t, opts, event_code, task, i, ptr, n):
           flat_results = [pack_async_copy_result(task, buffer, h)]
   return flat_results
 ```
-The trap if `not h.paired` prevents `write`s on the writable end of streams or
-futures that have not yet been lifted. The `copying_buffer` field serves as a
-boolean indication of whether an async `read` or `write` is already in
-progress, preventing multiple overlapping calls to `read` or `write`. (This
-restriction could be relaxed [in the future](Async.md#TODO) to allow greater
-pipeline parallelism.)
+The `trap_if(h.copying_buffer)` trap prevents multiple overlapping calls to
+`read` or `write`. (This restriction could be relaxed [in the
+future](Async.md#TODO) to allow greater pipeline parallelism.) The
+`trap_if(not h.paired)` in the synchronous case prevents what would otherwise
+be a deadlock, performing a blocking write when there is no reader.
 
 One subtle corner case handled by this code that is worth pointing out is that,
 between calling `h.copy()` and `h.copy()` returning, wasm guest code can call

design/mvp/canonical-abi/definitions.py

@@ -1144,7 +1144,6 @@ def lift_async_value(ReadableHandleT, WritableHandleT, cx, i, t):
       cx.inst.waitables.remove(i)
     case WritableHandleT():
       trap_if(h.paired)
-      assert(not h.copying_buffer)
       h.paired = True
       if contains_borrow(t):
         h.borrow_scope = cx.borrow_scope
@@ -1978,14 +1977,14 @@ async def async_copy(HandleT, BufferT, t, opts, event_code, task, i, ptr, n):
   h = task.inst.waitables.get(i)
   trap_if(not isinstance(h, HandleT))
   trap_if(h.t != t)
-  trap_if(not h.paired)
   trap_if(h.copying_buffer)
   cx = LiftLowerContext(opts, task.inst, h.borrow_scope)
   buffer = BufferT(cx, t, ptr, n)
   if h.stream.closed():
     flat_results = [pack_async_copy_result(task, buffer, h)]
   else:
     if opts.sync:
+      trap_if(not h.paired)
       await task.call_sync(h.copy, buffer)
       flat_results = [pack_async_copy_result(task, buffer, h)]
     else:

design/mvp/canonical-abi/run_tests.py

@@ -1234,11 +1234,19 @@ async def core_func(task, args):
     assert(len(args) == 0)
     [wsi] = await canon_stream_new(U8Type(), task)
     assert(wsi == 1)
-    retp = 4
+    [ret] = await canon_stream_write(U8Type(), opts, task, wsi, 0, 4)
+    assert(ret == definitions.BLOCKED)
+    retp = 8
+    [ret] = await canon_lower(opts, host_ft, host_import, task, [wsi, retp])
+    assert(ret == 0)
+    result = int.from_bytes(mem[retp : retp+4], 'little', signed=False)
+    assert(result == (wsi | 2**31))
     [ret] = await canon_lower(opts, host_ft, host_import, task, [wsi, retp])
     assert(ret == 0)
     result = int.from_bytes(mem[retp : retp+4], 'little', signed=False)
     assert(result == (wsi | 2**31))
+    [ret] = await canon_stream_cancel_write(True, task, wsi)
+    assert(ret == 0)
     [] = await canon_stream_close_writable(U8Type(), task, wsi, 0)
     return []