CABI: tidy stream code a bit (no change)

Author: lukewagner

design/mvp/CanonicalABI.md

@@ -3792,6 +3792,7 @@ the other end closed, or this end cancelled the copy via
 ```python
   def copy_event(why, revoke_buffer):
     revoke_buffer()
+    assert(e.copying)
     e.copying = False
     return (event_code, i, pack_copy_result(task, e, buffer, why))
 
@@ -3810,13 +3811,13 @@ synchronously returns how much was copied and how the operation ended to the
 caller. Otherwise, the built-in blocks:
 ```python
   else:
+    e.copying = True
     if opts.sync:
       await task.wait_on(e.wait_for_pending_event(), sync = True)
       code,index,payload = e.get_event()
       assert(code == event_code and index == i)
       return [payload]
     else:
-      e.copying = True
       return [BLOCKED]
 ```
 In the synchronous case, the caller synchronously waits for progress

design/mvp/canonical-abi/definitions.py

@@ -2197,6 +2197,7 @@ async def copy(EndT, BufferT, event_code, stream_or_future_t, opts, task, i, ptr
 
   def copy_event(why, revoke_buffer):
     revoke_buffer()
+    assert(e.copying)
     e.copying = False
     return (event_code, i, pack_copy_result(task, e, buffer, why))
 
@@ -2209,13 +2210,13 @@ def on_copy_done(why):
   if e.copy(task.inst, buffer, on_partial_copy, on_copy_done) == 'done':
     return [pack_copy_result(task, e, buffer, 'completed')]
   else:
+    e.copying = True
     if opts.sync:
       await task.wait_on(e.wait_for_pending_event(), sync = True)
       code,index,payload = e.get_event()
       assert(code == event_code and index == i)
       return [payload]
     else:
-      e.copying = True
       return [BLOCKED]
 
 BLOCKED   = 0xffff_ffff