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Clarify {stream,future}.cancel-{read,write} #593

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Merged
lukewagner merged 2 commits into from
Jan 9, 2026
Merged

Clarify {stream,future}.cancel-{read,write} #593

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7cd62c1
Trap on call to {stream,future}.cancel-{read,write} after a prior can…
lukewagner Jan 6, 2026
16fb8ce
Add comma
lukewagner Jan 7, 2026
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24 changes: 16 additions & 8 deletions design/mvp/CanonicalABI.md
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Original file line number Diff line number Diff line change
Expand Up @@ -1525,7 +1525,8 @@ class CopyState(Enum):
IDLE = 1
SYNC_COPYING = 2
ASYNC_COPYING = 3
DONE = 4
CANCELLING_COPY = 4
DONE = 5

class CopyEnd(Waitable):
state: CopyState
Expand All @@ -1537,7 +1538,12 @@ class CopyEnd(Waitable):
self.shared = shared

def copying(self):
return self.state == CopyState.SYNC_COPYING or self.state == CopyState.ASYNC_COPYING
match self.state:
case CopyState.IDLE | CopyState.DONE:
return False
case CopyState.SYNC_COPYING | CopyState.ASYNC_COPYING | CopyState.CANCELLING_COPY:
return True
assert(False)

def drop(self):
trap_if(self.copying())
Expand All @@ -1553,11 +1559,11 @@ class WritableStreamEnd(CopyEnd):
self.shared.write(inst, src, on_copy, on_copy_done)
```
As shown in `drop`, attempting to drop a readable or writable end while a copy
is in progress traps. This means that client code must take care to wait for
these operations to finish (potentially cancelling them via
`stream.cancel-{read,write}`) before dropping. The `SYNC_COPY` vs. `ASYNC_COPY`
distinction is tracked in the state to determine whether the copy operation can
be cancelled.
is in progress or in the process of being cancelled traps. This means that
client code must take care to wait for these operations to finish (potentially
cancelling them via `stream.cancel-{read,write}`) before dropping. The
`SYNC_COPY` vs. `ASYNC_COPY` distinction is tracked in the state to determine
whether the copy operation can be cancelled.

The polymorphic `copy` method dispatches to either `ReadableStream.read` or
`WritableStream.write` and allows the implementations of `stream.{read,write}`
Expand Down Expand Up @@ -4270,6 +4276,7 @@ def cancel_copy(EndT, event_code, stream_or_future_t, async_, thread, i):
trap_if(not isinstance(e, EndT))
trap_if(e.shared.t != stream_or_future_t.t)
trap_if(e.state != CopyState.ASYNC_COPYING)
e.state = CopyState.CANCELLING_COPY
if not e.has_pending_event():
e.shared.cancel()
if not e.has_pending_event():
Expand All @@ -4286,7 +4293,8 @@ unconditionally traps if it transitively attempts to make a synchronous call to
`cancel-read` or `cancel-write` (regardless of whether the cancellation would
have completed without blocking). There is also a trap if there is not
currently an async copy in progress (sync copies do not expect or check for
cancellation and thus cannot be cancelled).
cancellation and thus cannot be cancelled, and repeatedly cancelling the same
async copy after the first call blocked is not allowed).

The *first* check for `e.has_pending_event()` catches the case where the copy has
already racily finished, in which case we must *not* call `cancel()`. Calling
Expand Down
11 changes: 9 additions & 2 deletions design/mvp/canonical-abi/definitions.py
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Expand Up @@ -886,7 +886,8 @@ class CopyState(Enum):
IDLE = 1
SYNC_COPYING = 2
ASYNC_COPYING = 3
DONE = 4
CANCELLING_COPY = 4
DONE = 5

class CopyEnd(Waitable):
state: CopyState
Expand All @@ -898,7 +899,12 @@ def __init__(self, shared):
self.shared = shared

def copying(self):
return self.state == CopyState.SYNC_COPYING or self.state == CopyState.ASYNC_COPYING
match self.state:
case CopyState.IDLE | CopyState.DONE:
return False
case CopyState.SYNC_COPYING | CopyState.ASYNC_COPYING | CopyState.CANCELLING_COPY:
return True
assert(False)

def drop(self):
trap_if(self.copying())
Expand Down Expand Up @@ -2431,6 +2437,7 @@ def cancel_copy(EndT, event_code, stream_or_future_t, async_, thread, i):
trap_if(not isinstance(e, EndT))
trap_if(e.shared.t != stream_or_future_t.t)
trap_if(e.state != CopyState.ASYNC_COPYING)
e.state = CopyState.CANCELLING_COPY
if not e.has_pending_event():
e.shared.cancel()
if not e.has_pending_event():
Expand Down
5 changes: 5 additions & 0 deletions design/mvp/canonical-abi/run_tests.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2031,6 +2031,11 @@ def core_func(thread, args):
host_source.block_cancel()
[ret] = canon_stream_cancel_read(StreamType(U8Type()), True, thread, rsi)
assert(ret == definitions.BLOCKED)
try:
canon_stream_cancel_read(StreamType(U8Type()), True, thread, rsi)
assert(False)
except Trap:
pass
host_source.write([7,8])
host_source.unblock_cancel()
[seti] = canon_waitable_set_new(thread)
Expand Down