Replace 'sync' with 'cancellable' in wait/poll/yield

design/mvp/Async.md

@@ -58,6 +58,9 @@ additional goals and requirements for native async support:
 * Maintain meaningful cross-language call stacks (for the benefit of debugging,
   logging and tracing).
 * Provide mechanisms for applying and observing backpressure.
+* Allow non-reentrant synchronous and event-loop-driven core wasm code (that,
+  e.g., assumes a single global linear memory stack) to not have to worry about
+  additional reentrancy.
 
 
 ## High-level Approach
@@ -474,6 +477,16 @@ wants to accept new accept new export calls while waiting or not.
 See the [`canon_backpressure_set`] function and [`Task.enter`] method in the
 Canonical ABI explainer for the setting and implementation of backpressure.
 
+In addition to *explicit* backpressure set by wasm code, there is also an
+*implicit* source of backpressure used to protect non-reentrant core wasm
+code. In particular, when an export is lifted synchronously or using an
+`async callback`, a component-instance-wide lock is implicitly acquired every
+time core wasm is executed. By returning to the event loop after every event
+(instead of once at the end of the task), `async callback` exports release
+the lock between every event, allowing a higher degree of concurrency than
+synchronous exports. `async` (stackful) exports ignore the lock entirely and
+thus achieve the highest degree of (cooperative) concurrency.
+
 Once a task is allowed to start according to these backpressure rules, its
 arguments are lowered into the callee's linear memory and the task is in
 the "started" state.
@@ -607,6 +620,9 @@ defined by the Component Model:
 * Whenever a task yields or waits on (or polls) a waitable set with an already
   pending event, whether the task "blocks" and transfers execution to its async
   caller is nondeterministic.
+* If multiple tasks are waiting on [backpressure](#backpressure), and the
+  backpressure is disabled, the order in which these pending tasks (and new
+  tasks started while there are still pending tasks) start is nondeterministic.
 
 Despite the above, the following scenarios do behave deterministically:
 * If a component `a` asynchronously calls the export of another component `b`,

design/mvp/Binary.md

@@ -295,7 +295,7 @@ canon    ::= 0x00 0x00 f:<core:funcidx> opts:<opts> ft:<typeidx> => (canon lift
            | 0x05                                                => (canon task.cancel (core func)) 🔀
            | 0x0a 0x7f i:<u32>                                   => (canon context.get i32 i (core func)) 🔀
            | 0x0b 0x7f i:<u32>                                   => (canon context.set i32 i (core func)) 🔀
-           | 0x0c async?:<async>?                                => (canon yield async? (core func)) 🔀
+           | 0x0c cancel?:<cancel?>                              => (canon yield cancel? (core func)) 🔀
            | 0x06 async?:<async?>                                => (canon subtask.cancel async? (core func)) 🔀
            | 0x0d                                                => (canon subtask.drop (core func)) 🔀
            | 0x0e t:<typeidx>                                    => (canon stream.new t (core func)) 🔀
@@ -316,15 +316,17 @@ canon    ::= 0x00 0x00 f:<core:funcidx> opts:<opts> ft:<typeidx> => (canon lift
            | 0x1d opts:<opts>                                    => (canon error-context.debug-message opts (core func)) 📝
            | 0x1e                                                => (canon error-context.drop (core func)) 📝
            | 0x1f                                                => (canon waitable-set.new (core func)) 🔀
-           | 0x20 async?:<async>? m:<core:memidx>                => (canon waitable-set.wait async? (memory m) (core func)) 🔀
-           | 0x21 async?:<async>? m:<core:memidx>                => (canon waitable-set.poll async? (memory m) (core func)) 🔀
+           | 0x20 cancel?:<cancel?> m:<core:memidx>              => (canon waitable-set.wait cancel? (memory m) (core func)) 🔀
+           | 0x21 cancel?:<cancel?> m:<core:memidx>              => (canon waitable-set.poll cancel? (memory m) (core func)) 🔀
            | 0x22                                                => (canon waitable-set.drop (core func)) 🔀
            | 0x23                                                => (canon waitable.join (core func)) 🔀
            | 0x40 ft:<typeidx>                                   => (canon thread.spawn_ref ft (core func)) 🧵
            | 0x41 ft:<typeidx> tbl:<core:tableidx>               => (canon thread.spawn_indirect ft tbl (core func)) 🧵
            | 0x42                                                => (canon thread.available_parallelism (core func)) 🧵
 async?   ::= 0x00                                                =>
            | 0x01                                                => async
+cancel?  ::= 0x00                                                =>
+           | 0x01                                                => cancellable 🚟
 opts     ::= opt*:vec(<canonopt>)                                => opt*
 canonopt ::= 0x00                                                => string-encoding=utf8
            | 0x01                                                => string-encoding=utf16
@@ -505,6 +507,8 @@ named once.
 * The `0x00` variant of `importname'` and `exportname'` will be removed. Any
   remaining variant(s) will be renumbered or the prefix byte will be removed or
   repurposed.
+* Most built-ins should have a `<canonopt>*` immediate instead of an ad hoc
+  subset of `canonopt`s.
 
 
 [`core:byte`]: https://webassembly.github.io/spec/core/binary/values.html#binary-byte