Add 'non-blocking' function attribute

design/mvp/Async.md

@@ -151,7 +151,34 @@ component-level function that has been [lifted] from Core WebAssembly with the
 function that does not have the `async` option set (which is the default and
 only option prior to Preview 3). Thus, the sync/async distinction appears
 only independently in how a component-level function is *implemented* or
-*called*.
+*called*. This lack of distinction helps to avoid the classic ["What Color Is
+Your Function?"][Color] problem.
+
+Function types *may* be given an optional `non-blocking` function attribute to
+indicate that the caller should assume that the callee doesn't block. However,
+`non-blocking` is ignored at validation- and run-time and thus a `non-blocking`
+function may in fact block (and indeed there are valid use cases for doing so,
+such as in performance-insensitive virtualization scenarios). Therefore,
+`non-blocking` is primarily intended as a *hint* to inform source-language
+bindings to generate synchronous source-level function signatures (lifted and
+and lowered without setting the `async` Canonical ABI option). Importantly,
+since "blocking" is the default, interface authors are encouraged to apply
+`non-blocking` judiciously.
+
+Since in many languages `new` expressions cannot be async, `constructor`
+functions also imply `non-blocking`. In the future, [getters and setters] would
+also imply `non-blocking`. For example:
+```wit
+interface filesystem {
+  resource file {
+    constructor();                                  // non-blocking by default
+    is-closed: non-blocking func() -> bool;         // explicitly non-blocking
+    read: func(num-bytes: u32) -> result<list<u8>>; // blocking by default
+```
+If a resource-type has a potentially-blocking constructor, it can simply use
+`static new: func(...) -> my-resource` instead; `constructor` has no advantages
+beyond more-idiomatic bindings generation in some languages.
+
 
 ### Task
 
@@ -591,8 +618,6 @@ Native async support is being proposed incrementally. The following features
 will be added in future chunks roughly in the order list to complete the full
 "async" story, with a TBD cutoff between what's in [WASI Preview 3] and what
 comes after:
-* `nonblocking` function type attribute: allow a function to declare in its
-  type that it will not transitively do anything blocking
 * define what `async` means for `start` functions (top-level await + background
   tasks), along with cross-task coordination built-ins
 * `subtask.cancel`: allow a supertask to signal to a subtask that its result is
@@ -646,6 +671,7 @@ comes after:
 [Use Cases]: ../high-level/UseCases.md
 [Blast Zone]: FutureFeatures.md#blast-zones
 [Reentrance]: Explainer.md#component-invariants
+[Getters And Setters]: https://github.com/WebAssembly/component-model/issues/235
 
 [stack-switching]: https://github.com/WebAssembly/stack-switching/
 [JSPI]: https://github.com/WebAssembly/js-promise-integration/

design/mvp/Binary.md

@@ -214,7 +214,11 @@ valtype       ::= i:<typeidx>                             => i
                 | pvt:<primvaltype>                       => pvt
 resourcetype  ::= 0x3f 0x7f f?:<funcidx>?                 => (resource (rep i32) (dtor f)?)
                 | 0x3e 0x7f f:<funcidx> cb?:<funcidx>?    => (resource (rep i32) (dtor async f (callback cb)?))
-functype      ::= 0x40 ps:<paramlist> rs:<resultlist>     => (func ps rs)
+functype      ::= 0x40 sig:<funcsig>                      => (func sig)
+                | 0x3d attrs:<funcattrs> sig:<funcsig>    => (func attrs sig)
+funcattrs     ::= 0x00                                    =>
+                | 0x01                                    => non-blocking
+funcsig       ::= ps:<paramlist> rs:<resultlist>          => ps rs
 paramlist     ::= lt*:vec(<labelvaltype>)                 => (param lt)*
 resultlist    ::= 0x00 t:<valtype>                        => (result t)
                 | 0x01 0x00                               =>
@@ -386,7 +390,10 @@ Notes:
   export, respectively, in the same scope (component, component type or
   instance type).
 * Validation of `[constructor]` names requires that the `func` returns a
-  `(result (own $R))`, where `$R` is the resource labeled `r`.
+  `(result (own $R))`, where `$R` is the resource labeled `r`. Additionally,
+  the function type *should* have the `non-blocking` attribute. In a future
+  pre-1.0 breaking change, validation will *require* the `non-blocking`
+  attribute.
 * Validation of `[method]` names requires the first parameter of the function
   to be `(param "self" (borrow $R))`, where `$R` is the resource labeled `r`.
 * Validation of `[method]` and `[static]` names ensures that all field names
@@ -487,6 +494,8 @@ named once.
 * The opcodes (for types, canon built-ins, etc) should be re-sorted
 * The two `list` type codes should be merged into one with an optional immediate.
 * The `0x00` prefix byte of `importname'` and `exportname'` will be removed or repurposed.
+* Merge the two `functype` opcodes.
+* Require `non-blocking` on `[constructor]` functions.
 
 
 [`core:byte`]: https://webassembly.github.io/spec/core/binary/values.html#binary-byte

design/mvp/Explainer.md

@@ -560,7 +560,7 @@ defvaltype    ::= bool
 valtype       ::= <typeidx>
                 | <defvaltype>
 resourcetype  ::= (resource (rep i32) (dtor async? <funcidx> (callback <funcidx>)?)?)
-functype      ::= (func (param "<label>" <valtype>)* (result <valtype>)?)
+functype      ::= (func non-blocking? (param "<label>" <valtype>)* (result <valtype>)?)
 componenttype ::= (component <componentdecl>*)
 instancetype  ::= (instance <instancedecl>*)
 componentdecl ::= <importdecl>
@@ -783,7 +783,11 @@ shared-nothing functions, resources, components, and component instances:
 
 The `func` type constructor describes a component-level function definition
 that takes a list of uniquely-named `valtype` parameters and optionally returns
-a `valtype`.
+a `valtype`. Function types can optionally be annotated with a `non-blocking`
+attribute which has no semantic effect and is ignored at validation- and
+run-time, serving primarily as a hint that tells bindings generators to lift
+and lower without setting the `async` `canonopt` (see the [async
+explainer](Async.md#sync-and-async-functions) for more details).
 
 The `resource` type constructor creates a fresh type for each instance of the
 containing component (with "freshness" and its interaction with general

design/mvp/WIT.md

@@ -876,6 +876,7 @@ keyword ::= 'as'
           | 'include'
           | 'interface'
           | 'list'
+          | 'non-blocking'
           | 'option'
           | 'own'
           | 'package'
@@ -1295,7 +1296,7 @@ typedef-item ::= resource-item
 
 func-item ::= id ':' func-type ';'
 
-func-type ::= 'func' param-list result-list
+func-type ::= 'non-blocking'? 'func' param-list result-list
 
 param-list ::= '(' named-type-list ')'