runtime: add a runtime.wasiOnIdle function

This small patch provides a mechanism for bridging the Go scheduler with
WASIp3's concurrency model.

Note that I'm fairly new to Go and very open to feedback and alternative
approaches.  Please consider this as much an RFC as it is a PR.

Background

Both the `js` and `wasip1` target OSes define a `runtime.beforeIdle` function,
called by the scheduler if and when no goroutines are runnable.  In the case of
`js`, the Go scheduler yields to the JS event loop to await any async events it
might produce.  In the case of `wasip1` (and all other OSes), `beforeIdle` does
nothing since that platform has neither an event loop nor async events.

However, WASIp3 (due to be released early next year) _does_ support [concurrency
and asynchronous
I/O](https://github.com/WebAssembly/component-model/blob/main/design/mvp/Concurrency.md),
in which case it's useful for the Go scheduler to yield to the host once all
goroutines have gone idle, just like it does for JS.

Motivation

This patch is intended as a baby step towards full `GOOS=wasip3` support.
Unlike `GOOS=wasip1`, where blocking I/O operations block _all_ goroutines,
`GOOS=wasip3` can support asynchronous I/O operations which cooperate with the
Go scheduler, only blocking the goroutine doing the call and allowing any others
to continue running.  Internally, each such operation may either complete
immediately without blocking or return a `waitable` handle representing a
pending event.  We can associate a channel with that `waitable`, to be written
to once the host delivers the corresponding event.  The calling goroutine reads
from that channel before returning a value.

In order to support the above, each exported function needs to be able to wait
for all goroutines to reach an idle state, collect any accumulated `waitable`
handles, and return control to the host until one or more events are ready.
`runtime.wasiOnIdle` provides that capability by accepting a callback to be run
by `runtime.beforeIdle`.

Once `GOOS=wasip3` has been fully implemented, the above can be handled
internally by the compiler and runtime.  As a first step, though, I've created
[a bindings
generator](https://github.com/bytecodealliance/wit-bindgen/tree/main/crates/go)
which generates import and export glue code from the IDL in which the WASIp3
interfaces are defined.  That glue code handles bridging Go's scheduler to the
WASIp3 host event loop.  It's able to do this using standard goroutines and
channels, with no special integration with the Go scheduler _except_ for
`runtime.wasiOnIdle`, hence this patch.

Note that `wasiOnIdle` is private since it's not intended for general use; the
glue code mentioned above uses `go:linkname` to access it.  This use of
`go:linkname` is a temporary measure while we experiment with WASIp3 support
outside of the runtime.  The eventual goal is to encapsulate the
host<->scheduler interaction entirely within the Go runtime.

Concurrent imports and exports

WASIp3 is based on the WebAssembly [Component
Model](https://github.com/WebAssembly/component-model), which includes an IDL
(WebAssembly Interface Types, or WIT) and an ABI for expressing high-level
types, functions, and interfaces which can be used to represent both traditional
OS features (e.g. filesystem and network access) and high-level features such as
HTTP request handlers and database connections.  WIT can also be used to
represent custom, application specific APIs and then build components which
either implement or consume those APIs, analogous to how shared libraries work
on native OSes.

Consequently, `GOOS=wasip3` will ideally support creating both
"executable"-style applications with a single `func main` entrypoint and also
"library"-style components with one or more custom entrypoints and imports.
Fortunately, Go already has `go:wasmexport` and `go:wasmimport` directives to
support this.  The `wit-bindgen-go` project mentioned above builds upon those
directives to support exporting and importing _concurrent_ functions which may
suspend and resume as necessary (e.g. due to I/O) prior to producing a result.
Hypothetically, this support could be integrated into the compiler if there's
interest.

Author: dicej

src/runtime/lock_wasip1.go

@@ -105,8 +105,16 @@ func notetsleepg(n *note, ns int64) bool {
 	}
 }
 
+var onIdle = func() bool {
+	return false
+}
+
+func wasiOnIdle(callback func() bool) {
+	onIdle = callback
+}
+
 func beforeIdle(int64, int64) (*g, bool) {
-	return nil, false
+	return nil, onIdle()
 }
 
 func checkTimeouts() {}