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 [1] 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 [2] 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 [3], 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.

[1] https://github.com/WebAssembly/component-model/blob/main/design/mvp/Concurrency.md
[2] https://github.com/bytecodealliance/wit-bindgen/tree/main/crates/go
[3] https://github.com/WebAssembly/component-model

Signed-off-by: Joel Dice <[email protected]>

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() {}