internal/task: use asyncify on webassembly

This change implements a new "scheduler" for WebAssembly using binaryen's asyncify transform.
This is more reliable than the current "coroutines" transform, and works with non-Go code in the call stack.

runtime (js/wasm): handle scheduler nesting

If WASM calls into JS which calls back into WASM, it is possible for the scheduler to nest.
The event from the callback must be handled immediately, so the task cannot simply be deferred to the outer scheduler.
This creates a minimal scheduler loop which is used to handle such nesting.

compileopts/config.go

@@ -157,7 +157,7 @@ func (c *Config) OptLevels() (optLevel, sizeLevel int, inlinerThreshold uint) {
 // target.
 func (c *Config) FuncImplementation() string {
 	switch c.Scheduler() {
-	case "tasks":
+	case "tasks", "asyncify":
 		// A func value is implemented as a pair of pointers:
 		//     {context, function pointer}
 		// where the context may be a pointer to a heap-allocated struct

compileopts/options.go

@@ -8,7 +8,7 @@ import (
 
 var (
 	validGCOptions            = []string{"none", "leaking", "extalloc", "conservative"}
-	validSchedulerOptions     = []string{"none", "tasks", "coroutines"}
+	validSchedulerOptions     = []string{"none", "tasks", "coroutines", "asyncify"}
 	validSerialOptions        = []string{"none", "uart", "usb"}
 	validPrintSizeOptions     = []string{"none", "short", "full"}
 	validPanicStrategyOptions = []string{"print", "trap"}

compileopts/options_test.go

@@ -10,7 +10,7 @@ import (
 func TestVerifyOptions(t *testing.T) {
 
 	expectedGCError := errors.New(`invalid gc option 'incorrect': valid values are none, leaking, extalloc, conservative`)
-	expectedSchedulerError := errors.New(`invalid scheduler option 'incorrect': valid values are none, tasks, coroutines`)
+	expectedSchedulerError := errors.New(`invalid scheduler option 'incorrect': valid values are none, tasks, coroutines, asyncify`)
 	expectedPrintSizeError := errors.New(`invalid size option 'incorrect': valid values are none, short, full`)
 	expectedPanicStrategyError := errors.New(`invalid panic option 'incorrect': valid values are print, trap`)
 

compileopts/target.go

@@ -208,6 +208,11 @@ func LoadTarget(options *Options) (*TargetSpec, error) {
 	if err != nil {
 		return nil, err
 	}
+
+	if spec.Scheduler == "asyncify" {
+		spec.ExtraFiles = append(spec.ExtraFiles, "src/internal/task/task_asyncify_wasm.S")
+	}
+
 	return spec, nil
 }