internal/task: add Waiter for waiting for a flag from interrupts

This provides an abstraction to allow goroutines to wait for an event
from an interrupt, and for interrupts to send such an event and know
whether the goroutine is still working on the previous event.

src/internal/task/waiter.go

@@ -0,0 +1,99 @@
+package task
+
+import (
+	"runtime/interrupt"
+	"unsafe"
+)
+
+// A waiter waits for an interrupt to fire. Call Wait() from a goroutine to
+// pause it, and call Resume() from an interrupt handler to resume the
+// goroutine.
+type Waiter struct {
+	waiting *Task
+	lock    PMutex
+}
+
+var resumeTaskSentinel = (*Task)(unsafe.Pointer(uintptr(2)))
+var workingTaskSentinel = (*Task)(unsafe.Pointer(uintptr(1)))
+
+// Wait from a main goroutine until an interrupt calls Resume(). It will also
+// return if the interrupt called Resume() before the Wait() call (to avoid a
+// race condition).
+func (w *Waiter) Wait() {
+	if interrupt.In() {
+		runtimePanic("Waiter: called Wait from interrupt")
+	}
+
+	mask := interrupt.Disable()
+	w.lock.Lock()
+	switch w.waiting {
+	case nil, workingTaskSentinel:
+		w.waiting = Current()
+		w.lock.Unlock()
+		interrupt.Restore(mask)
+		Pause()
+	case resumeTaskSentinel:
+		// Marked as 'resume now', can return immediately.
+		w.waiting = workingTaskSentinel
+		w.lock.Unlock()
+		interrupt.Restore(mask)
+	default:
+		w.lock.Unlock()
+		interrupt.Restore(mask)
+		runtimePanic("Waiter: task is waiting already")
+	}
+}
+
+// Resume a waiting goroutine from an interrupt handler. If there is a goroutine
+// waiting, it will resume. If not, the next one that calls Wait() will
+// immediately resume.
+func (w *Waiter) Resume() {
+	if !interrupt.In() {
+		runtimePanic("Waiter: called Resume outside interrupt")
+	}
+
+	waiting := w.waiting
+	switch waiting {
+	case nil, workingTaskSentinel:
+		w.waiting = resumeTaskSentinel
+	case resumeTaskSentinel:
+		// Called Resume() twice in a row, this may indicate a bug at the caller
+		// site.
+	default:
+		// Schedule the given task to resume.
+		// If it's not yet paused, it will immediately resume on the next call
+		// to Pause().
+		w.waiting = workingTaskSentinel
+		scheduleTask(waiting)
+	}
+}
+
+// Return true if Done has not been called after a Resume call.
+// This is typically used to indicate the task outside the interrupt is still
+// being worked on.
+func (w *Waiter) Working() bool {
+	switch w.waiting {
+	case workingTaskSentinel, resumeTaskSentinel:
+		return true
+	default: // nil, <*task.Task>
+		return false
+	}
+}
+
+// Done can be called outside interrupt context to indicate the task this waiter
+// was waiting for is done, and Working() can return false. It is entirely
+// optional, if not called Working will continue to return true until it is
+// blocked in Wait() but the waiter will function normally otherwise.
+func (w *Waiter) Done() {
+	if interrupt.In() {
+		runtimePanic("Waiter: called Done from interrupt")
+	}
+
+	mask := interrupt.Disable()
+	w.lock.Lock()
+	if w.waiting == workingTaskSentinel {
+		w.waiting = nil
+	}
+	w.lock.Unlock()
+	interrupt.Restore(mask)
+}