Lesson 29 · Cancelling a task - Build a Green-Thread Scheduler

Cancelling a task

Sometimes work should stop before it finishes - a timeout fires, a result is no longer needed. Cancellation removes a task from the runtime wherever it is - waiting in the run queue, parked on a wait queue, or asleep on a timer. Today you build it cleanly, so a cancelled task simply never runs again.

The goal

Add Cancel, which removes a task from the run queue or its wait queue and marks it finished.

Start here - the target

TO DO

Scenario: A cancelled task stops running

Giventask 1 that yields repeatedly, and task 2 that cancels task 1 on its first step then finishes, spawned in that order

WhenRun is called

Thentask 1 runs once, is cancelled, and never runs again; Run ends cleanly with no deadlock and the run trace is [1, 2]

Andcancelling a task that is currently blocked removes it from its wait queue so it is not left parked forever

Background

Cancellation is the ability to stop a green thread on demand. A task might be in one of three places when you cancel it: sitting in the run queue waiting its turn, parked on a wait queue, or asleep in the timer set. Cancel has to find it wherever it is and pull it out, then mark it finished and drop the live count so the runtime does not think work is still outstanding.

To remove a parked task efficiently, each task remembers where it blocked - set that parkedIn reference when it blocks, clear it when it wakes. Here task 2 cancels task 1 after task 1 has run just once; task 1 is removed from the run queue and never stepped again, so Run ends cleanly with trace [1, 2] and no deadlock (the live count went back to zero). Cancelling a blocked task is the important edge: it must be unlinked from its wait queue, or it would sit there forever and later trip the deadlock detector. One thing cancellation leaves dangling, though - anyone waiting on the cancelled task to finish. That is the next lesson.

Make it work

// record where a task is parked so Cancel can find it; a task is in exactly
// ONE place at a time (a run-queue lane, OR t.parkedIn, OR the timer set)
func (s *Scheduler) Cancel(t *Task) {
  if t.done { return }          // already finished or cancelled: no-op
  remove t from its run-queue lane, or from t.parkedIn.waiters, or from s.timers
  t.done = true
  s.live--                      // it no longer counts as alive
}
// also mark t.done = true when a task's step returns Done, so Cancel on an
// already-finished task is a safe no-op

CheckpointDONE

A task can be cancelled from anywhere and stops running. Commit and stop here.