Fix bug in Dispatch executor preventing long sleeps

[rauhul]

Dispatch executor's swift_task_enqueueGlobalWithDelayImpl adds the requested delay to the current time and enqueues the job to run at that time. This has a bug when using a very large delay on the order of 2^63 to 2^64 nanoseconds because dispatch_time(), which is used to preform the now + delay calculation, returns DISPATCH_TIME_FOREVER. However, dispatch_after(), which is used to actually enqueue the job, has undefined behavior for this time; the result in practice is that the job is immediately run.

This change resolves the bug by leaking the job if the result of dispatch_time is DISPATCH_TIME_FOREVER. This should effectively be the same as the job getting delayed forever.

Radar-Id: rdar://problem/89455918

[rauhul]

@DougGregor I don't know the details of Task cancellation so I can't tell if leaking the job to handle this exceptional case is actually valid. Do you have any ideas?

[ktoso]

Hm... pretty weird.

Could we perhaps test that the run does not at least run immediately like the bug suggested; by sleeping "forever" and then causing a cancellation after e.g. 2 seconds or so and see that it properly is cancelled? Just droping a task on the floor is not right, it still must be able to get cancelled like that after all.

[ktoso]

Seems this might supersede my WIP over here which I didn't finish back then: #69118

Resolves #59174

[ktoso]

Just leaking is too naive; we need to support cancellation on such task:

let t = Task {
  try? await Task.sleep(.max)
  exit(0)
}
t.cancel()

should be cancelled and return properly -- that's a test-case worth adding