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package main

import (
	"fmt"
	"os"
	"runtime/debug"
	"time"
)

func main() {
	debug.SetTraceback("system")
	if len(os.Args) == 1 {
		panic("before timers")
	}
	for i := 0; i < 10000; i++ {
		time.AfterFunc(time.Duration(5*time.Second), func() {
			fmt.Println("Hello!")
		})
	}
	panic("after timers")
}

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go run afterfunc.go 2>&1 | grep "^goroutine" | wc -l
4

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go run afterfunc.go after 2>&1 | grep "^goroutine" | wc -l
5

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goroutine 5 [syscall]:
runtime.notetsleepg(0x5014b8, 0x12a043838, 0x0)
        /home/moroz/go/src/runtime/lock_futex.go:205 +0x42 fp=0xc42002bf40 sp=0xc42002bf10
runtime.timerproc()
        /home/moroz/go/src/runtime/time.go:209 +0x2ec fp=0xc42002bfc0 sp=0xc42002bf40
runtime.goexit()
        /home/moroz/go/src/runtime/asm_amd64.s:2160 +0x1 fp=0xc42002bfc8 sp=0xc42002bfc0
created by runtime.addtimerLocked
        /home/moroz/go/src/runtime/time.go:116 +0xed

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func NewTimer(d Duration) *Timer {
    c := make(chan Time, 1)
    t := &Timer{
        C: c,
        r: runtimeTimer{
            when: when(d),
            f:    sendTime,
            arg:  c,
        },
    }
    startTimer(&t.r)
    return t
}

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// Add a timer to the heap and start or kick the timerproc if the new timer is
// earlier than any of the others.
func addtimerLocked(t *timer) {
	// when must never be negative; otherwise timerproc will overflow
	// during its delta calculation and never expire other runtime·timers.
	if t.when < 0 {
		t.when = 1<<63 - 1
	}
	t.i = len(timers.t)
	timers.t = append(timers.t, t)
	// maintain heap invariant
	siftupTimer(t.i)
	// new time is on top
	if t.i == 0 {
		// siftup moved to top: new earliest deadline.
		if timers.sleeping {
			// wake up sleeping goroutine, put to sleep with notetsleepg in timerproc()
			timers.sleeping = false
			notewakeup(&timers.waitnote)
		}
		if timers.rescheduling {
			// run parked goroutine, put to sleep with goparkunlock in timerproc()
			timers.rescheduling = false
			goready(timers.gp, 0)
		}
	}
	if !timers.created {
		// run timerproc() goroutine only once
		timers.created = true
		go timerproc()
	}
}

// Timerproc runs the time-driven events.
// It sleeps until the next event in the timers heap.
// If addtimer inserts a new earlier event, addtimerLocked wakes timerproc early.
func timerproc() {
	// set timer goroutine
	timers.gp = getg()
	// forever loop
	for {
		lock(&timers.lock)
		// mark goroutine not sleeping
		timers.sleeping = false
		now := nanotime()
		delta := int64(-1)
		// iterate over timers in heap starting from [0]
		for {
			// there are no more timers, exit iterating loop
			if len(timers.t) == 0 {
				delta = -1
				break
			}
			t := timers.t[0]
			delta = t.when - now
			if delta > 0 {
				break
			}
			// t.period means that it's ticker, so change when and move down
			// in heap to execute it again after t.period.
			if t.period > 0 {
				// leave in heap but adjust next time to fire
				t.when += t.period * (1 + -delta/t.period)
				siftdownTimer(0)
			} else {
				// remove from heap
				// this is just removing from heap operation:
				// - swap first(extremum) with last
				// - set last to nil
				// - maintain heap: move first to its true place with siftdownTimer.
				last := len(timers.t) - 1
				if last > 0 {
					timers.t[0] = timers.t[last]
					timers.t[0].i = 0
				}
				timers.t[last] = nil
				timers.t = timers.t[:last]
				if last > 0 {
					siftdownTimer(0)
				}
				// set i to -1, so concurrent deltimer won't do anything to
				// heap.
				t.i = -1 // mark as removed
			}
			f := t.f
			arg := t.arg
			seq := t.seq
			unlock(&timers.lock)
			if raceenabled {
				raceacquire(unsafe.Pointer(t))
			}
			// call timer function without lock
			f(arg, seq)
			lock(&timers.lock)
		}
		// if delta < 0 - timers is empty, set "rescheduling" and park timers
		// goroutine. It will sleep here until "goready" call in addtimerLocked.
		if delta < 0 || faketime > 0 {
			// No timers left - put goroutine to sleep.
			timers.rescheduling = true
			goparkunlock(&timers.lock, "timer goroutine (idle)", traceEvGoBlock, 1)
			continue
		}
		// At least one timer pending. Sleep until then.
		// If we have some timers in heap, we're sleeping until it's time to
		// spawn soonest of them. notetsleepg will sleep for `delta` period or
		// until notewakeup in addtimerLocked.
		// notetsleepg fills timers.waitnote structure and put goroutine to sleep for some time.
		// timers.waitnote can be used to wakeup this goroutine with notewakeup.
		timers.sleeping = true
		noteclear(&timers.waitnote)
		unlock(&timers.lock)
		notetsleepg(&timers.waitnote, delta)
	}
}