rm go.work
cp go.mod.21 go.mod
go mod edit -godebug default=go1.20 -godebug asynctimerchan=0
go list -f '{{.Module.GoVersion}} {{.DefaultGODEBUG}}'
stdout panicnil=1
stdout asynctimerchan=0

# Go 1.21 go.work with godebug default=go1.20
cp go.work.21 go.work
go list -f '{{.Module.GoVersion}} {{.DefaultGODEBUG}}'
! stdout panicnil # go.work wins
stdout asynctimerchan=1 # go.work wins
go work edit -godebug default=go1.20 -godebug asynctimerchan=0

	if asynctimerchan.Value() == "1" {
		return nil
	}

	// Otherwise pass to runtime.
	// This handles asynctimerchan=0, which is the default Go 1.23 behavior,
	// as well as asynctimerchan=2, which is like asynctimerchan=1
	// but implemented entirely by the runtime.
	// The only reason to use asynctimerchan=2 is for debugging
	// a problem fixed by asynctimerchan=1: it enables the new

is in a module with a `go.mod` `go` line using Go 1.23.0 or later.
When Go 1.23 builds older programs, the old behaviors remain in effect.
The new [GODEBUG setting](/doc/godebug) [`asynctimerchan=1`](/pkg/time/#NewTimer)
can be used to revert back to asynchronous channel behaviors

	// that do an os.Setenv in main.init or main.main.
	asynctimerchan atomic.Int32
}

var dbgvars = []*dbgVar{
	{name: "adaptivestackstart", value: &debug.adaptivestackstart},
	{name: "asyncpreemptoff", value: &debug.asyncpreemptoff},
	{name: "asynctimerchan", atomic: &debug.asynctimerchan},
	{name: "cgocheck", value: &debug.cgocheck},
	{name: "clobberfree", value: &debug.clobberfree},

		default=go1.21
		panicnil=1
		asynctimerchan=0
	)

The special key `default` indicates a Go version to take unspecified
settings from. This allows setting the GODEBUG defaults separately
from the Go language version in the module.
In this example, the program is asking for Go 1.21 semantics and
then asking for the old pre-Go 1.21 `panic(nil)` behavior and the
new Go 1.23 `asynctimerchan=0` behavior.

	run(t, "NewTickerStop", func() { NewTicker(Hour).Stop() })
}

func TestChan(t *testing.T) {
	for _, name := range []string{"0", "1", "2"} {
		t.Run("asynctimerchan="+name, func(t *testing.T) {
			t.Setenv("GODEBUG", "asynctimerchan="+name)
			t.Run("Timer", func(t *testing.T) {
				tim := NewTimer(10000 * Second)
				testTimerChan(t, tim, tim.C, name == "0")
			})
			t.Run("Ticker", func(t *testing.T) {

// (Otherwise the runtime/metrics test will fail.)
//
// Note: After adding entries to this table, update the list in doc/godebug.md as well.
// (Otherwise the test in this package will fail.)
var All = []Info{
	{Name: "asynctimerchan", Package: "time", Changed: 23, Old: "1", Opaque: true},
	{Name: "execerrdot", Package: "os/exec"},
	{Name: "gocachehash", Package: "cmd/go"},
	{Name: "gocachetest", Package: "cmd/go"},

	arg    any
	seq    uintptr

	// If non-nil, the timers containing t.
	ts *timers

	// sendLock protects sends on the timer's channel.
	// Not used for async (pre-Go 1.23) behavior when debug.asynctimerchan.Load() != 0.
	sendLock mutex
}

// init initializes a newly allocated timer t.
// Any code that allocates a timer must call t.init before using it.
// The arg and f can be set during init, or they can be nil in init

func reflect_chanrecv(c *hchan, nb bool, elem unsafe.Pointer) (selected bool, received bool) {
	return chanrecv(c, elem, !nb)
}

func chanlen(c *hchan) int {
	if c == nil {
		return 0
	}
	async := debug.asynctimerchan.Load() != 0
	if c.timer != nil && async {
		c.timer.maybeRunChan()
	}
	if c.timer != nil && !async {
		// timer channels have a buffered implementation
		// but present to users as unbuffered, so that we can