time: add After

Permits one to easily put a timeout in a select:

select {
case <-ch:
	// foo
case <-time.After(1e6):
	// bar
}

R=r, rog, rsc, sameer1, PeterGo, iant, nigeltao_gnome
CC=golang-dev
https://golang.org/cl/2321043

src/pkg/time/sleep.go

@@ -9,18 +9,38 @@ import (
 	"syscall"
 )
 
-// Sleep pauses the current goroutine for at least ns nanoseconds. Higher resolution
-// sleeping may be provided by syscall.Nanosleep on some operating systems.
+// Sleep pauses the current goroutine for at least ns nanoseconds.
+// Higher resolution sleeping may be provided by syscall.Nanosleep 
+// on some operating systems.
 func Sleep(ns int64) os.Error {
-	// TODO(cw): use monotonic-time once it's available
+	_, err := sleep(Nanoseconds(), ns)
+	return err
+}
+
+// After waits at least ns nanoseconds before sending the current time
+// on the returned channel.
+func After(ns int64) <-chan int64 {
 	t := Nanoseconds()
+	ch := make(chan int64, 1)
+	go func() {
+		t, _ = sleep(t, ns)
+		ch <- t
+	}()
+	return ch
+}
+
+// sleep takes the current time and a duration,
+// pauses for at least ns nanoseconds, and
+// returns the current time and an error.
+func sleep(t, ns int64) (int64, os.Error) {
+	// TODO(cw): use monotonic-time once it's available
 	end := t + ns
 	for t < end {
 		errno := syscall.Sleep(end - t)
 		if errno != 0 && errno != syscall.EINTR {
-			return os.NewSyscallError("sleep", errno)
+			return 0, os.NewSyscallError("sleep", errno)
 		}
 		t = Nanoseconds()
 	}
-	return nil
+	return t, nil
 }

src/pkg/time/sleep_test.go

@@ -24,3 +24,15 @@ func TestSleep(t *testing.T) {
 		t.Fatalf("Sleep(%d) slept for only %d ns", delay, duration)
 	}
 }
+
+func TestAfter(t *testing.T) {
+	const delay = int64(100e6)
+	start := Nanoseconds()
+	end := <-After(delay)
+	if duration := Nanoseconds() - start; duration < delay {
+		t.Fatalf("After(%d) slept for only %d ns", delay, duration)
+	}
+	if min := start + delay; end < min {
+		t.Fatalf("After(%d) expect >= %d, got %d", delay, min, end)
+	}
+}