unix: add support for timerfd syscalls on Linux

timerfd_create, timerfd_gettime and timerfd_settime syscalls have been added.
Fixes golang/go#38733

Change-Id: I306d68103b6efb2515c74f384646210c4b68f66e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/230798
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_linux_test.go

@@ -19,6 +19,7 @@ import (
 	"strings"
 	"testing"
 	"time"
+	"unsafe"
 
 	"golang.org/x/sys/unix"
 )
@@ -663,3 +664,53 @@ func TestPrctlRetInt(t *testing.T) {
 		t.Fatalf("unexpected return from prctl; got %v, expected %v", v, 1)
 	}
 }
+
+func TestTimerfd(t *testing.T) {
+	var now unix.Timespec
+	if err := unix.ClockGettime(unix.CLOCK_REALTIME, &now); err != nil {
+		t.Fatalf("ClockGettime: %v", err)
+	}
+
+	tfd, err := unix.TimerfdCreate(unix.CLOCK_REALTIME, 0)
+	if err == unix.ENOSYS {
+		t.Skip("timerfd_create system call not implemented")
+	} else if err != nil {
+		t.Fatalf("TimerfdCreate: %v", err)
+	}
+	defer unix.Close(tfd)
+
+	var timeSpec unix.ItimerSpec
+	if err := unix.TimerfdGettime(tfd, &timeSpec); err != nil {
+		t.Fatalf("TimerfdGettime: %v", err)
+	}
+
+	if timeSpec.Value.Nsec != 0 || timeSpec.Value.Sec != 0 {
+		t.Fatalf("TimerfdGettime: timer is already set, but shouldn't be")
+	}
+
+	timeSpec = unix.ItimerSpec{
+		Interval: unix.NsecToTimespec(int64(time.Millisecond)),
+		Value:    now,
+	}
+
+	if err := unix.TimerfdSettime(tfd, unix.TFD_TIMER_ABSTIME, &timeSpec, nil); err != nil {
+		t.Fatalf("TimerfdSettime: %v", err)
+	}
+
+	const totalTicks = 10
+	const bufferLength = 8
+
+	buffer := make([]byte, bufferLength)
+
+	var count uint64 = 0
+	for count < totalTicks {
+		n, err := unix.Read(tfd, buffer)
+		if err != nil {
+			t.Fatalf("Timerfd: %v", err)
+		} else if n != bufferLength {
+			t.Fatalf("Timerfd: got %d bytes from timerfd, expected %d bytes", n, bufferLength)
+		}
+
+		count += *(*uint64)(unsafe.Pointer(&buffer))
+	}
+}