unix: implement getitimer(2) and setitimer(2) on Linux

Wrap the low-level system calls with a more idiomatic Go API and a set of
constants to indicate which timer should be queried or modified.

man 2 getitimer indicates that these system calls are obsolete as of
POSIX.1-2008, but the code I am working on has not been ported to the modern
timer_gettime(2) and timer_settime(2) APIs as of yet.

Change-Id: I91482e141047846cadf47aa2417b8770955986bf
Reviewed-on: https://go-review.googlesource.com/c/sys/+/384054
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dominik Honnef <dominik@honnef.co>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Matt Layher <mdlayher@gmail.com>

unix/syscall_linux.go

@@ -14,6 +14,7 @@ package unix
 import (
 	"encoding/binary"
 	"syscall"
+	"time"
 	"unsafe"
 )
 
@@ -2314,6 +2315,52 @@ type RemoteIovec struct {
 //sys	shmdt(addr uintptr) (err error)
 //sys	shmget(key int, size int, flag int) (id int, err error)
 
+//sys	getitimer(which int, currValue *Itimerval) (err error)
+//sys	setitimer(which int, newValue *Itimerval, oldValue *Itimerval) (err error)
+
+// MakeItimerval creates an Itimerval from interval and value durations.
+func MakeItimerval(interval, value time.Duration) Itimerval {
+	return Itimerval{
+		Interval: NsecToTimeval(interval.Nanoseconds()),
+		Value:    NsecToTimeval(value.Nanoseconds()),
+	}
+}
+
+// A value which may be passed to the which parameter for Getitimer and
+// Setitimer.
+type ItimerWhich int
+
+// Possible which values for Getitimer and Setitimer.
+const (
+	ItimerReal    ItimerWhich = ITIMER_REAL
+	ItimerVirtual ItimerWhich = ITIMER_VIRTUAL
+	ItimerProf    ItimerWhich = ITIMER_PROF
+)
+
+// Getitimer wraps getitimer(2) to return the current value of the timer
+// specified by which.
+func Getitimer(which ItimerWhich) (Itimerval, error) {
+	var it Itimerval
+	if err := getitimer(int(which), &it); err != nil {
+		return Itimerval{}, err
+	}
+
+	return it, nil
+}
+
+// Setitimer wraps setitimer(2) to arm or disarm the timer specified by which.
+// It returns the previous value of the timer.
+//
+// If the Itimerval argument is the zero value, the timer will be disarmed.
+func Setitimer(which ItimerWhich, it Itimerval) (Itimerval, error) {
+	var prev Itimerval
+	if err := setitimer(int(which), &it, &prev); err != nil {
+		return Itimerval{}, err
+	}
+
+	return prev, nil
+}
+
 /*
  * Unimplemented
  */
@@ -2333,7 +2380,6 @@ type RemoteIovec struct {
 // GetMempolicy
 // GetRobustList
 // GetThreadArea
-// Getitimer
 // Getpmsg
 // IoCancel
 // IoDestroy