lock.go

package gannoy

import (
	"io"
	"os/exec"
	"strings"
	"syscall"

	"github.com/coreos/go-semver/semver"
	"regexp"
)

type Locker interface {
	ReadLock(uintptr, int64, int64) error
	WriteLock(uintptr, int64, int64) error
	UnLock(uintptr, int64, int64) error
}

func newLocker() Locker {
	bytes, err := exec.Command("uname", "-sr").Output()
	if err != nil {
		return Flock{}
	}
	if validateKernel(bytes) {
		return Fcntl{}
	}
	return Flock{}
}

func validateKernel(bytes []byte) bool {
	kernel := strings.Split(strings.TrimRight(string(bytes), "\n"), " ")
	nk := normalizeKernelVersion(kernel[1])
	if kernel[0] == "Linux" && !semver.New(nk).LessThan(*semver.New("3.15.0")) {
		return true
	}
	return false
}

func normalizeKernelVersion(v string) string {
	re := regexp.MustCompile(".elrepo.x86_64|.el7.x86_64")
	return re.ReplaceAllString(v, "")
}

// Only Linux and kernel version 3.15 or later.
// This depends on open file description lock (F_OFD_SETLKW).
type Fcntl struct {
}

const F_OFD_SETLKW = 38

func (f Fcntl) ReadLock(fd uintptr, start, len int64) error {
	return f.fcntl(syscall.F_RDLCK, fd, start, len)
}

func (f Fcntl) WriteLock(fd uintptr, start, len int64) error {
	return f.fcntl(syscall.F_WRLCK, fd, start, len)
}

func (f Fcntl) UnLock(fd uintptr, start, len int64) error {
	return f.fcntl(syscall.F_UNLCK, fd, start, len)
}

func (f Fcntl) fcntl(typ int16, fd uintptr, start, len int64) error {
	return syscall.FcntlFlock(fd, F_OFD_SETLKW, &syscall.Flock_t{
		Start:  start,
		Len:    len,
		Type:   typ,
		Whence: io.SeekStart,
	})
}

type Flock struct {
}

func (f Flock) ReadLock(fd uintptr, start, len int64) error {
	return f.flock(fd, syscall.LOCK_SH)
}

func (f Flock) WriteLock(fd uintptr, start, len int64) error {
	return f.flock(fd, syscall.LOCK_EX)
}

func (f Flock) UnLock(fd uintptr, start, len int64) error {
	return f.flock(fd, syscall.LOCK_UN)
}

func (f Flock) flock(fd uintptr, how int) error {
	return syscall.Flock(int(fd), how)
}