Lazily init /dev/null fd

Fixes an issue that the init() func in splice.go panics if
/dev is not yet mounted.

This allows supporting go-based init binaries which set up all the
filesystems on the machine, including both /dev as well as
FUSE-mounted dirs.

Change-Id: I74dab18effe4df410689ae7a2ef43670fcab7bd8

splice/pair_linux.go

@@ -40,7 +40,7 @@ func (p *Pair) WriteTo(fd uintptr, n int) (int, error) {
 const _SPLICE_F_NONBLOCK = 0x2
 
 func (p *Pair) discard() {
-	_, err := syscall.Splice(p.r, nil, int(devNullFD), nil, int(p.size), _SPLICE_F_NONBLOCK)
+	_, err := syscall.Splice(p.r, nil, devNullFD(), nil, int(p.size), _SPLICE_F_NONBLOCK)
 	if err == syscall.EAGAIN {
 		// all good.
 	} else if err != nil {

splice/splice.go

@@ -11,6 +11,7 @@ import (
 	"io/ioutil"
 	"log"
 	"os"
+	"sync"
 	"syscall"
 )
 
@@ -30,8 +31,10 @@ func MaxPipeSize() int {
 // Since Linux 2.6.11, the pipe capacity is 65536 bytes.
 const DefaultPipeSize = 16 * 4096
 
-// We empty pipes by splicing to /dev/null.
-var devNullFD uintptr
+var (
+	devNullFDOnce  sync.Once
+	devNullFDValue int
+)
 
 func init() {
 	content, err := ioutil.ReadFile("/proc/sys/fs/pipe-max-size")
@@ -51,13 +54,18 @@ func init() {
 	resizable = resizable && (errNo == 0)
 	r.Close()
 	w.Close()
+}
 
+// We empty pipes by splicing to /dev/null.
+func devNullFD() int {
+	devNullFDOnce.Do(func() {
 		fd, err := syscall.Open("/dev/null", os.O_WRONLY, 0)
 		if err != nil {
-		log.Panicf("splice: %v", err)
+			panic(fmt.Sprintf("failed to open /dev/null: %s", err))
 		}
-
-	devNullFD = uintptr(fd)
+		devNullFDValue = fd
+	})
+	return devNullFDValue
 }
 
 // copy & paste from syscall.