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splice.go
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splice.go
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package splice
// Routines for efficient file to file copying.
import (
"fmt"
"io/ioutil"
"log"
"os"
"syscall"
)
var maxPipeSize int
var resizable bool
func Resizable() bool {
return resizable
}
func MaxPipeSize() int {
return maxPipeSize
}
// From manpage on ubuntu Lucid:
//
// Since Linux 2.6.11, the pipe capacity is 65536 bytes.
const DefaultPipeSize = 16 * 4096
func init() {
content, err := ioutil.ReadFile("/proc/sys/fs/pipe-max-size")
if err != nil {
maxPipeSize = DefaultPipeSize
} else {
fmt.Sscan(string(content), &maxPipeSize)
}
r, w, err := os.Pipe()
if err != nil {
log.Panicf("cannot create pipe: %v", err)
}
sz, errNo := fcntl(r.Fd(), F_GETPIPE_SZ, 0)
resizable = (errNo == 0)
_, errNo = fcntl(r.Fd(), F_SETPIPE_SZ, 2*sz)
resizable = resizable && (errNo == 0)
r.Close()
w.Close()
}
// copy & paste from syscall.
func fcntl(fd uintptr, cmd int, arg int) (val int, errno syscall.Errno) {
r0, _, e1 := syscall.Syscall(syscall.SYS_FCNTL, fd, uintptr(cmd), uintptr(arg))
val = int(r0)
errno = syscall.Errno(e1)
return
}
const F_SETPIPE_SZ = 1031
const F_GETPIPE_SZ = 1032
func newSplicePair() (p *Pair, err error) {
p = &Pair{}
p.r, p.w, err = os.Pipe()
if err != nil {
return nil, err
}
errNo := syscall.Errno(0)
for _, f := range []*os.File{p.r, p.w} {
_, errNo = fcntl(f.Fd(), syscall.F_SETFL, syscall.O_NONBLOCK)
if errNo != 0 {
p.Close()
return nil, os.NewSyscallError("fcntl setfl", errNo)
}
}
p.size, errNo = fcntl(p.r.Fd(), F_GETPIPE_SZ, 0)
if errNo == syscall.EINVAL {
p.size = DefaultPipeSize
return p, nil
}
if errNo != 0 {
p.Close()
return nil, os.NewSyscallError("fcntl getsize", errNo)
}
return p, nil
}