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// Copyright 2024 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
//go:build darwin || dragonfly || freebsd || linux || solaris
package poll
import (
"io"
"runtime"
"syscall"
)
// SendFile wraps the sendfile system call.
//
// It copies data from src (a file descriptor) to dstFD,
// starting at the current position of src.
// It updates the current position of src to after the
// copied data.
//
// If size is zero, it copies the rest of src.
// Otherwise, it copies up to size bytes.
//
// The handled return parameter indicates whether SendFile
// was able to handle some or all of the operation.
// If handled is false, sendfile was unable to perform the copy,
// has not modified the source or destination,
// and the caller should perform the copy using a fallback implementation.
func SendFile(dstFD *FD, src int, size int64) (n int64, err error, handled bool) {
if goos := runtime.GOOS; goos == "linux" || goos == "android" {
// Linux's sendfile doesn't require any setup:
// It sends from the current position of the source file and
// updates the position of the source after sending.
return sendFile(dstFD, src, nil, size)
}
// Non-Linux sendfile implementations don't use the current position of the source file,
// so we need to look up the position, pass it explicitly, and adjust it after
// sendfile returns.
start, err := ignoringEINTR2(func() (int64, error) {
return syscall.Seek(src, 0, io.SeekCurrent)
})
if err != nil {
return 0, err, false
}
pos := start
n, err, handled = sendFile(dstFD, src, &pos, size)
if n > 0 {
ignoringEINTR2(func() (int64, error) {
return syscall.Seek(src, start+n, io.SeekStart)
})
}
return n, err, handled
}
// sendFile wraps the sendfile system call.
func sendFile(dstFD *FD, src int, offset *int64, size int64) (written int64, err error, handled bool) {
defer func() {
TestHookDidSendFile(dstFD, src, written, err, handled)
}()
if err := dstFD.writeLock(); err != nil {
return 0, err, false
}
defer dstFD.writeUnlock()
if err := dstFD.pd.prepareWrite(dstFD.isFile); err != nil {
return 0, err, false
}
dst := dstFD.Sysfd
for {
// Some platforms support passing 0 to read to the end of the source,
// but all platforms support just writing a large value.
//
// Limit the maximum size to fit in an int32, to avoid any possible overflow.
chunk := 1<<31 - 1
if size > 0 {
chunk = int(min(size-written, int64(chunk)))
}
var n int
n, err = sendFileChunk(dst, src, offset, chunk, written)
if n > 0 {
written += int64(n)
}
switch err {
case nil:
// We're done if sendfile copied no bytes
// (we're at the end of the source)
// or if we have a size limit and have reached it.
//
// If sendfile copied some bytes and we don't have a size limit,
// try again to see if there is more data to copy.
if n == 0 || (size > 0 && written >= size) {
return written, nil, true
}
case syscall.EAGAIN:
// *BSD and Darwin can return EAGAIN with n > 0,
// so check to see if the write has completed.
// So far as we know all other platforms only
// return EAGAIN when n == 0, but checking is harmless.
if size > 0 && written >= size {
return written, nil, true
}
if err = dstFD.pd.waitWrite(dstFD.isFile); err != nil {
return written, err, true
}
case syscall.EINTR:
// Retry.
case syscall.ENOSYS, syscall.EOPNOTSUPP, syscall.EINVAL:
// ENOSYS indicates no kernel support for sendfile.
// EINVAL indicates a FD type that does not support sendfile.
//
// On Linux, copy_file_range can return EOPNOTSUPP when copying
// to a NFS file (issue #40731); check for it here just in case.
return written, err, written > 0
default:
// We want to handle ENOTSUP like EOPNOTSUPP.
// It's a pain to put it as a switch case
// because on Linux systems ENOTSUP == EOPNOTSUPP,
// so the compiler complains about a duplicate case.
if err == syscall.ENOTSUP {
return written, err, written > 0
}
// Not a retryable error.
return written, err, true
}
}
}
func sendFileChunk(dst, src int, offset *int64, size int, written int64) (n int, err error) {
switch runtime.GOOS {
case "linux", "android":
// The offset is always nil on Linux.
n, err = syscall.Sendfile(dst, src, offset, size)
case "solaris", "illumos":
// Trust the offset, not the return value from sendfile.
start := *offset
n, err = syscall.Sendfile(dst, src, offset, size)
n = int(*offset - start)
// A quirk on Solaris/illumos: sendfile claims to support out_fd
// as a regular file but returns EINVAL when the out_fd
// is not a socket of SOCK_STREAM, while it actually sends
// out data anyway and updates the file offset.
//
// Another quirk: sendfile transfers data and returns EINVAL when being
// asked to transfer bytes more than the actual file size. For instance,
// the source file is wrapped in an io.LimitedReader with larger size
// than the actual file size.
//
// To handle these cases we ignore EINVAL if any call to sendfile was
// able to send data.
if err == syscall.EINVAL && (n > 0 || written > 0) {
err = nil
}
default:
start := *offset
n, err = syscall.Sendfile(dst, src, offset, size)
if n > 0 {
// The BSD implementations of syscall.Sendfile don't
// update the offset parameter (despite it being a *int64).
//
// Trust the return value from sendfile, not the offset.
*offset = start + int64(n)
}
}
return
}
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