1 files changed, 194 insertions, 0 deletions
diff --git a/src/internal/poll/splice_linux.go b/src/internal/poll/splice_linux.go
new file mode 100644
index 0000000..01baf14
--- /dev/null
+++ b/src/internal/poll/splice_linux.go
@@ -0,0 +1,194 @@
+// Copyright 2018 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.
+
+package poll
+
+import (
+	"internal/syscall/unix"
+	"sync/atomic"
+	"syscall"
+	"unsafe"
+)
+
+const (
+	// spliceNonblock makes calls to splice(2) non-blocking.
+	spliceNonblock = 0x2
+
+	// maxSpliceSize is the maximum amount of data Splice asks
+	// the kernel to move in a single call to splice(2).
+	maxSpliceSize = 4 << 20
+)
+
+// Splice transfers at most remain bytes of data from src to dst, using the
+// splice system call to minimize copies of data from and to userspace.
+//
+// Splice creates a temporary pipe, to serve as a buffer for the data transfer.
+// src and dst must both be stream-oriented sockets.
+//
+// If err != nil, sc is the system call which caused the error.
+func Splice(dst, src *FD, remain int64) (written int64, handled bool, sc string, err error) {
+	prfd, pwfd, sc, err := newTempPipe()
+	if err != nil {
+		return 0, false, sc, err
+	}
+	defer destroyTempPipe(prfd, pwfd)
+	var inPipe, n int
+	for err == nil && remain > 0 {
+		max := maxSpliceSize
+		if int64(max) > remain {
+			max = int(remain)
+		}
+		inPipe, err = spliceDrain(pwfd, src, max)
+		// The operation is considered handled if splice returns no
+		// error, or an error other than EINVAL. An EINVAL means the
+		// kernel does not support splice for the socket type of src.
+		// The failed syscall does not consume any data so it is safe
+		// to fall back to a generic copy.
+		//
+		// spliceDrain should never return EAGAIN, so if err != nil,
+		// Splice cannot continue.
+		//
+		// If inPipe == 0 && err == nil, src is at EOF, and the
+		// transfer is complete.
+		handled = handled || (err != syscall.EINVAL)
+		if err != nil || (inPipe == 0 && err == nil) {
+			break
+		}
+		n, err = splicePump(dst, prfd, inPipe)
+		if n > 0 {
+			written += int64(n)
+			remain -= int64(n)
+		}
+	}
+	if err != nil {
+		return written, handled, "splice", err
+	}
+	return written, true, "", nil
+}
+
+// spliceDrain moves data from a socket to a pipe.
+//
+// Invariant: when entering spliceDrain, the pipe is empty. It is either in its
+// initial state, or splicePump has emptied it previously.
+//
+// Given this, spliceDrain can reasonably assume that the pipe is ready for
+// writing, so if splice returns EAGAIN, it must be because the socket is not
+// ready for reading.
+//
+// If spliceDrain returns (0, nil), src is at EOF.
+func spliceDrain(pipefd int, sock *FD, max int) (int, error) {
+	if err := sock.readLock(); err != nil {
+		return 0, err
+	}
+	defer sock.readUnlock()
+	if err := sock.pd.prepareRead(sock.isFile); err != nil {
+		return 0, err
+	}
+	for {
+		n, err := splice(pipefd, sock.Sysfd, max, spliceNonblock)
+		if err == syscall.EINTR {
+			continue
+		}
+		if err != syscall.EAGAIN {
+			return n, err
+		}
+		if err := sock.pd.waitRead(sock.isFile); err != nil {
+			return n, err
+		}
+	}
+}
+
+// splicePump moves all the buffered data from a pipe to a socket.
+//
+// Invariant: when entering splicePump, there are exactly inPipe
+// bytes of data in the pipe, from a previous call to spliceDrain.
+//
+// By analogy to the condition from spliceDrain, splicePump
+// only needs to poll the socket for readiness, if splice returns
+// EAGAIN.
+//
+// If splicePump cannot move all the data in a single call to
+// splice(2), it loops over the buffered data until it has written
+// all of it to the socket. This behavior is similar to the Write
+// step of an io.Copy in userspace.
+func splicePump(sock *FD, pipefd int, inPipe int) (int, error) {
+	if err := sock.writeLock(); err != nil {
+		return 0, err
+	}
+	defer sock.writeUnlock()
+	if err := sock.pd.prepareWrite(sock.isFile); err != nil {
+		return 0, err
+	}
+	written := 0
+	for inPipe > 0 {
+		n, err := splice(sock.Sysfd, pipefd, inPipe, spliceNonblock)
+		// Here, the condition n == 0 && err == nil should never be
+		// observed, since Splice controls the write side of the pipe.
+		if n > 0 {
+			inPipe -= n
+			written += n
+			continue
+		}
+		if err != syscall.EAGAIN {
+			return written, err
+		}
+		if err := sock.pd.waitWrite(sock.isFile); err != nil {
+			return written, err
+		}
+	}
+	return written, nil
+}
+
+// splice wraps the splice system call. Since the current implementation
+// only uses splice on sockets and pipes, the offset arguments are unused.
+// splice returns int instead of int64, because callers never ask it to
+// move more data in a single call than can fit in an int32.
+func splice(out int, in int, max int, flags int) (int, error) {
+	n, err := syscall.Splice(in, nil, out, nil, max, flags)
+	return int(n), err
+}
+
+var disableSplice unsafe.Pointer
+
+// newTempPipe sets up a temporary pipe for a splice operation.
+func newTempPipe() (prfd, pwfd int, sc string, err error) {
+	p := (*bool)(atomic.LoadPointer(&disableSplice))
+	if p != nil && *p {
+		return -1, -1, "splice", syscall.EINVAL
+	}
+
+	var fds [2]int
+	// pipe2 was added in 2.6.27 and our minimum requirement is 2.6.23, so it
+	// might not be implemented. Falling back to pipe is possible, but prior to
+	// 2.6.29 splice returns -EAGAIN instead of 0 when the connection is
+	// closed.
+	const flags = syscall.O_CLOEXEC | syscall.O_NONBLOCK
+	if err := syscall.Pipe2(fds[:], flags); err != nil {
+		return -1, -1, "pipe2", err
+	}
+
+	if p == nil {
+		p = new(bool)
+		defer atomic.StorePointer(&disableSplice, unsafe.Pointer(p))
+
+		// F_GETPIPE_SZ was added in 2.6.35, which does not have the -EAGAIN bug.
+		if _, _, errno := syscall.Syscall(unix.FcntlSyscall, uintptr(fds[0]), syscall.F_GETPIPE_SZ, 0); errno != 0 {
+			*p = true
+			destroyTempPipe(fds[0], fds[1])
+			return -1, -1, "fcntl", errno
+		}
+	}
+
+	return fds[0], fds[1], "", nil
+}
+
+// destroyTempPipe destroys a temporary pipe.
+func destroyTempPipe(prfd, pwfd int) error {
+	err := CloseFunc(prfd)
+	err1 := CloseFunc(pwfd)
+	if err == nil {
+		return err1
+	}
+	return err
+}