// 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 (
	
	
	
	
	
)

const (
	// spliceNonblock doesn't make the splice itself necessarily nonblocking
	// (because the actual file descriptors that are spliced from/to may block
	// unless they have the O_NONBLOCK flag set), but it makes the splice pipe
	// operations nonblocking.
	spliceNonblock = 0x2

	// maxSpliceSize is the maximum amount of data Splice asks
	// the kernel to move in a single call to splice(2).
	// We use 1MB as Splice writes data through a pipe, and 1MB is the default maximum pipe buffer size,
	// which is determined by /proc/sys/fs/pipe-max-size.
	maxSpliceSize = 1 << 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 gets a pipe buffer from the pool or creates a new one if needed, 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 (,  *FD,  int64) ( int64,  bool,  string,  error) {
	, ,  := getPipe()
	if  != nil {
		return 0, false, , 
	}
	defer putPipe()
	var ,  int
	for  == nil &&  > 0 {
		 := maxSpliceSize
		if int64() >  {
			 = int()
		}
		,  = spliceDrain(.wfd, , )
		// 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.
		 =  || ( != syscall.EINVAL)
		if  != nil ||  == 0 {
			break
		}
		.data += 

		,  = splicePump(, .rfd, )
		if  > 0 {
			 += int64()
			 -= int64()
			.data -= 
		}
	}
	if  != nil {
		return , , "splice", 
	}
	return , 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( int,  *FD,  int) (int, error) {
	if  := .readLock();  != nil {
		return 0, 
	}
	defer .readUnlock()
	if  := .pd.prepareRead(.isFile);  != nil {
		return 0, 
	}
	for {
		// In theory calling splice(2) with SPLICE_F_NONBLOCK could end up an infinite loop here,
		// because it could return EAGAIN ceaselessly when the write end of the pipe is full,
		// but this shouldn't be a concern here, since the pipe buffer must be sufficient for
		// this data transmission on the basis of the workflow in Splice.
		,  := splice(, .Sysfd, , spliceNonblock)
		if  == syscall.EINTR {
			continue
		}
		if  != syscall.EAGAIN {
			return , 
		}
		if .pd.pollable() {
			if  := .pd.waitRead(.isFile);  != nil {
				return , 
			}
		}
	}
}

// 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( *FD,  int,  int) (int, error) {
	if  := .writeLock();  != nil {
		return 0, 
	}
	defer .writeUnlock()
	if  := .pd.prepareWrite(.isFile);  != nil {
		return 0, 
	}
	 := 0
	for  > 0 {
		// In theory calling splice(2) with SPLICE_F_NONBLOCK could end up an infinite loop here,
		// because it could return EAGAIN ceaselessly when the read end of the pipe is empty,
		// but this shouldn't be a concern here, since the pipe buffer must contain inPipe size of
		// data on the basis of the workflow in Splice.
		,  := splice(.Sysfd, , , spliceNonblock)
		if  == syscall.EINTR {
			continue
		}
		// Here, the condition n == 0 && err == nil should never be
		// observed, since Splice controls the write side of the pipe.
		if  > 0 {
			 -= 
			 += 
			continue
		}
		if  != syscall.EAGAIN {
			return , 
		}
		if .pd.pollable() {
			if  := .pd.waitWrite(.isFile);  != nil {
				return , 
			}
		}
	}
	return , 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( int,  int,  int,  int) (int, error) {
	,  := syscall.Splice(, nil, , nil, , )
	return int(), 
}

type splicePipeFields struct {
	rfd  int
	wfd  int
	data int
}

type splicePipe struct {
	splicePipeFields

	// We want to use a finalizer, so ensure that the size is
	// large enough to not use the tiny allocator.
	_ [24 - unsafe.Sizeof(splicePipeFields{})%24]byte
}

// splicePipePool caches pipes to avoid high-frequency construction and destruction of pipe buffers.
// The garbage collector will free all pipes in the sync.Pool periodically, thus we need to set up
// a finalizer for each pipe to close its file descriptors before the actual GC.
var splicePipePool = sync.Pool{New: newPoolPipe}

func newPoolPipe() any {
	// Discard the error which occurred during the creation of pipe buffer,
	// redirecting the data transmission to the conventional way utilizing read() + write() as a fallback.
	 := newPipe()
	if  == nil {
		return nil
	}
	runtime.SetFinalizer(, destroyPipe)
	return 
}

// getPipe tries to acquire a pipe buffer from the pool or create a new one with newPipe() if it gets nil from the cache.
//
// Note that it may fail to create a new pipe buffer by newPipe(), in which case getPipe() will return a generic error
// and system call name splice in a string as the indication.
func getPipe() (*splicePipe, string, error) {
	 := splicePipePool.Get()
	if  == nil {
		return nil, "splice", syscall.EINVAL
	}
	return .(*splicePipe), "", nil
}

func putPipe( *splicePipe) {
	// If there is still data left in the pipe,
	// then close and discard it instead of putting it back into the pool.
	if .data != 0 {
		runtime.SetFinalizer(, nil)
		destroyPipe()
		return
	}
	splicePipePool.Put()
}

// newPipe sets up a pipe for a splice operation.
func newPipe() *splicePipe {
	var  [2]int
	if  := syscall.Pipe2([:], syscall.O_CLOEXEC|syscall.O_NONBLOCK);  != nil {
		return nil
	}

	// Splice will loop writing maxSpliceSize bytes from the source to the pipe,
	// and then write those bytes from the pipe to the destination.
	// Set the pipe buffer size to maxSpliceSize to optimize that.
	// Ignore errors here, as a smaller buffer size will work,
	// although it will require more system calls.
	unix.Fcntl([0], syscall.F_SETPIPE_SZ, maxSpliceSize)

	return &splicePipe{splicePipeFields: splicePipeFields{rfd: [0], wfd: [1]}}
}

// destroyPipe destroys a pipe.
func destroyPipe( *splicePipe) {
	CloseFunc(.rfd)
	CloseFunc(.wfd)
}