// Copyright 2017 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 (
	"io"
	"sync/atomic"
	"syscall"
	"syscall/zx"
	"syscall/zx/mxnet"
	"syscall/zx/zxsocket"
	"unsafe"
)

// FD is a file descriptor. The net and os packages embed this type in
// a larger type representing a network connection or OS file.
type FD struct {
	// Lock sysfd and serialize access to Read and Write methods.
	fdmu fdMutex

	// System file descriptor. Immutable until Close.
	Sysfd int

	// I/O poller.
	pd pollDesc

	// Whether this is a streaming descriptor, as opposed to a
	// packet-based descriptor like a UDP socket.
	IsStream bool

	// Whether a zero byte read indicates EOF. This is false for a
	// message based socket connection.
	ZeroReadIsEOF bool

	// Whether this is a normal file.
	isFile bool

	// Whether in blocking mode
	isBlocking uint32
}

// Init initializes the FD. The Sysfd field should already be set.
// This can be called multiple times on a single FD.
// The net argument is a network name from the net package (e.g., "tcp"),
// or "file".
// Set pollable to true if fd should be managed by runtime netpoll.
func (fd *FD) Init(net string, pollable bool) error {
	// We don't actually care about the various network types.
	if net == "file" {
		fd.isFile = true
	}
	if !pollable {
		fd.isBlocking = 1
		return nil
	}
	err := fd.pd.init(fd)
	if err != nil {
		fd.isBlocking = 1
	}
	return err
}

// see fd.Close() for reasoning.
func close(fd *FD) error {
	sysfd := atomic.SwapInt64((*int64)(unsafe.Pointer(&fd.Sysfd)), -1)
	if sysfd != -1 {
		return syscall.Close(int(sysfd))
	}
	return nil
}

// Destroy closes the file descriptor. This is called when there are
// no remaining references.
func (fd *FD) destroy() error {
	// Poller may want to unregister fd in readiness notification mechanism,
	// so this must be executed before closing the fd.
	fd.pd.close()

	return close(fd)
}

// Close closes the FD. The underlying file descriptor is closed by the
// destroy method when there are no remaining references.
func (fd *FD) Close() error {
	if !fd.fdmu.increfAndClose() {
		return errClosing(fd.isFile)
	}

	fd.pd.evict()
	// Note: On other implementations, evict will unblock sibling refs, however,
	// that can't be done yet here. Instead, we have to forcefully close the
	// connection, sadly ahead of destroy, in order to unblock others.
	err := close(fd)

	fd.decref()

	return err
}

// Shutdown wraps the shutdown network call.
func (fd *FD) Shutdown(how int) error {
	if err := fd.incref(); err != nil {
		return err
	}
	defer fd.decref()
	panic("TODO shutdown")
	//return syscall.Shutdown(fd.Sysfd, how) // TODO
}

func (fd *FD) Read(p []byte) (int, error) {
	if err := fd.readLock(); err != nil {
		return 0, err
	}
	defer fd.readUnlock()
	if len(p) == 0 {
		// If the caller wanted a zero byte read, return immediately
		// without trying (but after acquiring the readLock).
		return 0, nil
	}
	if err := fd.pd.prepareRead(fd.isFile); err != nil {
		return 0, err
	}
	// TODO: pollable() read
	n, err := syscall.Read(fd.Sysfd, p)
	err = fd.eofError(n, err)
	return n, err
}

// Pread wraps the pread system call.
func (fd *FD) Pread(p []byte, off int64) (int, error) {
	// Call incref, not readLock, because since pread specifies the
	// offset it is independent from other reads.
	// Similarly, using the poller doesn't make sense for pread.
	if err := fd.incref(); err != nil {
		return 0, err
	}
	n, err := syscall.Pread(fd.Sysfd, p, off)
	if err != nil {
		n = 0
	}
	fd.decref()
	err = fd.eofError(n, err)
	return n, err
}

// Write implements io.Writer.
func (fd *FD) Write(p []byte) (int, error) {
	if err := fd.writeLock(); err != nil {
		return 0, err
	}
	defer fd.writeUnlock()
	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
		return 0, err
	}
	var nn int
	for {
		n, err := syscall.Write(fd.Sysfd, p[nn:])
		if n > 0 {
			nn += n
		}
		if nn == len(p) {
			return nn, err
		}
		if err != nil {
			return nn, err
		}
		if n == 0 {
			return nn, io.ErrUnexpectedEOF
		}
	}
}

// Pwrite wraps the pwrite system call.
func (fd *FD) Pwrite(p []byte, off int64) (int, error) {
	// Call incref, not writeLock, because since pwrite specifies the
	// offset it is independent from other writes.
	// Similarly, using the poller doesn't make sense for pwrite.
	if err := fd.incref(); err != nil {
		return 0, err
	}
	defer fd.decref()
	var nn int
	for {
		n, err := syscall.Pwrite(fd.Sysfd, p[nn:], off+int64(nn))
		if n > 0 {
			nn += n
		}
		if nn == len(p) {
			return nn, err
		}
		if err != nil {
			return nn, err
		}
		if n == 0 {
			return nn, io.ErrUnexpectedEOF
		}
	}
}

// Accept wraps the accept network call.
func (fd *FD) Accept() (*zxsocket.Socket, error) {
	if err := fd.readLock(); err != nil {
		return nil, err
	}
	defer fd.readUnlock()

	if err := fd.pd.prepareRead(fd.isFile); err != nil {
		return nil, err
	}

	if s, ok := syscall.FDIOForFD(fd.Sysfd).(*zxsocket.Socket); ok {
		code, err := s.Accept(0)
		if code == syscall.EAGAIN {
			// TODO: non-blocking support, pass this to the poller
			if obs, err := s.Wait(mxnet.MXSIO_SIGNAL_INCOMING|zx.SignalSocketPeerClosed, zx.TimensecInfinite); err != nil {
				return nil, err
			} else if obs&zx.SignalSocketPeerClosed != 0 {
				return nil, &zx.Error{Status: zx.ErrPeerClosed, Text: "zxsocket"}
			} else if obs&mxnet.MXSIO_SIGNAL_INCOMING != 0 {
				// fallthrough
			} else {
				panic("unreachable")
			}
			code, err = s.Accept(0)
		}
		if err != nil {
			return nil, err
		}
		if code != 0 {
			return nil, syscall.Errno(code)
		}
		var h zx.Handle
		if err := s.Socket.Accept(&h); err != nil {
			return nil, err
		}
		return zxsocket.NewSocket(zx.Socket(h)), nil
	}
	return nil, syscall.EBADF
}

// Seek wraps syscall.Seek.
func (fd *FD) Seek(offset int64, whence int) (int64, error) {
	if err := fd.incref(); err != nil {
		return 0, err
	}
	defer fd.decref()
	return syscall.Seek(fd.Sysfd, offset, whence)
}

// ReadDirent wraps syscall.ReadDirent.
// We treat this like an ordinary system call rather than a call
// that tries to fill the buffer.
func (fd *FD) ReadDirent(buf []byte) (int, error) {
	if err := fd.incref(); err != nil {
		return 0, err
	}
	defer fd.decref()
	return syscall.ReadDirent(fd.Sysfd, buf)
}

// Fstat wraps syscall.Fstat
func (fd *FD) Fstat(s *syscall.Stat_t) error {
	if err := fd.incref(); err != nil {
		return err
	}
	defer fd.decref()
	return syscall.Fstat(fd.Sysfd, s)
}

// eofError returns io.EOF when fd is available for reading end of
// file.
func (fd *FD) eofError(n int, err error) error {
	if n == 0 && err == nil && fd.ZeroReadIsEOF {
		return io.EOF
	}
	return err
}

// RawControl invokes the user-defined function f for a non-IO
// operation.
func (fd *FD) RawControl(f func(uintptr)) error {
	if err := fd.incref(); err != nil {
		return err
	}
	defer fd.decref()
	f(uintptr(fd.Sysfd))
	return nil
}

// RawRead invokes the user-defined function f for a read operation.
func (fd *FD) RawRead(f func(uintptr) bool) error {
	if err := fd.readLock(); err != nil {
		return err
	}
	defer fd.readUnlock()
	if err := fd.pd.prepareRead(fd.isFile); err != nil {
		return err
	}
	for {
		if f(uintptr(fd.Sysfd)) {
			return nil
		}
		if err := fd.pd.waitRead(fd.isFile); err != nil {
			return err
		}
	}
}

// RawWrite invokes the user-defined function f for a write operation.
func (fd *FD) RawWrite(f func(uintptr) bool) error {
	if err := fd.writeLock(); err != nil {
		return err
	}
	defer fd.writeUnlock()
	if err := fd.pd.prepareWrite(fd.isFile); err != nil {
		return err
	}
	for {
		if f(uintptr(fd.Sysfd)) {
			return nil
		}
		if err := fd.pd.waitWrite(fd.isFile); err != nil {
			return err
		}
	}
}

// SetBlocking puts the file into blocking mode.
func (fd *FD) SetBlocking() error {
	if err := fd.incref(); err != nil {
		return err
	}
	defer fd.decref()
	// Atomic store so that concurrent calls to SetBlocking
	// do not cause a race condition. isBlocking only ever goes
	// from 0 to 1 so there is no real race here.
	atomic.StoreUint32(&fd.isBlocking, 1)
	return syscall.SetNonblock(fd.Sysfd, false)
}