// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

//go:build linux

package nfsfs

import (
	"syscall"
)

// The whence values of the hole seeking of Linux.
const (
	seekData = 3
	seekHole = 4
)

// SeekHole finds the next hole at or after the offset. The eof flag
// reports that none of the requested content follows: every file
// carries a virtual hole at its end, so a dense tail answers the file
// size with eof set, RFC 7862 section 15.11. ErrBeyondEOF answers a
// request that starts past the end.
func (l *Local) SeekHole(h Handle, offset int64) (int64, bool, error) {
	return l.seek(h, offset, seekHole)
}

// SeekData finds the next data byte at or after the offset. The eof
// flag reports that no data follows the offset, and the answer names
// the file size; ErrBeyondEOF answers a request that starts past the
// end.
func (l *Local) SeekData(h Handle, offset int64) (int64, bool, error) {
	return l.seek(h, offset, seekData)
}

func (l *Local) seek(h Handle, offset int64, whence int) (int64, bool, error) {
	// The seek opens the same revalidated, unfollowed descriptor every
	// other data path uses, so a name swapped for a link never serves
	// bytes from outside the export.
	f, _, err := l.openVerified(h, syscall.O_RDONLY)
	if err != nil {
		return 0, false, err
	}
	defer f.Close()
	fd := int(f.Fd())
	size, err := fstatSize(fd)
	if err != nil {
		return 0, false, err
	}
	if offset > size {
		return 0, false, ErrBeyondEOF
	}
	at, err := syscall.Seek(fd, offset, whence)
	if err == syscall.ENXIO {
		// The range from the offset to the end carries none of the
		// requested content: the answer is the end of the file with the
		// eof flag set.
		return size, true, nil
	}
	if err != nil {
		return 0, false, err
	}
	if at >= size {
		// The virtual hole at the end of every file: the eof flag tells
		// the client the search is over.
		return size, true, nil
	}
	return at, false, nil
}

// Allocate reserves a range with fallocate in the default mode, which
// grows the file to offset plus length, the behaviour RFC 7862 section
// 15.1 requires of ALLOCATE; Deallocate punches a hole into the range and
// keeps the size. The handle is opened without following a final symlink
// and revalidated against the descriptor before anything is reserved.
func (l *Local) Allocate(h Handle, offset, length int64) error {
	return l.fallocate(h, offset, length, 0)
}

// Deallocate punches a hole into the range and keeps the file size.
func (l *Local) Deallocate(h Handle, offset, length int64) error {
	return l.fallocate(h, offset, length, 0x03) // KEEP_SIZE|PUNCH_HOLE
}

func (l *Local) fallocate(h Handle, offset, length int64, mode uint32) error {
	kind, _, _, _, err := l.resolve(h)
	if err != nil {
		return err
	}
	if kind != typeFile {
		return ErrIsDir
	}
	f, _, err := l.openVerified(h, syscall.O_WRONLY)
	if err != nil {
		return err
	}
	defer f.Close()
	_, _, errno := syscall.Syscall6(syscall.SYS_FALLOCATE, f.Fd(), uintptr(mode),
		uintptr(offset), uintptr(length), 0, 0)
	if errno != 0 {
		return errno
	}
	return nil
}

// fstatSize reads the size of the open file.
func fstatSize(fd int) (int64, error) {
	var st syscall.Stat_t
	if err := syscall.Fstat(fd, &st); err != nil {
		return 0, err
	}
	return st.Size, nil
}