package main

import (
	"fmt"
	"os"
)

// PathInfo describes a source/dest path as seen locally by whichever
// process (control agent, sink, source-stream) actually opens it.
type PathInfo struct {
	Exists   bool
	IsDevice bool
	Size     int64
}

// alignmentFor returns the offset/length alignment a path's handle requires
// for positioned reads and writes. Linux block devices accept ordinary
// buffered pread/pwrite at any alignment, so there is nothing to round to.
func alignmentFor(string) int64 { return 1 }

// canElevate reports whether a permission failure opening a device is worth
// retrying under `sudo` (see --sudo).
func canElevate() bool { return true }

func statPath(path string) (PathInfo, error) {
	fi, err := os.Stat(path)
	if err != nil {
		if os.IsNotExist(err) {
			return PathInfo{Exists: false}, nil
		}
		return PathInfo{}, err
	}
	mode := fi.Mode()
	isDevice := mode&os.ModeDevice != 0 && mode&os.ModeCharDevice == 0
	size := fi.Size()
	if isDevice {
		sz, err := blockDeviceSize(path)
		if err != nil {
			return PathInfo{}, fmt.Errorf("stat block device %s: %w", path, err)
		}
		size = sz
	}
	return PathInfo{Exists: true, IsDevice: isDevice, Size: size}, nil
}

// prepareDest makes the destination ready to receive exactly targetSize
// bytes: a regular file is created if missing and truncated (grown or
// shrunk) to targetSize; a block device is only validated to be large
// enough (it can't be resized) — the caller is expected to have already
// capped targetSize at the device's own size when it is the limiting side.
func prepareDest(path string, targetSize int64) error {
	info, err := statPath(path)
	if err != nil {
		return err
	}
	if info.Exists && info.IsDevice {
		if targetSize > info.Size {
			return fmt.Errorf("destination device %s is only %d bytes, need %d", path, info.Size, targetSize)
		}
		return nil
	}
	f, err := os.OpenFile(path, os.O_CREATE|os.O_WRONLY, 0644)
	if err != nil {
		return fmt.Errorf("open/create %s: %w", path, err)
	}
	defer f.Close()
	if err := f.Truncate(targetSize); err != nil {
		return fmt.Errorf("truncate %s to %d: %w", path, targetSize, err)
	}
	return nil
}