Pad/internal/io/pool/real/filesystem.go

package real

import (
	"errors"
	"io"
	"os"
	"path/filepath"
	"strconv"
	"strings"
	"sync/atomic"
	"time"

	"pad/internal/io/pool/types"
)

// RealFileSystem implements the pool.FileSystem interface using the real OS filesystem.
type RealFileSystem struct {
	WorkingDir string
}

// NewRealFileSystem creates a RealFileSystem rooted at the given working directory.
// If workingDir is empty, it defaults to "/".
func NewRealFileSystem(workingDir string) *RealFileSystem {
	if workingDir == "" {
		workingDir = "/"
	}
	abs, err := filepath.Abs(workingDir)
	if err != nil {
		abs = workingDir
	}
	return &RealFileSystem{WorkingDir: abs}
}

func (fs *RealFileSystem) ReadDir(path string) ([]types.DirEntry, error) {
	entries, err := os.ReadDir(filepath.Join(fs.WorkingDir, path))
	if err != nil {
		return nil, err
	}
	var res []types.DirEntry
	for _, e := range entries {
		res = append(res, &realDirEntry{e})
	}
	return res, nil
}

func (fs *RealFileSystem) DirExists(path string) bool {
	info, err := os.Stat(filepath.Join(fs.WorkingDir, path))
	return err == nil && info.IsDir()
}

func (fs *RealFileSystem) FileExists(path string) bool {
	info, err := os.Stat(filepath.Join(fs.WorkingDir, path))
	return err == nil && !info.IsDir()
}

func (fs *RealFileSystem) ReadFile(path string) ([]byte, error) {
	return os.ReadFile(filepath.Join(fs.WorkingDir, path))
}

func (fs *RealFileSystem) ReadFileAt(path string, offset, size int) ([]byte, error) {
	fullPath := filepath.Join(fs.WorkingDir, path)
	f, err := os.Open(fullPath)
	if err != nil {
		return nil, err
	}
	defer f.Close()
	buf := make([]byte, size)
	// ReadAt may return a SHORT read with a nil or io.EOF error (POSIX
	// allows it; the Android FUSE layer does it), so loop until the whole
	// range is read or the file truly ends. A silently short result would
	// truncate a chunk and shift every byte offset after it — taps and IME
	// commits would then land at the wrong buffer position (text corruption).
	total := 0
	for total < size {
		n, err := f.ReadAt(buf[total:], int64(offset)+int64(total))
		total += n
		if err != nil {
			if err == io.EOF || errors.Is(err, io.ErrUnexpectedEOF) {
				break
			}
			return nil, err
		}
	}
	return buf[:total], nil
}

func (fs *RealFileSystem) WriteFile(path string, content []byte) error {
	// Simple write for backward compatibility if needed,
	// but defer to Atomic implementation.
	return fs.WriteFileAtomic(path, content)
}

// tmpSeq numbers the per-call temp files so two writes never share a staging
// path. Combined with the owner-side per-file write serialization this makes
// same-file write interleaving structurally impossible.
var tmpSeq atomic.Uint64

func (fs *RealFileSystem) WriteFileAtomic(path string, content []byte) error {
	// Atomic write implementation
	fullPath := filepath.Join(fs.WorkingDir, path)
	// Temp file in the same directory as the target to ensure same filesystem
	// rename. Unique per call (".<name>.tmp.<pid>.<seq>"): even a concurrent
	// write or a crashed process's leftover cannot interleave with this one
	// on the staging file, and rename promotes only this call's complete
	// content.
	base := filepath.Base(fullPath)
	tmpPath := filepath.Join(filepath.Dir(fullPath),
		"."+base+".tmp."+strconv.Itoa(os.Getpid())+"."+strconv.FormatUint(tmpSeq.Add(1), 10))

	if err := os.MkdirAll(filepath.Dir(tmpPath), 0755); err != nil {
		return err
	}

	if err := os.WriteFile(tmpPath, content, 0644); err != nil {
		os.Remove(tmpPath)
		return err
	}

	fs.removeStaleTemps(filepath.Dir(fullPath), base, tmpPath)

	if err := os.Rename(tmpPath, fullPath); err != nil {
		os.Remove(tmpPath)
		return err
	}

	// The Android shared-storage layer (FUSE backed by MediaProvider
	// metadata) re-serves the PREVIOUS file's mtime after a rename: its
	// media-scan DB row for the path is not updated when the staging file
	// is renamed into place (logcat: "Database update failed while
	// renaming"). Change detectors that combine inotify events with a
	// stat compare (syncthing) then treat the new content as unmodified
	// and skip it until the next full rescan — up to an hour of "edited
	// but not synced" on a phone. Setting the timestamps explicitly after
	// the rename publishes the current mtime through the FUSE layer, and
	// the timestamp update is the event the watcher acts on immediately
	// (verified on device: a plain rename's mtime is reverted to the old
	// file's, while utimensat sticks and triggers the sync at once).
	if err := os.Chtimes(fullPath, time.Now(), time.Now()); err != nil {
		// The content is already safely in place; the timestamps are a
		// best-effort hint for external change detectors, not data.
		return nil
	}
	return nil
}

// removeStaleTemps best-effort removes leftover staging files for the same
// file: any ".<base>.tmp.<pid>.<seq>" that is not the caller's own temp, plus
// the legacy deterministic name ".<base>.tmp" from older app versions. A
// live write never owns either: same-file writes are serialized by the logic
// owner, and a live temp always carries the caller's own pid/seq (excluded
// via keepTmp). Errors are ignored.
func (fs *RealFileSystem) removeStaleTemps(dir, base, keepTmp string) {
	entries, err := os.ReadDir(dir)
	if err != nil {
		return
	}
	keep := filepath.Base(keepTmp)
	prefix := "." + base + ".tmp."
	legacy := "." + base + ".tmp"
	for _, e := range entries {
		name := e.Name()
		if name == keep {
			continue // this call's own live temp file
		}
		if name == legacy || strings.HasPrefix(name, prefix) {
			os.Remove(filepath.Join(dir, name))
		}
	}
}

func (fs *RealFileSystem) DeleteFile(path string) error {
	return os.Remove(filepath.Join(fs.WorkingDir, path))
}

func (fs *RealFileSystem) CreateDir(path string) error {
	return os.MkdirAll(filepath.Join(fs.WorkingDir, path), 0755)
}

// realDirEntry wraps os.DirEntry
type realDirEntry struct {
	entry os.DirEntry
}

func (e *realDirEntry) Name() string               { return e.entry.Name() }
func (e *realDirEntry) IsDir() bool                { return e.entry.IsDir() }
func (e *realDirEntry) Info() (os.FileInfo, error) { return e.entry.Info() }