From c481632a2853eb3540056f13707c2670e38948c6 Mon Sep 17 00:00:00 2001
From: Matt Joiner <anacrolix@gmail.com>
Date: Wed, 7 May 2025 09:37:25 +1000
Subject: [PATCH] Harden read/write part file ordering

---
 storage/file-piece.go |  6 +++---
 storage/file.go       | 32 +++++++++++++++++++++++++++-----
 2 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/storage/file-piece.go b/storage/file-piece.go
index 985cca92..7b25fdd3 100644
--- a/storage/file-piece.go
+++ b/storage/file-piece.go
@@ -64,9 +64,9 @@ func (me *filePieceImpl) Completion() Completion {
 		for i, extent := range me.t.segmentLocater.LocateIter(me.extent()) {
 			noFiles = false
 			file := me.t.files[i]
-			s, err := os.Stat(file.safeOsPath)
+			s, err := os.Stat(file.partFilePath())
 			if errors.Is(err, fs.ErrNotExist) {
-				s, err = os.Stat(file.partFilePath())
+				s, err = os.Stat(file.safeOsPath)
 			}
 			if err != nil {
 				me.logger().Warn(
@@ -185,7 +185,7 @@ func (me *filePieceImpl) onFileNotComplete(f file) (err error) {
 	// Ensure the file is writable
 	err = os.Chmod(f.safeOsPath, info.Mode().Perm()|(filePerm&0o222))
 	if err != nil {
-		err = fmt.Errorf("setting file to read-only: %w", err)
+		err = fmt.Errorf("setting file writable: %w", err)
 		return
 	}
 	return
diff --git a/storage/file.go b/storage/file.go
index 42ef20c5..3cf5f62b 100644
--- a/storage/file.go
+++ b/storage/file.go
@@ -222,10 +222,13 @@ type fileTorrentImplIO struct {
 
 // Returns EOF on short or missing file.
 func (fst fileTorrentImplIO) readFileAt(file file, b []byte, off int64) (n int, err error) {
-	f, err := os.Open(file.safeOsPath)
-	if fst.fts.partFiles() && errors.Is(err, fs.ErrNotExist) {
+	var f *os.File
+	if fst.fts.partFiles() {
 		f, err = os.Open(file.partFilePath())
 	}
+	if errors.Is(err, fs.ErrNotExist) {
+		f, err = os.Open(file.safeOsPath)
+	}
 	if errors.Is(err, fs.ErrNotExist) {
 		// File missing is treated the same as a short file. Should we propagate this through the
 		// interface now that fs.ErrNotExist is a thing?
@@ -268,13 +271,32 @@ func (fst fileTorrentImplIO) ReadAt(b []byte, off int64) (n int, err error) {
 	return
 }
 
+func (fst fileTorrentImplIO) openForWrite(file file) (f *os.File, err error) {
+	p := fst.fts.pathForWrite(file)
+	f, err = os.OpenFile(p, os.O_WRONLY|os.O_CREATE, filePerm)
+	if err == nil {
+		return
+	}
+	if errors.Is(err, fs.ErrNotExist) {
+		err = os.MkdirAll(filepath.Dir(p), dirPerm)
+		if err != nil {
+			return
+		}
+	} else if errors.Is(err, fs.ErrPermission) {
+		err = os.Chmod(p, filePerm)
+		if err != nil {
+			return
+		}
+	}
+	f, err = os.OpenFile(p, os.O_WRONLY|os.O_CREATE, filePerm)
+	return
+}
+
 func (fst fileTorrentImplIO) WriteAt(p []byte, off int64) (n int, err error) {
 	// log.Printf("write at %v: %v bytes", off, len(p))
 	fst.fts.segmentLocater.Locate(segments.Extent{off, int64(len(p))}, func(i int, e segments.Extent) bool {
-		name := fst.fts.pathForWrite(fst.fts.files[i])
-		os.MkdirAll(filepath.Dir(name), dirPerm)
 		var f *os.File
-		f, err = os.OpenFile(name, os.O_WRONLY|os.O_CREATE, filePerm)
+		f, err = fst.openForWrite(fst.fts.files[i])
 		if err != nil {
 			return false
 		}
-- 
2.51.0