From 33aca73bb941012760c36806f168924690393ff4 Mon Sep 17 00:00:00 2001
From: Sergey Matveev <stargrave@stargrave.org>
Date: Mon, 28 Nov 2022 17:32:31 +0300
Subject: [PATCH] Per-file locks

---
 storage/file.go | 72 +++++++++++++++++++++++++++++--------------------
 1 file changed, 43 insertions(+), 29 deletions(-)

diff --git a/storage/file.go b/storage/file.go
index 674a506b..7a5bc33d 100644
--- a/storage/file.go
+++ b/storage/file.go
@@ -15,30 +15,35 @@ import (
 	"github.com/anacrolix/torrent/segments"
 )
 
+type fdCacheEntry struct {
+	fd *os.File
+	sync.Mutex
+}
+
 var (
-	fdRCache        = map[string]*os.File{}
+	fdRCache        = map[string]*fdCacheEntry{}
 	fdRCacheM       sync.Mutex
-	fdWCache        = map[string]*os.File{}
+	fdWCache        = map[string]*fdCacheEntry{}
 	fdWCacheM       sync.Mutex
 	fdMkdirAllCache = map[string]struct{}{}
 	fdCacheCleanerM sync.Once
 )
 
 func fdCacheCleaner() {
-	for range time.Tick(10 * time.Second) {
-		fdRCacheM.Lock()
-		for _, v := range fdRCache {
-			v.Close()
-		}
-		fdRCache = make(map[string]*os.File)
-		fdRCacheM.Unlock()
-
-		fdWCacheM.Lock()
-		for _, v := range fdWCache {
-			v.Close()
+	cleaner := func(c *map[string]*fdCacheEntry, m *sync.Mutex) {
+		m.Lock()
+		prev := fdRCache
+		*c = make(map[string]*fdCacheEntry)
+		m.Unlock()
+		for _, v := range prev {
+			v.Lock()
+			v.fd.Close()
+			v.Unlock()
 		}
-		fdWCache = make(map[string]*os.File)
-		fdWCacheM.Unlock()
+	}
+	for range time.Tick(10 * time.Second) {
+		cleaner(&fdRCache, &fdRCacheM)
+		cleaner(&fdWCache, &fdWCacheM)
 	}
 }
 
@@ -173,25 +178,30 @@ type fileTorrentImplIO struct {
 // Returns EOF on short or missing file.
 func (fst *fileTorrentImplIO) readFileAt(file file, b []byte, off int64) (n int, err error) {
 	fdRCacheM.Lock()
-	defer fdRCacheM.Unlock()
-	f := fdRCache[file.path]
-	if f == nil {
-		f, err = os.Open(file.path)
+	centry := fdRCache[file.path]
+	if centry == nil {
+		var fd *os.File
+		fd, err = os.Open(file.path)
 		if os.IsNotExist(err) {
 			// File missing is treated the same as a short file.
 			err = io.EOF
-			return
 		}
 		if err != nil {
+			fdRCacheM.Unlock()
 			return
 		}
-		fdRCache[file.path] = f
+		centry = &fdCacheEntry{fd: fd}
+		fdRCache[file.path] = centry
 	}
+	fdRCacheM.Unlock()
 	// Limit the read to within the expected bounds of this file.
 	if int64(len(b)) > file.length-off {
 		b = b[:file.length-off]
 	}
-	return f.ReadAt(b, off)
+	centry.Lock()
+	n, err = centry.fd.ReadAt(b, off)
+	centry.Unlock()
+	return
 }
 
 // Only returns EOF at the end of the torrent. Premature EOF is ErrUnexpectedEOF.
@@ -218,18 +228,22 @@ func (fst fileTorrentImplIO) WriteAt(p []byte, off int64) (n int, err error) {
 			fdMkdirAllCache[filepath.Dir(name)] = struct{}{}
 		}
 		fdWCacheM.Lock()
-		defer fdWCacheM.Unlock()
-		f := fdWCache[name]
-		if f == nil {
-			f, err = os.OpenFile(name, os.O_WRONLY|os.O_CREATE, 0o666)
+		centry := fdWCache[name]
+		if centry == nil {
+			var fd *os.File
+			fd, err = os.OpenFile(name, os.O_WRONLY|os.O_CREATE, 0o666)
 			if err != nil {
+				fdWCacheM.Unlock()
 				return false
 			}
-			fdWCache[name] = f
+			centry = &fdCacheEntry{fd: fd}
+			fdWCache[name] = centry
 		}
+		fdWCacheM.Unlock()
 		var n1 int
-		n1, err = f.WriteAt(p[:e.Length], e.Start)
-		// log.Printf("%v %v wrote %v: %v", i, e, n1, err)
+		centry.Lock()
+		n1, err = centry.fd.WriteAt(p[:e.Length], e.Start)
+		centry.Unlock()
 		n += n1
 		p = p[n1:]
 		if err == nil && int64(n1) != e.Length {
-- 
2.44.0