From d8611ecf2dc8eff8c7f766b4c6a755fed3feac37 Mon Sep 17 00:00:00 2001
From: Matt Joiner <anacrolix@gmail.com>
Date: Mon, 11 Apr 2022 15:44:24 +1000
Subject: [PATCH] Fix races around Reader and Torrent.Drop when Torrent is
 closed

---
 client.go  | 3 ---
 reader.go  | 8 ++++++++
 torrent.go | 7 +++++--
 3 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/client.go b/client.go
index d5f96c0e..ee8261f3 100644
--- a/client.go
+++ b/client.go
@@ -1352,9 +1352,6 @@ func (cl *Client) dropTorrent(infoHash metainfo.Hash, wg *sync.WaitGroup) (err e
 		return
 	}
 	err = t.close(wg)
-	if err != nil {
-		panic(err)
-	}
 	delete(cl.torrents, infoHash)
 	return
 }
diff --git a/reader.go b/reader.go
index c2f3de89..9670ff09 100644
--- a/reader.go
+++ b/reader.go
@@ -245,12 +245,20 @@ func (r *reader) readOnceAt(ctx context.Context, b []byte, pos int64) (n int, er
 			err = nil
 			return
 		}
+		if r.t.closed.IsSet() {
+			err = fmt.Errorf("reading from closed torrent: %w", err)
+			return
+		}
 		r.t.cl.lock()
 		// I think there's a panic here caused by the Client being closed before obtaining this
 		// lock. TestDropTorrentWithMmapStorageWhileHashing seems to tickle occasionally in CI.
 		func() {
 			// Just add exceptions already.
 			defer r.t.cl.unlock()
+			if r.t.closed.IsSet() {
+				// Can't update because Torrent's piece order is removed from Client.
+				return
+			}
 			// TODO: Just reset pieces in the readahead window. This might help
 			// prevent thrashing with small caches and file and piece priorities.
 			r.log(log.Fstr("error reading torrent %s piece %d offset %d, %d bytes: %v",
diff --git a/torrent.go b/torrent.go
index 7e6ec24c..6a75eff3 100644
--- a/torrent.go
+++ b/torrent.go
@@ -153,7 +153,7 @@ type Torrent struct {
 	// Torrent sources in use keyed by the source string.
 	activeSources sync.Map
 	sourcesLogger log.Logger
-	
+
 	smartBanCache smartBanCache
 }
 
@@ -850,7 +850,10 @@ func (t *Torrent) numPiecesCompleted() (num pieceIndex) {
 }
 
 func (t *Torrent) close(wg *sync.WaitGroup) (err error) {
-	t.closed.Set()
+	if !t.closed.Set() {
+		err = errors.New("already closed")
+		return
+	}
 	if t.storage != nil {
 		wg.Add(1)
 		go func() {
-- 
2.44.0