From 4ed4483bbc49b8b72f9bb0d3034d4e94c1f82125 Mon Sep 17 00:00:00 2001
From: Nick Craig-Wood <nick@craig-wood.com>
Date: Thu, 6 Jun 2024 09:39:00 +0100
Subject: [PATCH] vfs: fix fatal error: sync: unlock of unlocked mutex in
 panics

Before this change a panic could be overwritten with the message

    fatal error: sync: unlock of unlocked mutex

This was because we temporarily unlocked the mutex, but failed to lock
it again if there was a panic.

This is code is never the cause of an error but it masks the
underlying error by overwriting the panic cause.

See: https://forum.rclone.org/t/serve-webdav-is-crashing-fatal-error-sync-unlock-of-unlocked-mutex/46300
---
 vfs/vfscache/item.go | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/vfs/vfscache/item.go b/vfs/vfscache/item.go
index 6cc68d1e5..702847719 100644
--- a/vfs/vfscache/item.go
+++ b/vfs/vfscache/item.go
@@ -573,6 +573,15 @@ func (item *Item) open(o fs.Object) (err error) {
 	return err
 }
 
+// Calls f with mu unlocked, re-locking mu if a panic is raised
+//
+// mu must be locked when calling this function
+func unlockMutexForCall(mu *sync.Mutex, f func()) {
+	mu.Unlock()
+	defer mu.Lock()
+	f()
+}
+
 // Store stores the local cache file to the remote object, returning
 // the new remote object. objOld is the old object if known.
 //
@@ -589,9 +598,9 @@ func (item *Item) _store(ctx context.Context, storeFn StoreFn) (err error) {
 	// Object has disappeared if cacheObj == nil
 	if cacheObj != nil {
 		o, name := item.o, item.name
-		item.mu.Unlock()
-		o, err := operations.Copy(ctx, item.c.fremote, o, name, cacheObj)
-		item.mu.Lock()
+		unlockMutexForCall(&item.mu, func() {
+			o, err = operations.Copy(ctx, item.c.fremote, o, name, cacheObj)
+		})
 		if err != nil {
 			if errors.Is(err, fs.ErrorCantUploadEmptyFiles) {
 				fs.Errorf(name, "Writeback failed: %v", err)