vfs: fix "file already exists" error for stale cache files

Before this change if a file was uploaded through a mount, then
deleted externally, trying to upload that file again could give EEXIST
"file already exists".

This was because the file already existing in the cache was confusing
rclone into thinking it already had the file.

The fix is to check that if rclone has a stale cache file then to
ignore it in this situation.

See: https://forum.rclone.org/t/rclone-cant-reuse-filenames/20400

vfs/read_write.go

@@ -36,7 +36,7 @@ func newRWFileHandle(d *Dir, f *File, flags int) (fh *RWFileHandle, err error) {
 	// get an item to represent this from the cache
 	item := d.vfs.cache.Item(f.Path())
 
-	exists := f.exists() || item.Exists()
+	exists := f.exists() || (item.Exists() && !item.WrittenBack())
 
 	// if O_CREATE and O_EXCL are set and if path already exists, then return EEXIST
 	if flags&(os.O_CREATE|os.O_EXCL) == os.O_CREATE|os.O_EXCL && exists {

vfs/vfscache/item.go

@@ -808,6 +808,13 @@ func (item *Item) _checkObject(o fs.Object) error {
 	return nil
 }
 
+// WrittenBack checks to see if the item has been written back or not
+func (item *Item) WrittenBack() bool {
+	item.mu.Lock()
+	defer item.mu.Unlock()
+	return item.info.Fingerprint != ""
+}
+
 // remove the cached file
 //
 // call with lock held