vfs: Ignore ECLOSED in Setattr when truncating file handles

Before this change file handles could get closed while the truncate
the file handles loop was running.

This would mean that ocassionally an ECLOSED (which is translated into
EBADF by cmd/mount) would spuriously be returned if Release happened
to happen in the middle of a Truncate call (Setattr called with
size=0).

This change ignores the ECLOSED while truncating file handles.

See: https://forum.rclone.org/t/writes-to-wasabi-mount-failing-with-bad-file-descriptor-intermittently/26321

vfs/file.go

@@ -746,18 +746,23 @@ func (f *File) Truncate(size int64) (err error) {
 	copy(writers, f.writers)
 	f.mu.Unlock()
 
-	// FIXME: handle closing writer
-
 	// If have writers then call truncate for each writer
 	if len(writers) != 0 {
+		var openWriters = len(writers)
 		fs.Debugf(f.Path(), "Truncating %d file handles", len(writers))
 		for _, h := range writers {
 			truncateErr := h.Truncate(size)
-			if truncateErr != nil {
+			if truncateErr == ECLOSED {
+				// Ignore ECLOSED since file handle can get closed while this is running
+				openWriters--
+			} else if truncateErr != nil {
 				err = truncateErr
 			}
 		}
-		return err
+		// If at least one open writer return here
+		if openWriters > 0 {
+			return err
+		}
 	}
 
 	// if o is nil it isn't valid yet