In mongodb/mongo-php-library#387, a user reported a case where reading a GridFS file produced a "Cursor not found" exception after some period. This was caused by reading a GridFS file over an extended period of time without advancing to a new batch of chunk documents, after which the server killed the cursor. Supporting a noCursorTimeout option on the openDownloadStream() (and related) method would address this. The downloadToStream() (and related) method shouldn't need such an option, as it reads the file as fast as possible (a cursor timeout might only be hit if writing to the destination stream was blocking).