def openCommittedBlobFile(oid, serial, blob=None):
    """Return a file for committed data for the given object id and serial

        If a blob is provided, then a BlobFile object is returned,
        otherwise, an ordinary file is returned.  In either case, the
        file is opened for binary reading.

        This method is used to allow storages that cache blob data to
        make sure that data are available at least long enough for the
        file to be opened.
        """
