.

    If `upperboundcomp` is not None, this is the expected maximal gain from
    compression for the data content.

    `concurrencychecker` is an optional function that receives 3 arguments: a
    file handle, a filename, and an expected position. It should check whether
    the current position in the file handle is valid, and log/warn/fail (by
    raising).

    See mercurial/revlogutils/contants.py for details about the content of an
    index entry.
    c