from ..i18n import _

from .. import error

def get_checker(ui, revlog_name=b'changelog'):

    """Get a function that checks file handle position is as expected.

    This is used to ensure that files haven't been modified outside of our

    knowledge (such as on a networked filesystem, if `hg debuglocks` was used,

    or writes to .hg that ignored locks happened).

    Due to revlogs supporting a concept of buffered, delayed, or diverted

    writes, we're allowing the files to be shorter than expected (the data may

    not have been written yet), but they can't be longer.

    Please note that this check is not perfect; it can't detect all cases (there

    may be false-negatives/false-OKs), but it should never claim there's an

    issue when there isn't (false-positives/false-failures).

    """

    vpos = ui.config(b'debug', b'revlog.verifyposition.' + revlog_name)

    # Avoid any `fh.tell` cost if this isn't enabled.

    if not vpos or vpos not in [b'log', b'warn', b'fail']:

        return None

    def _checker(fh, fn, expected):

        if fh.tell() <= expected:

            return

        msg = _(b'%s: file cursor at position %d, expected %d')

        # Always log if we're going to warn or fail.

        ui.log(b'debug', msg + b'\n', fn, fh.tell(), expected)

        if vpos == b'warn':

            ui.warn((msg + b'\n') % (fn, fh.tell(), expected))

        elif vpos == b'fail':

            raise error.RevlogError(msg % (fn, fh.tell(), expected))

    return _checker