procutil: make stream detection in make_line_buffered more correct and strict In make_line_buffered(), we don’t want to wrap the stream if we know that lines get flushed to the underlying raw stream already. Previously, the heuristic was too optimistic. It assumed that any stream which is not an instance of io.BufferedIOBase doesn’t need wrapping. However, there are buffered streams that aren’t instances of io.BufferedIOBase, like Mercurial’s own winstdout. The new logic is different in two ways: First, only for the check, if unwraps any combination of WriteAllWrapper and winstdout. Second, it skips wrapping the stream only if it is an instance of io.RawIOBase (or already wrapped). If it is an instance of io.BufferedIOBase, it gets wrapped. In any other case, the function raises an exception. This ensures that, if an unknown stream is passed or we add another wrapper in the future, we don’t wrap the stream if it’s already line buffered or not wrap the stream if it’s not line buffered. In fact, this was already helpful during development of this change. Without it, I possibly would have forgot that WriteAllWrapper needs to be ignored for the check, leading to unnecessary wrapping if stdout is unbuffered. The alternative would have been to always wrap unknown streams. However, I don’t think that anyone would benefit from being less strict. We can expect streams from the standard library to be subclassing either io.RawIOBase or io.BufferedIOBase, so running Mercurial in the standard way should not regress by this change. Py2exe might replace sys.stdout and sys.stderr, but that currently breaks Mercurial anyway and also these streams don’t claim to be interactive, so this function is not called for them.

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