On Tue, Nov 30, 2021 at 2:14 AM Jeff King [off-list ref] wrote:

  - flushing causes us to block. This implies our stdout is connected to
    a pipe or socket, and the other side is not expecting to read. A
    plausible case here is a client sending us a big input which we find
    to be bogus (maybe index-pack checking an incoming pack). We call
    die() to complain about the input, but the client is still writing.
    In the current code, we'd write out our error and then exit; the
    client would get SIGPIPE or a write() error and abort. But with a
    flush here, we could block writing back to the client, and now we're
    in a deadlock; they are trying to write to us but we are no longer
    reading, and we are blocked trying to get out a little bit of
    irrelevant stdout data.

Of the two, the deadlock case worries me more, just because it would be
quiet subtle and racy. As I said, I think we may be OK, but my reasoning
there is pretty hand-wavy.

Flushing stdout only if it is attached to a terminal:

    if (isatty(1))
        fflush(stdout);

should address this potential-deadlock concern, I think(?). It's
rather ugly, though, and entering the realm of
too-much-special-casing; it feels like it has the potential of heading
down a rabbit hole where we find more cases which need to be handled
specially.