Re: [RFC PATCH] vreportf: ensure sensible ordering of normal and error output

[Jeff King <peff@peff.net>]

On Tue, Nov 30, 2021 at 02:14:25AM -0500, Jeff King 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.
> 
>     I _think_ we're probably OK here. The scenario above means that the
>     caller is already doing asynchronous I/O via stdio and is subject to
>     deadlock. Because the segment of buffer we try to flush here _could_
>     have been flushed already under the hood, which would have caused
>     the same blocking. A careful caller might be using select() or
>     similar to decide when it is OK to write, but I find it highly
>     unlikely they'd be using stdio in that case.
> 
> 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.

Thinking on this a bit more: I guess as soon as we exit libc would call
the equivalent of fflush(NULL) anyway, and try that same flush. So in a
sense this is just ordering a bit differently, and not introducing any
new problems. (Unless libc is clever enough to avoid blocking, but that
doesn't seem like something we could or should rely on in general).

-Peff