Re: [PATCH v2 1/1] vreportf: Fix interleaving issues, remove 4096 limitation

[Jeff King <peff@peff.net>]

On Fri, Oct 25, 2019 at 04:02:36PM +0200, Johannes Schindelin wrote:

> ... and indeed, I verified that this patch fixes the problem:
> 
> -- snip --
> diff --git a/usage.c b/usage.c
> index 2fdb20086bd..7f5bdfb0f40 100644
> --- a/usage.c
> +++ b/usage.c
> @@ -10,13 +10,16 @@ void vreportf(const char *prefix, const char *err, va_list params)
>  {
>  	char msg[4096];
>  	char *p;
> -
> -	vsnprintf(msg, sizeof(msg), err, params);
> +	size_t off = strlcpy(msg, prefix, sizeof(msg));
> +	int ret = vsnprintf(msg + off, sizeof(msg) - off, err, params);
>  	for (p = msg; *p; p++) {
>  		if (iscntrl(*p) && *p != '\t' && *p != '\n')
>  			*p = '?';
>  	}
> -	fprintf(stderr, "%s%s\n", prefix, msg);
> +	if (ret > 0) {
> +		msg[off + ret] = '\n'; /* we no longer need a NUL */
> +		write_in_full(2, msg, off + ret + 1);
> +	}
>  }

Heh. This is quite similar to what I posted in:

  https://public-inbox.org/git/20190828145412.GB14432@sigill.intra.peff.net/

though I missed the cleverness with "we no longer need a NUL" to get an
extra byte. ;)

> > except truncation to 4096. Then I would expect a patch to increase
> > buffer size to 8192 in the next couple years. And if you also try to
> > solve truncation, it will get you very close to my code.
> 
> My point is: I don't want to "fix" truncation. I actually think of it as
> a feature. An error message that is longer than the average news article
> I read is too long, period.

Yeah. As the person responsible for many of the "avoid truncation" works
referenced in the original patch, I have come to the conclusion that it
is not worth the complexity. Even when we do manage to produce a
gigantic error message correctly, it's generally not very readable.

That's basically what I came here to say, and I was pleased to find that
you had already argued for it quite well. So I'll just add my support
for the direction you've taken the conversation.

I _do_ wish we could do the truncation more intelligently. I'd much
rather see:

  error: unable to open 'absurdly-long-file-name...': permission denied

than:

  error: unable to open 'absurdly-long-file-name-that-goes-on-forever-and-ev

But I don't think it's possible without reimplementing snprintf
ourselves.

-Peff