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From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: Junio C Hamano <gitster@pobox.com>
Cc: "Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>,
	git@vger.kernel.org, "Jeff King" <peff@peff.net>,
	"SZEDER Gábor" <szeder.dev@gmail.com>,
	"Alexandr Miloslavskiy" <alexandr.miloslavskiy@syntevo.com>
Subject: Re: [PATCH v3 1/1] vreportf(): avoid relying on stdio buffering
Date: Wed, 30 Oct 2019 10:13:26 +0100 (CET)	[thread overview]
Message-ID: <nycvar.QRO.7.76.6.1910300957370.46@tvgsbejvaqbjf.bet> (raw)
In-Reply-To: <xmqqo8xz3sa2.fsf@gitster-ct.c.googlers.com>

Hi Junio,

On Wed, 30 Oct 2019, Junio C Hamano wrote:

> "Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
> writes:
>
> > Also please note that we `fflush(stderr)` here to help when running in a
> > Git Bash on Windows: in this case, `stderr` is not actually truly
> > unbuffered, and needs the extra help.
>
> I do not think you have to single out Windows in this case; if we
> are writing directly to file descriptor #2, as long as there have
> been something written by the process to the standard error stream
> before our caller called us, we must flush it to make sure what
> we are going to write out will come after, no matter what platform
> we are on.
>
>     The process may have written to the standard error stream and
>     there may be something left in the buffer kept in the stdio
>     layer.  Call fflush(stderr) before writing the message we
>     prepare in this function.
>
> or something along that line would be sufficient.

I replaced the paragraph with a slightly edited version of this. Thanks.

> > diff --git a/usage.c b/usage.c
> > index 2fdb20086b..471efb2de9 100644
> > --- a/usage.c
> > +++ b/usage.c
> > @@ -9,14 +9,21 @@
> >  void vreportf(const char *prefix, const char *err, va_list params)
> >  {
> >  	char msg[4096];
> > -	char *p;
> > +	size_t off = strlcpy(msg, prefix, sizeof(msg));
> > +	char *p, *pend = msg + sizeof(msg);
> >
> > -	vsnprintf(msg, sizeof(msg), err, params);
> > -	for (p = msg; *p; p++) {
> > +	p = off < pend - msg ? msg + off : pend - 1;
> > +	if (vsnprintf(p, pend - p, err, params) < 0)
> > +		*p = '\0'; /* vsnprintf() failed, clip at prefix */
> > +
> > +	for (; p != pend - 1 && *p; p++) {
>
> It may make the result much simpler to start with something like:
>
> 	size_t prefix_len = strlen(prefix);
>
> 	if (sizeof(msg) <= prefix_len) {
>  		fprintf(stderr, "BUG!!! too long a prefix '%s'\n", prefix);
> 		abort();
> 	}
> 	memcpy(msg, prefix, prefix_len);
> 	p = msg + prefix_len;
>
> as we agreed that we won't have prefix that will fill or overflow
> msg[] based on your earlier reading of the callers like BUG-fl.

Makes sense. I was a bit blinded by my endeavor to keep the diffstat
small.

> That way, we probably may be able to even lose the pend pointer and
> arithmetic around it.

I tried this, and it still looks more obvious to me with `pend`.

> >  		if (iscntrl(*p) && *p != '\t' && *p != '\n')
> >  			*p = '?';
> >  	}
>
> Thanks.  With this flow it is crystal clear that the prefix is shown
> as-is, while the payload is sanitized.

Yes ;-)

Thanks,
Dscho

> > -	fprintf(stderr, "%s%s\n", prefix, msg);
> > +
> > +	*(p++) = '\n'; /* we no longer need a NUL */
> > +	fflush(stderr);
> > +	write_in_full(2, msg, p - msg);
> >  }
> >
> >  static NORETURN void usage_builtin(const char *err, va_list params)
>
>

  reply	other threads:[~2019-10-30  9:13 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-28 15:00 [PATCH 0/1] Fix t5516 flakiness in Visual Studio builds Johannes Schindelin via GitGitGadget
2019-10-28 15:00 ` [PATCH 1/1] vreportf(): avoid buffered write in favor of unbuffered one Johannes Schindelin via GitGitGadget
2019-10-29  3:18   ` Junio C Hamano
2019-10-29 12:30     ` Johannes Schindelin
2019-10-29 13:49       ` Jeff King
2019-10-29 14:13         ` Johannes Schindelin
2019-10-29 14:32           ` Jeff King
2019-10-29 20:09             ` Johannes Schindelin
2019-10-30  1:43               ` Junio C Hamano
2019-10-29 16:44         ` Junio C Hamano
2019-10-29 10:38   ` SZEDER Gábor
2019-10-29 12:38     ` Johannes Schindelin
2019-10-29 13:52       ` Jeff King
2019-10-29 14:18         ` Johannes Schindelin
2019-10-29 13:37 ` [PATCH v2 0/1] Fix t5516 flakiness in Visual Studio builds Johannes Schindelin via GitGitGadget
2019-10-29 13:37   ` [PATCH v2 1/1] vreportf(): avoid relying on stdio buffering Johannes Schindelin via GitGitGadget
2019-10-29 14:21     ` Alexandr Miloslavskiy
2019-10-29 19:57       ` Johannes Schindelin
2019-10-29 20:09         ` Jeff King
2019-10-29 20:24           ` Alexandr Miloslavskiy
2019-10-29 20:11         ` Alexandr Miloslavskiy
2019-10-29 20:01   ` [PATCH v3 0/1] Fix t5516 flakiness in Visual Studio builds Johannes Schindelin via GitGitGadget
2019-10-29 20:01     ` [PATCH v3 1/1] vreportf(): avoid relying on stdio buffering Johannes Schindelin via GitGitGadget
2019-10-29 20:32       ` Jeff King
2019-10-30  8:54         ` Johannes Schindelin
2019-10-31  6:24           ` Jeff King
2019-10-31 10:26             ` Johannes Schindelin
2019-10-31 15:48               ` Jeff King
2019-11-01 18:41                 ` Johannes Schindelin
2019-10-30  2:01       ` Junio C Hamano
2019-10-30  9:13         ` Johannes Schindelin [this message]
2019-10-30 10:44     ` [PATCH v4 0/1] Fix t5516 flakiness in Visual Studio builds Johannes Schindelin via GitGitGadget
2019-10-30 10:44       ` [PATCH v4 1/1] vreportf(): avoid relying on stdio buffering Johannes Schindelin via GitGitGadget
2019-11-02  4:05         ` Junio C Hamano

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