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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>,
	Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>
Subject: Re: [PATCH 1/1] vreportf(): avoid buffered write in favor of unbuffered one
Date: Tue, 29 Oct 2019 13:30:51 +0100 (CET)	[thread overview]
Message-ID: <nycvar.QRO.7.76.6.1910291222500.46@tvgsbejvaqbjf.bet> (raw)
In-Reply-To: <xmqqeeyw6xyr.fsf@gitster-ct.c.googlers.com>

Hi Junio,

On Tue, 29 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.
>
> Yuck.  So on all systems, vreportf() now totally bypasses stdio?

Yep ;-)

> Also, this is only to help output from us that goes via vreportf() and
> other codepaths in us that use stdio to write to the standard error
> stream can still get mixed on Windows (I think the answer is yes,
> because we wouldn't need fflush() in this patch if we are covering all
> writes to the standard error stream)?

Yes, `write()` can get interrupted, so there is still a slight chance of
interleaving.

However, with `fprintf()`, apparently the MSVC runtime essentially
writes and flushes one character at a time, which will make it _much_
more likely that two competing processes write interleaved messages to
`stderr`.

> By the way, I'd retitle the patch to highlight that we are still doing
> buffered write, if I were doing this topic.  We are just avoiding some
> implementations of stdio that do not give us buffering and doing the
> buffering ourselves.
>
>     Subject: vreportf(): don't rely on stdio buffering
>
> or something like that.

Good idea.

> > Co-authored-by: Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>
> > Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> > ---
> >  usage.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/usage.c b/usage.c
> > index 2fdb20086b..4328894dce 100644
> > --- a/usage.c
> > +++ b/usage.c
> > @@ -10,13 +10,19 @@ 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));
>
> Like snprintf(3) the strlcpy() and strlcat() functions return the
> total length of the string they tried to create.  For strlcpy() that
> means the length of src.

True (I misread `compat/strlcpy.c` and forgot to consult the
documentation). This length can be longer than `msg`, of course.

> So "off" would be strlen(prefix), which could be longer than
> sizeof(msg)?  Then what happens to this vsnprintf() we see below?

A problem.

I `git grep`ed and saw that only very short `prefix`es are hard-coded.
So that is a hypothetical concern.

However, Alex also indicated his discomfort with this, so I will change
the code to account for a `prefix` that is too long (the entire error
message will be clipped away in that case, which is unfortunate, but to
be expected).

> > +	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);
>
> Strictly speaking this is a breaking change in that control
> sequences in prefix are now clobbered.  Does any caller call this
> function with prefix like "^M\033[K<some string>" to overwrite the
> last output line with the new message?  If not, then probably we do
> not have to worry about it (and reusing msg[] does feel attractive).

Such a sequence would not exactly be a prefix, but okay, I changed the
code to replace only characters in the non-prefix part. For good
measure, I also detect `NUL`s in that part and shorten `ret` in that
case (think `die("This was an\0unintentional NUL")`).

Thanks for the review!
Dscho

> > +	if (ret > 0) {
> > +		if (off + ret > sizeof(msg) - 1)
> > +			ret = sizeof(msg) - 1 - off;
> > +		msg[off + ret] = '\n'; /* we no longer need a NUL */
> > +		fflush(stderr);
> > +		xwrite(2, msg, off + ret + 1);
> > +	}
> >  }
> >
> >  static NORETURN void usage_builtin(const char *err, va_list params)
>

  reply	other threads:[~2019-10-29 12:31 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 [this message]
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
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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