Re: [PATCH 1/1] vreportf(): avoid buffered write in favor of unbuffered one

> On Mon, Oct 28, 2019 at 03:00:59PM +0000, Johannes Schindelin via GitGitGadget wrote:
> > From: Johannes Schindelin <johannes.schindelin@gmx.de>
> >
> > The MSVC runtime behavior differs from glibc's with respect to
> > `fprintf(stderr, ...)` in that the former writes out the message
> > character by character.
> >
> > In t5516, this leads to a funny problem where a `git fetch` process as
> > well as the `git upload-pack` process spawned by it _both_ call `die()`
> > at the same time. The output can look like this:
> >
> > 	fatal: git uploadfata-lp: raemcokte :error:  upload-pnot our arcef k6: n4ot our ea4cr1e3f 36d45ea94fca1398e86a771eda009872d63adb28598f6a9
> > 	8e86a771eda009872d6ab2886
>
> Heh.

> > Let's avoid this predicament altogether by rendering the entire message,
> > including the prefix and the trailing newline, into the buffer we
> > already have (and which is still fixed size) and then write it out via
> > `write_in_full()`.
>
> s/write_in_full/xwrite/ perhaps?  Both the cover letter and the patch
> below use xwrite().

> > The history of `vreportf()` with regard to this issue includes the
> > following commits:
> >
> > d048a96e (2007-11-09) - 'char msg[256]' is introduced to avoid interleaving
> > 389d1767 (2009-03-25) - Buffer size increased to 1024 to avoid truncation
> > 625a860c (2009-11-22) - Buffer size increased to 4096 to avoid truncation
> > f4c3edc0 (2015-08-11) - Buffer removed to avoid truncation
> > b5a9e435 (2017-01-11) - Reverts f4c3edc0 to be able to replace control
> >                         chars before sending to stderr
> > 9ac13ec9 (2006-10-11) - Another attempt to solve interleaving.
> >                         This is seemingly related to d048a96e.
> > 137a0d0e (2007-11-19) - Addresses out-of-order for display()
> > 34df8aba (2009-03-10) - Switches xwrite() to fprintf() in recv_sideband()
> >                         to support UTF-8 emulation
> > eac14f89 (2012-01-14) - Removes the need for fprintf() for UTF-8 emulation,
> >                         so it's safe to use xwrite() again
> > 5e5be9e2 (2016-06-28) - recv_sideband() uses xwrite() again
> >
> > Note that we need to be careful to handle the return value of
> > `vsnprintf()` that indicates the _desired_ byte count.
> >
> > 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.
> >
> > 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));
> > +	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) {
> > +		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)
> > --
> > gitgitgadget
>