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

From: SZEDER Gábor <[email-masked]>
Date: Tue, 29 Oct 2019 11:38:42 +0100

On Mon, Oct 28, 2019 at 03:00:59PM +0000, Johannes Schindelin via GitGitGadget wrote:
> From: Johannes Schindelin <[email-masked]>
> 
> 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 <[email-masked]>
> Signed-off-by: Johannes Schindelin <[email-masked]>
> ---
>  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