Re: [PATCH 1/1] vreportf(): avoid buffered write in favor of unbuffered one
- From
SZEDER Gábor <szeder.dev@gmail.com>
- Date
- Oct 29, 2019, 10:38 UTC
- Message-ID
- <20191029103842.GV4348@szeder.dev>
- In-Reply-To
- <455026ce3ef2b2d7cfecfc4b4bf5b588eebddcfe.1572274859.git.gitgitgadget@gmail.com>
On Mon, Oct 28, 2019 at 03:00:59PM +0000, Johannes Schindelin via GitGitGadget wrote:
Show 12 quoted lines
> 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().
Show 60 quoted lines
> 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