Re: [PATCH v2 1/1] vreportf(): avoid relying on stdio buffering

> On 29.10.2019 14:37, Johannes Schindelin via GitGitGadget wrote:
>
> > -	vsnprintf(msg, sizeof(msg), err, params);
> > -	for (p = msg; *p; p++) {
> > +	p = msg + off < pend ? msg + off : pend - 1;
>
> According to my understanding, this is undefined behavior:

> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf
> ... Unless both pointers point to elements of the same array object, or one
> past the last element of the array object, the behavior is undefined.
>
> > The MSVC runtime behavior differs from glibc's with respect to
> > `fprintf(stderr, ...)` in that the former writes out the message
> > character by character.
>
> Can you please preserve the research text about fprintf() behavior on
> different platforms that I provided before? Change formatting to what you
> think fits best.

>
> fprintf() has problems with any buffering settings
> --------------------------------------------------
> 1) If `stderr` is fully buffered:
>    the ordering of `stdout` and `stderr` messages could be wrong,
>    because `stderr` output waits for the buffer to become full.
> 2) If `stderr` has any type of buffering:
>    buffer has fixed size, which could lead to interleaved buffer blocks
>    when two threads/processes write at the same time.
> 3) If `stderr` is not buffered:
>    Some implementations, such as VC++ and MinGW, literally disable
>    buffering and `fprintf()` will output char-by-char, which leads to
>    unreadable char-interleaved writes if two processes write to
>    `stderr` at the same time (threads are OK because `fprintf()`
>    usually locks `FILE*` in current process).
> 4) If stderr is line buffered: MinGW/VC++ will enable full buffering
>    instead. See MSDN for `setvbuf()`.
>
> fprintf() behavior in git, per platform
> ---------------------------------------
> 1) libc - large outputs can be block-interleaved
>    fprintf() enables temporary stream buffering.
>    Code references:
>        buffered_vfprintf()
> 2) VC++ - char-interleaved
>    fprintf() enables temporary stream buffering, but only if stream was
>    not set to no buffering. This has no effect, because stderr is not
>    buffered by default, and git takes an extra step to ensure that in
>    `swap_osfhnd()`.
>    Code references:
>        _iob[_IOB_ENTRIES]
>        __acrt_stdio_temporary_buffering_guard
>        has_any_buffer()
> 3) MinGW - char-interleaved (console), full buffering (file)
>    `fprintf()` obeys `stderr` buffering. But it uses old MSVCRT.DLL,
>    which eventually calls `_flsbuf()`, which enables buffering unless
>    `isatty(stderr)` or buffering is disabled. Buffering is not disabled
>    by default for `stderr`. Therefore, buffering is enabled only for
>    file-redirected `stderr`.
>    Code references:
>        __mingw_vfprintf()
>        __pformat_wcputs()
>        _fputc_nolock()
>        _flsbuf()
>        _iob[_IOB_ENTRIES]
>
>