Re: [PATCH v2 1/2] utf8.c: Prepare workaround for iconv under macOS 14/15

> From: Torsten Bögershausen <tboegi@web.de>
>
> MacOS14 (Sonoma) has started to ship an iconv library with bugs.
> The same bugs exists even in MacOS 15 (Sequoia)
>
> A bug report running the Git test suite says:
>
> three tests of t3900 fail on macOS 26.1 for me:
>
>   not ok 17 - ISO-2022-JP should be shown in UTF-8 now
>   not ok 25 - ISO-2022-JP should be shown in UTF-8 now
>   not ok 38 - commit --fixup into ISO-2022-JP from UTF-8
>
> Here's the verbose output of the first one:
>
> ----- snip! -----

> expecting success of 3900.17 'ISO-2022-JP should be shown in UTF-8 now':
>                 compare_with ISO-2022-JP "$TEST_DIRECTORY"/t3900/2-UTF-8.txt
>
> --- /Users/x/src/git/t/t3900/2-UTF-8.txt 2024-10-01 19:43:24.605230684 +0000
> +++ current     2025-12-08 21:52:45.786161909 +0000

> @@ -1,4 +1,4 @@
>  はれひほふ
>
>  しているのが、いるので。
> -濱浜ほれぷりぽれまびぐりろへ。
> +濱浜ほれぷりぽれまび$0$j$m$X!#
> not ok 17 - ISO-2022-JP should be shown in UTF-8 now
> 1..17
> ----- snap! -----

> compare_with runs git show to display a commit message, which in this
> case here was encoded using ISO-2022-JP and is supposed to be reencoded
> to UTF-8, but git show only does that half-way -- the "$0$j$m$X!#" part
> is from the original ISO-2022-JP representation.
>
> That botched conversion is done by utf8.c::reencode_string_iconv().  It
> calls iconv(3) to do the actual work, initially with an output buffer of
> the same size as the input.  If the output needs more space the function
> enlarges the buffer and calls iconv(3) again.
>
> iconv(3) won't tell us how much space it needs, but it will report what
> part it already managed to convert, so we can increase the buffer and
> continue from there.  ISO-2022-JP has escape codes for switching between
> character sets, so it's a stateful encoding.  I guess the iconv(3) on my
> machine forgets the state at the end of part one and then messes up part
> two.
>
> [end of citation]
>
> Working around the buggy iconv shipped with the OS can be done in
> two  ways:
> a) Link Git against a different version of iconv
> b) Improve the handling when iconv needs a larger output buffer
>
> a) is already done by default when either Fink [1] or MacPorts [2]
>    or Homebrew [3] is installed.
> b) is implemented here, in case that no fixed iconv is available:
>    When the output buffer is too short, increase it (as before)
>    and start from scratch (this is new).
>
> This workound needs to be enabled with
> '#define ICONV_RESTART_RESET'
> and a makefile knob will be added in the next commit
>
> Suggested-by: René Scharfe <l.s.r@web.de>
> Signed-off-by: Torsten Bögershausen <tboegi@web.de>
>
> [1] https://www.finkproject.org/
> [2] https://www.macports.org/
> [3] https://brew.sh/
>
> Signed-off-by: Torsten Bögershausen <tboegi@web.de>
> ---
>  utf8.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/utf8.c b/utf8.c
> index 35a0251939..96460cc414 100644
> --- a/utf8.c
> +++ b/utf8.c
> @@ -515,6 +515,19 @@ char *reencode_string_iconv(const char *in, size_t insz, iconv_t conv,
>  			out = xrealloc(out, outalloc);
>  			outpos = out + sofar;
>  			outsz = outalloc - sofar - 1;
> +#ifdef ICONV_RESTART_RESET
> +			/*
> +			 * If iconv(3) messes up piecemeal conversions
> +			 * then restore the original pointers, sizes,
> +			 * and converter state, then retry converting
> +			 * the full string using the reallocated buffer.
> +			 */
> +			insz += cp - (iconv_ibp)in; /* Restore insz */
> +			cp = (iconv_ibp)in;         /* original start value */
> +			outpos = out + bom_len;     /* original start value */
> +			outsz = outalloc - bom_len - 1; /* new len */
> +			iconv(conv, NULL, NULL, NULL, NULL); /* reset iconv machinery */
> +#endif
>  		}
>  		else {
>  			*outpos = '\0';