utf8.c: Prepare workaround for iconv under macOS 14/15

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.t    xt

    --- /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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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';