The behaviour of upstream iconv() when faced with a character which is
valid in the input encoding, but not representable in the output
encoding, is implementation defined:

http://pubs.opengroup.org/onlinepubs/9699919799/



Specifically:

   If iconv() encounters a character in the input buffer that is valid,
   but for which an identical character does not exist in the target
   codeset, iconv() shall perform an implementation-defined conversion
   on this character.

This behaviour was being exposed in our g_iconv() wrapper and also in
g_convert_with_iconv() — but users of g_convert_with_iconv() (both the
GLib unit tests, and the implementation of g_convert_with_fallback())
were assuming that iconv() would return EILSEQ if faced with an
unrepresentable character.

On platforms like NetBSD, this is not the case: NetBSD’s iconv()
finishes the conversion successfully, and outputs a string containing
replacement characters. It signals those replacements in its return
value from iconv(), which is positive (specifically, non-zero) in such a
case.

Let’s codify the existing assumed behaviour of g_convert_with_iconv(),
documenting that it will return G_CONVERT_ERROR_INVALID_SEQUENCE if
faced with an unrepresentable character. As g_iconv() is a thin wrapper
around iconv(), leave the behaviour there implementation-defined (but
document it as such).

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=790698