bug#74922: Fwd: bug#74922: 29.4; copy_string_contents doesn't always produce a valid utf-8

[Eli Zaretskii <eliz@gnu.org>]

> From: Evgeny Kurnevsky <kurnevsky@gmail.com>
> Date: Tue, 17 Dec 2024 13:31:57 +0000
> 
> Yes, that's a binary file that is not an utf-8 string. From the comment in module_copy_string_contents
> implementation I guessed that in such cases emacs should signal an error, but instead it just passes this
> invalid string to the dynamic library which caused this bug in emacs-module-rs (see
> https://ubolonton.github.io/emacs-module-rs/latest/type-conversions.html#strings ). So if it's expected then
> maybe it should be explicitly said in the docs of copy_string_contents here
> https://www.gnu.org/software/emacs/manual/html_node/elisp/Module-Values.html ? It just says that it stores
> the utf-8 encoded text which makes an impression that it's an always valid utf-8 string.

I could look into the internals, but I actually wonder why the module
doesn't check the text before relying on such subtle behaviors.  We
didn't document the fact that it signals an error for a reason.

So: why cannot the module code or the application which uses it test
up from that the string it copies is human-readable text, nit some
binary junk?