> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Sat, 23 Dec 2017 16:20:04 +0000
> Cc: phst@google.com, emacs-devel@gnu.org
>
> static void
> json_check_utf8 (Lisp_Object string)
> {
>   eassert (!STRING_MULTIBYTE (string));
>   struct coding_system coding;
>   setup_coding_system (Qutf_8_unix, &coding);
>   /* We initialize only the fields that check_utf_8 accesses.  */
>   coding.src_pos = 0;
>   coding.src_pos_byte = 0;
>   coding.src_chars = SCHARS (string);
>   coding.src_bytes = SBYTES (string);
>   coding.src_object = string;
>   coding.eol_seen = EOL_SEEN_NONE;
>   CHECK_TYPE (check_utf_8 (&coding) >= 0, Qutf_8_string_p, string);
> }
>
> This apparently fails to detect the unibyte string "u\xFFv" as invalid UTF-8.

How can that be?  0xFF shouldn't pass any of the
UTF_8_n_OCTET_LEADING_P tests, so it should end up in 'else', where
the function returns -1.  Right?

You also need to initialize the head_ascii member, I think.  Maybe its
being uninitialized explains the failure.