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decode_utf8: add an enumeration of failure reasons.

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Now you can optionally get back an enum value indicating whether the
character was successfully decoded, or whether U+FFFD was substituted
due to some kind of problem, and if the latter, what problem.

For a start, this allows distinguishing 'real' U+FFFD (encoded
legitimately in the input) from one invented by the decoder. Also, it
allows the recipient of the decode to treat failures differently,
either by passing on a useful error report to the user (as
utf8_unknown_char now does) or by doing something special.

In particular, there are two distinct error codes for a truncated
UTF-8 encoding, depending on whether it was truncated by the end of
the input or by encountering a non-continuation byte. The former code
means that the string is not legal UTF-8 _as it is_, but doesn't rule
out it being a (bytewise) prefix of a legal UTF-8 string - so if a
client is receiving UTF-8 data a byte at a time, they can treat that
error code specially and not make it a fatal error.
This commit is contained in:
Simon Tatham
2023-02-17 16:39:09 +00:00
parent 9d308b39da
commit 9e01de7c2b
6 changed files with 147 additions and 45 deletions
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8
utils/unicode-known.c
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@ -40,7 +40,13 @@ char *utf8_unknown_char(ptrlen input)
BinarySource_BARE_INIT_PL(src, input);
for (size_t nchars = 0; get_avail(src); nchars++) {
unsigned c = decode_utf8(src);
DecodeUTF8Failure err;
unsigned c = decode_utf8(src, &err);
if (err != DUTF8_SUCCESS)
return dupprintf(
"cannot normalise this string: UTF-8 decoding error "
"at character position %"SIZEu", byte position %"SIZEu": %s",
nchars, src->pos, decode_utf8_error_strings[err]);
if (!known(c))
return dupprintf(
"cannot stably normalise this string: code point %04X "