Fix overread in JSON parsing errors for incomplete byte sequences

json_lex_string() relies on pg_encoding_mblen_bounded() to point to the
end of a JSON string when generating an error message, and the input it
uses is not guaranteed to be null-terminated.

It was possible to walk off the end of the input buffer by a few bytes
when the last bytes consist of an incomplete multi-byte sequence, as
token_terminator would point to a location defined by
pg_encoding_mblen_bounded() rather than the end of the input.  This
commit switches token_terminator so as the error uses data up to the
end of the JSON input.

More work should be done so as this code could rely on an equivalent of
report_invalid_encoding() so as incorrect byte sequences can show in
error messages in a readable form.  This requires work for at least two
cases in the JSON parsing API: an incomplete token and an invalid escape
sequence.  A more complete solution may be too invasive for a backpatch,
so this is left as a future improvement, taking care of the overread
first.

A test is added on HEAD as test_json_parser makes this issue
straight-forward to check.

Note that pg_encoding_mblen_bounded() no longer has any callers.  This
will be removed on HEAD with a separate commit, as this is proving to
encourage unsafe coding.

Author: Jacob Champion
Discussion: https://postgr.es/m/CAOYmi+ncM7pwLS3AnKCSmoqqtpjvA8wmCdoBtKA3ZrB2hZG6zA@mail.gmail.com
Backpatch-through: 13

Author: michaelpq

src/common/jsonapi.c

@@ -1689,8 +1689,8 @@ json_lex_string(JsonLexContext *lex)
 	} while (0)
 #define FAIL_AT_CHAR_END(code) \
 	do { \
-		lex->token_terminator = \
-			s + pg_encoding_mblen_bounded(lex->input_encoding, s); \
+		char	   *term = s + pg_encoding_mblen(lex->input_encoding, s); \
+		lex->token_terminator = (term <= end) ? term : end; \
 		return code; \
 	} while (0)
 

src/test/modules/test_json_parser/t/002_inline.pl

@@ -127,4 +127,12 @@ sub test
 	'"\\\\\\\\\\\\\\"',
 	error => qr/Token ""\\\\\\\\\\\\\\"" is invalid/);
 
+# Case with three bytes: double-quote, backslash and <f5>.
+# Both invalid-token and invalid-escape are possible errors, because for
+# smaller chunk sizes the incremental parser skips the string parsing when
+# it cannot find an ending quote.
+test("incomplete UTF-8 sequence",
+	"\"\\\x{F5}",
+	error => qr/(Token|Escape sequence) ""?\\\x{F5}" is invalid/);
+
 done_testing();