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In the Snowball dictionary, don't try to stem excessively-long words.

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If the input word exceeds 1000 bytes, don't pass it to the stemmer;
just return it as-is after case folding.  Such an input is surely
not a word in any human language, so whatever the stemmer might
do to it would be pretty dubious in the first place.  Adding this
restriction protects us against a known recursion-to-stack-overflow
problem in the Turkish stemmer, and it seems like good insurance
against any other safety or performance issues that may exist in
the Snowball stemmers.  (I note, for example, that they contain no
CHECK_FOR_INTERRUPTS calls, so we really don't want them running
for a long time.)  The threshold of 1000 bytes is arbitrary.

An alternative definition could have been to treat such words as
stopwords, but that seems like a bigger break from the old behavior.

Per report from Egor Chindyaskin and Alexander Lakhin.
Thanks to Olly Betts for the recommendation to fix it this way.

Discussion: https://postgr.es/m/1661334672.728714027@f473.i.mail.ru
This commit is contained in:
Tom Lane 2022-08-31 10:42:05 -04:00
parent 0101f770a0
commit 1058555a5e
1 changed files with 17 additions and 1 deletions
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18
src/backend/snowball/dict_snowball.c
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@ -275,8 +275,24 @@ dsnowball_lexize(PG_FUNCTION_ARGS)
char *txt = lowerstr_with_len(in, len);
TSLexeme *res = palloc0(sizeof(TSLexeme) * 2);
if (*txt == '\0' || searchstoplist(&(d->stoplist), txt))
/*
* Do not pass strings exceeding 1000 bytes to the stemmer, as they're
* surely not words in any human language. This restriction avoids
* wasting cycles on stuff like base64-encoded data, and it protects us
* against possible inefficiency or misbehavior in the stemmer. (For
* example, the Turkish stemmer has an indefinite recursion, so it can
* crash on long-enough strings.) However, Snowball dictionaries are
* defined to recognize all strings, so we can't reject the string as an
* unknown word.
*/
if (len > 1000)
{
/* return the lexeme lowercased, but otherwise unmodified */
res->lexeme = txt;
}
else if (*txt == '\0' || searchstoplist(&(d->stoplist), txt))
{
/* empty or stopword, so report as stopword */
pfree(txt);
}
else