I'll send a note to the git repo owner as well, but it's worth
mentioning to any potential writer of custom ranking function.
Background here: We implemented a custom ranking function, let's call it
XRank. An end user complained that a query along these lines was
crashing Sqlite:
CREATE VIRTUAL TABLE t USING fts4 (a,b,tokenize=porter);
INSERT INTO T(docid,a,b) VALUES(1,"gumby", "pokey");
INSERT INTO T(docid,a,b) VALUES(2,"blockhead", "pokey");
INSERT INTO T(docid, a,b) VALUES(2,"goo", "gumby");
SELECT * FROM t WHERE docid IN (1,2) ORDER BY XRank(matchinfo(t)) DESC;
Now once I thought about it, this is an absurd query -- matchinfo
returns an empty array if there's no MATCH predicate. But the standard
examples of custom rank functions ASSUME that there will be non-empty
match info:
static void rankfunc(sqlite3_context *pCtx, int nVal, sqlite3_value
**apVal){
unsigned int *aMatchinfo = (unsigned int *)sqlite3_value_blob(apVal[0]);
nPhrase = aMatchinfo[0]; // SEGFAULT HERE, sqlite3_value_blob returns
NULL.
}
So I added a test of aMatchinfo in my custom function for NULL:
if(aMatchInfo == 0) {
sqlite3_result_error(cx, "matchinfo blob data is NULL", -1);
return;
}
This null test is missing from the "Appendix A" custom ranking function
example, as contained in this git repo:
https://git.daplie.com/coolaj86/sqlite3-fts4-rank
So a naive implementer -- me for example, before this week -- will
implement this in such a way that a query SQLite has no objection to
will crash any program with SQLite embedded.
So this suggests A) Appendix A needs to be updated B) Any code based on
this example needs to be updated.
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