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Fix performance issue in new regex match-all detection code.

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Commit 824bf7190 introduced a new search of the NFAs generated by
regex compilation.  I failed to think hard about the performance
characteristics of that search, with the predictable outcome
that it's bad: weird regexes can trigger exponential search time.
Worse, there's no check-for-interrupt in that code, so you can't
even cancel the query if this happens.

Fix by introducing memo-ization of the search results, so that any one
NFA state need be examined in detail just once.  This potentially uses
a lot of memory, but we can bound the memory usage by putting a limit
on the number of states for which we'll try to prove match-all-ness.
That is sane because we already have a limit (DUPINF) on the maximum
finite string length that a matchall regex can match; and patterns
that involve much more than DUPINF states would probably exceed that
limit anyway.

Also, rearrange the logic so that we check the basic is-the-graph-
all-RAINBOW-arcs property before we start the recursive search to
determine path lengths.  This will ensure that we fall out quickly
whenever the NFA couldn't possibly be matchall.

Also stick in a check-for-interrupt, just in case these measures
don't completely eliminate the risk of slowness.

Discussion: https://postgr.es/m/3483895.1619898362@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2021-05-03 11:42:31 -04:00
parent b94409a02f
commit f68970e33f
4 changed files with 378 additions and 175 deletions
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3
src/backend/regex/regcomp.c
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@ -182,8 +182,7 @@ static void markreachable(struct nfa *, struct state *, struct state *, struct s
static void markcanreach(struct nfa *, struct state *, struct state *, struct state *);
static long analyze(struct nfa *);
static void checkmatchall(struct nfa *);
static bool checkmatchall_recurse(struct nfa *, struct state *,
bool, int, bool *);
static bool checkmatchall_recurse(struct nfa *, struct state *, bool **);
static bool check_out_colors_match(struct state *, color, color);
static bool check_in_colors_match(struct state *, color, color);
static void compact(struct nfa *, struct cnfa *);