automata: don't use the backtracker when it can't search any haystack#1376
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kimjune01 wants to merge 1 commit into
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automata: don't use the backtracker when it can't search any haystack#1376kimjune01 wants to merge 1 commit into
kimjune01 wants to merge 1 commit into
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The bounded backtracker needs `len(NFA states) * (len(haystack) + 1)` bits of visited capacity. When the NFA is big enough relative to that capacity, it cannot search even an empty haystack. `BoundedBacktracker::max_haystack_len` saturates to `0` in that case, which is indistinguishable from "an empty haystack is fine." The meta engine's length check is `len > max_haystack_len()`, so an empty haystack passed it, the search returned `HaystackTooLong`, and `BoundedBacktrackerEngine::is_match` unwrapped that error and panicked. The meta engine now declines to build the backtracker at all when it lacks the capacity for a zero-length haystack, which restores the invariant that this engine is only reachable for haystacks it can actually search. Note that only the empty haystack panicked. For any longer haystack the existing length check already rejected the backtracker. Fixes rust-lang#1344
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I picked this up with a failing on main, pass on branch. It seems that the haystack length being expressed by 0 can sometimes be overloaded with what it means. I added a case to disambiguate.
The new test panics on master and passes here.
cargo test -p regex-automataand-p regexare green,cargo fmt --checkis clean.Fixes #1344