fix(regex): a built header must carry every program its pattern needs - #9801
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The three compiled-program caches cap independently and clear wholesale, and compile_and_cache_regex_checked returns early on a REGEX_CACHE hit, so a pattern whose real program lives in FANCY_CACHE (its REGEX_CACHE entry being the never-match placeholder) lost that program permanently once FANCY_CACHE overflowed while the placeholder survived. lookup_fancy_regex treats a built header as authoritative and site_cache::install_programs memoizes the triple against the pattern text, so the consequence is not one bad header but every later construction of that literal. Repair the missing program before publishing and before memoizing. The same shape applies to REPEAT_MATCHER_CACHE, where the wrong answer is the linear engine's capture assignment instead of ECMA-262's.
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Included review availability: Your plan provides up to 8 included reviews per hour; 5 remain after this review. 📝 WalkthroughWalkthroughThe regex runtime now shares a never-match placeholder constant and repairs missing fancy-regex and repeat-matcher programs before publishing cached headers. A regression test covers independent fancy-cache eviction, and the changelog documents the cache-coherence fix. ChangesRegex cache coherence
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to This change repairs regex cache coherence so lookbehind and specialized capture behavior remain correct after independent cache eviction. The targeted regression coverage supports merge readiness with no remaining actionable risk. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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Inline comments:
In `@crates/perry-runtime/src/regex/tests.rs`:
- Around line 1775-1777: Correct the cache-behavior documentation near the test
to state that a missing fancy or repeat program is rebuilt before the header is
published, rather than claiming all three caches clear together. Preserve the
description of independent eviction behavior and ensure the explanation matches
the test setup.
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changelog.d/regex-program-cache-coherence.mdcrates/perry-runtime/src/regex.rscrates/perry-runtime/src/regex/lazy.rscrates/perry-runtime/src/regex/tests.rs
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…es, engine prototype switch Rebased onto main after PerryTS#9764 landed as ddbe0b1; the site cache, header-authoritative lookups and find-only global `test` are main's now and are gone from here. The program-cache coherence fix moved to PerryTS#9801. Three changes remain. * The capture-group cliff. `repeat_matcher::capture_layout` takes a pattern off the linear engine when ECMA-262's RepeatMatcher capture semantics are observable — a capture directly under a quantifier, or a capture inside a negative lookaround. That routing is a correctness requirement, but the engine it routes to is a classical backtracker with no step budget, so adding parentheses fell from linear time to exponential (`/^(a+)+$/.test("a"*28 + "!")`: 16,522 ms; node 4,798 ms). 6.3 % of 4,463 real literals take that route. Both engines accept the same LANGUAGE and differ only in capture ASSIGNMENT, so `linear_rules_out_match` asks the linear program first and a subject it rules out — which is what every ReDoS input is — never reaches the backtracker. This removes the reachable exponential case; it does not bound the worst case (that needs the step budget open upstream as ridiculousfish/regress#177). * Allocation-free cache probes. The three compiled-program caches were `HashMap<(String, String), _>`, so every probe allocated two Strings and copied the pattern text, once per RegExp OBJECT. `ProgramKey = (Arc<str>, Arc<str>)` makes a probe two refcount increments; the remaining materialisations are cold (`RegExp.prototype.compile`, the syntax-error fallback). * `PERRY_REGEX_ENGINE=regress`, off by default: routes every pattern through the ECMAScript backtracker and installs a shared never-match placeholder as the standard program, so the tier-0 engine architecture can be measured in a real binary. Not a supported configuration — the backtracker has no budget. Tests: quantified_capture_pattern_does_not_backtrack_on_a_non_matching_subject, plus the `capture_layout` predicate assertions rewritten around the `(layout, needed)` pair. Claude-Session: https://claude.ai/code/session_01YPfnmWZmSpSWpmnoXvH8z2
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…es, engine prototype switch Rebased onto main after PerryTS#9764 landed as ddbe0b1; the site cache, header-authoritative lookups and find-only global `test` are main's now and are gone from here. The program-cache coherence fix moved to PerryTS#9801. Three changes remain. * The capture-group cliff. `repeat_matcher::capture_layout` takes a pattern off the linear engine when ECMA-262's RepeatMatcher capture semantics are observable — a capture directly under a quantifier, or a capture inside a negative lookaround. That routing is a correctness requirement, but the engine it routes to is a classical backtracker with no step budget, so adding parentheses fell from linear time to exponential (`/^(a+)+$/.test("a"*28 + "!")`: 16,522 ms; node 4,798 ms). 6.3 % of 4,463 real literals take that route. Both engines accept the same LANGUAGE and differ only in capture ASSIGNMENT, so `linear_rules_out_match` asks the linear program first and a subject it rules out — which is what every ReDoS input is — never reaches the backtracker. This removes the reachable exponential case; it does not bound the worst case (that needs the step budget open upstream as ridiculousfish/regress#177). * Allocation-free cache probes. The three compiled-program caches were `HashMap<(String, String), _>`, so every probe allocated two Strings and copied the pattern text, once per RegExp OBJECT. `ProgramKey = (Arc<str>, Arc<str>)` makes a probe two refcount increments; the remaining materialisations are cold (`RegExp.prototype.compile`, the syntax-error fallback). * `PERRY_REGEX_ENGINE=regress`, off by default: routes every pattern through the ECMAScript backtracker and installs a shared never-match placeholder as the standard program, so the tier-0 engine architecture can be measured in a real binary. Not a supported configuration — the backtracker has no budget. Tests: quantified_capture_pattern_does_not_backtrack_on_a_non_matching_subject, plus the `capture_layout` predicate assertions rewritten around the `(layout, needed)` pair. Claude-Session: https://claude.ai/code/session_01YPfnmWZmSpSWpmnoXvH8z2
added 2 commits
September 5, 2026 13:37
… shipped The comment described the group-clear approach that was built first and dropped — it closes the route into the incoherent state but cannot repair a header already in it, which is why the test still failed against it. The fix that shipped repairs the header in `build_and_install_programs` before publishing it and before `site_cache::install_programs` memoizes the triple. Claude-Session: https://claude.ai/code/session_01YPfnmWZmSpSWpmnoXvH8z2
changelog.d/README.md asks for `<PR-number>-<slug>.md`; the fragment landed unnumbered. Rename only — the entry text and the code are unchanged. Claude-Session: https://claude.ai/code/session_014UZWia6L37DpA93VLtNK9m
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proggeramlug
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Sep 5, 2026
…es, engine prototype switch Rebased onto main after PerryTS#9764 landed as ddbe0b1; the site cache, header-authoritative lookups and find-only global `test` are main's now and are gone from here. The program-cache coherence fix moved to PerryTS#9801. Three changes remain. * The capture-group cliff. `repeat_matcher::capture_layout` takes a pattern off the linear engine when ECMA-262's RepeatMatcher capture semantics are observable — a capture directly under a quantifier, or a capture inside a negative lookaround. That routing is a correctness requirement, but the engine it routes to is a classical backtracker with no step budget, so adding parentheses fell from linear time to exponential (`/^(a+)+$/.test("a"*28 + "!")`: 16,522 ms; node 4,798 ms). 6.3 % of 4,463 real literals take that route. Both engines accept the same LANGUAGE and differ only in capture ASSIGNMENT, so `linear_rules_out_match` asks the linear program first and a subject it rules out — which is what every ReDoS input is — never reaches the backtracker. This removes the reachable exponential case; it does not bound the worst case (that needs the step budget open upstream as ridiculousfish/regress#177). * Allocation-free cache probes. The three compiled-program caches were `HashMap<(String, String), _>`, so every probe allocated two Strings and copied the pattern text, once per RegExp OBJECT. `ProgramKey = (Arc<str>, Arc<str>)` makes a probe two refcount increments; the remaining materialisations are cold (`RegExp.prototype.compile`, the syntax-error fallback). * `PERRY_REGEX_ENGINE=regress`, off by default: routes every pattern through the ECMAScript backtracker and installs a shared never-match placeholder as the standard program, so the tier-0 engine architecture can be measured in a real binary. Not a supported configuration — the backtracker has no budget. Tests: quantified_capture_pattern_does_not_backtrack_on_a_non_matching_subject, plus the `capture_layout` predicate assertions rewritten around the `(layout, needed)` pair. Claude-Session: https://claude.ai/code/session_01YPfnmWZmSpSWpmnoXvH8z2
proggeramlug
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…ache keys The rebase onto `fbce42de6` (PerryTS#9801) auto-merged `lazy.rs` without a conflict, and the result did not compile: PerryTS#9801's "repair before publishing" block inserts into `FANCY_CACHE` and `REPEAT_MATCHER_CACHE`, whose key this branch changed from `(String, String)` to `ProgramKey = (Arc<str>, Arc<str>)`. One side added a writer, the other side changed the type those writers use, and git had nothing to complain about — the same shape as the `family_append_fresh` hazard, caught here only because the change is visible to the type checker. Both values are already `Arc<str>` in that scope, so the repair path now clones two refcounts instead of copying the pattern text twice. Also drops this branch's `NEVER_MATCH_SOURCE`: PerryTS#9801 landed the identical constant as `NEVER_MATCH_PATTERN`, documented for the same reason, and `linear_rules_out_match` now uses main's. Claude-Session: https://claude.ai/code/session_014UZWia6L37DpA93VLtNK9m
proggeramlug
pushed a commit
to proggeramlug/perry
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Sep 5, 2026
…ache keys The rebase onto `fbce42de6` (PerryTS#9801) auto-merged `lazy.rs` without a conflict, and the result did not compile: PerryTS#9801's "repair before publishing" block inserts into `FANCY_CACHE` and `REPEAT_MATCHER_CACHE`, whose key this branch changed from `(String, String)` to `ProgramKey = (Arc<str>, Arc<str>)`. One side added a writer, the other side changed the type those writers use, and git had nothing to complain about — the same shape as the `family_append_fresh` hazard, caught here only because the change is visible to the type checker. Both values are already `Arc<str>` in that scope, so the repair path now clones two refcounts instead of copying the pattern text twice. Also drops this branch's `NEVER_MATCH_SOURCE`: PerryTS#9801 landed the identical constant as `NEVER_MATCH_PATTERN`, documented for the same reason, and `linear_rules_out_match` now uses main's. Claude-Session: https://claude.ai/code/session_014UZWia6L37DpA93VLtNK9m
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A wrong answer, not a slowdown
A regex literal using lookbehind, lookahead-with-captures or a backreference
can stop matching permanently, and a literal with a capture group under a
quantifier can silently report the wrong captures. Both are reachable on
maintoday.Mechanism
REGEX_CACHE,FANCY_CACHE,REPEAT_MATCHER_CACHE) are capped at 512 entries each and eachclear()swholesale and independently on overflow.
compile_and_cache_regex_checkedreturns early wheneverREGEX_CACHEalready holds the pattern, so it never re-runs the fancy-regex or
repeat-matcher build.
fancy-regexaccepts, theREGEX_CACHEentry is thenever-match placeholder
[^\s\S]— the real program is the one inFANCY_CACHE. So onceFANCY_CACHEclears while that placeholdersurvives,
get_or_compile_regexhands back a program that matches nothingand nothing rebuilds the fallback.
lookup_fancy_regextreats a built header as authoritative — a nullfancy_ptrbeside a non-nullregex_ptris the answer — andsite_cache::install_programsmemoizes thatProgramstriple against thepattern text. So the damage is not one bad header: every later
construction of the same literal is born with it, until the site-cache
entry is evicted.
REPEAT_MATCHER_CACHEhas the same shape with a quieter symptom: the patternstill matches, but with the linear engine's capture assignment instead of
ECMA-262's RepeatMatcher semantics — which is the entire reason that engine is
consulted.
Attribution, stated precisely
Steps 1–3 pre-date
ddbe0b126(#9764): before it,lookup_fancy_regexfellback to a
FANCY_CACHEprobe, so a header built in the bad window was wronguntil the caches refilled and could recover afterwards.
ddbe0b126removesthat fallback (correctly — it was a full pattern copy and hash on every exec)
and adds
install_programs, which memoizes the incomplete triple. So it didnot introduce the incoherence; it turned a transient wrong answer into a
persistent one. It should be fixed either way, and now rather than later.
The fix
lazy::build_and_install_programsrepairs a missing program before publishingthe header and before memoizing the triple:
came back, rebuild the fancy one;
repeat_matcher::compileisa byte scan that returns immediately unless a capture group sits under a
quantifier or inside a negative lookaround, so it costs nothing for the
patterns that do not need it.
A built header therefore always carries every program its pattern needs, which
is exactly the invariant the header-authoritative lookups and the construction
cache already assume. No cache policy changes, no eviction behaviour changes.
I also tried the narrower fix of clearing the three caches as a group. It is
strictly weaker: it closes the eviction route to the bad state but cannot
repair the state, so the invariant the readers depend on still is not
established — the test below still fails under it.
Test
a_single_program_cache_clear_cannot_disarm_a_lookbehind_literal: build/(?<=foo)bar/, match it, dropFANCY_CACHE(what its independent overflowdoes) while the
REGEX_CACHEplaceholder survives, reset the site cache so afresh literal site cannot answer from the first header's programs, and build
the literal again. It asserts the new header's
fancy_ptris non-null andthat the regex still matches.
a built header must carry every program its pattern needstest result: ok. 99 passed; 0 failed(
cargo test -p perry-runtime --release regex -- --test-threads=1)Summary by CodeRabbit
Bug Fixes
Documentation