[JSC] DateCache: ask ICU where a run of one local time offset ends instead of assuming none changes twice in 19 days - #618
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline finding, I checked the reworked bisection loop in DSTCache::localTimeOffset: the i == 0 iteration probes millisecondsFromEpoch itself, so each of the three branches returns and the ASSERT_NOT_REACHED() fallthrough is genuinely unreachable. The (m_before->end, m_after->start) invariant is preserved when a third-offset probe replaces either endpoint, and leastRecentlyUsed with two excludes still has ≥30 of 32 entries to pick from, so it cannot return null.
Extended reasoning...
The inline comment covers the test-runtime convention issue; the note above records the algorithmic checks I ran on the C++ side so a human reviewer knows those specific concerns (loop termination, invariant preservation across the new third-offset branch, and LRU null-safety with two excluded entries) were already looked at and hold up.
WalkthroughChangesDST offset cache handling
Merge Risk: 🟡 Moderate · up to The change targets incorrect timezone offsets and debug assertions after closely spaced transitions, but the new test does not guarantee that such a transition window is exercised. The regression test should be strengthened before merge. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Inline comments:
In `@JSTests/stress/date-timezone-offset-cache-two-transitions-within-19-days.js`:
- Line 134: Strengthen the scenario validation around the returned sweep data
from the timezone test helpers so the test requires at least one scenario with
two observable transitions within 19 days. Count adjacent DST state changes even
when their numeric offsets are equal, and fail setup or validation when every
scenario lacks this regression condition.
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JSTests/stress/date-timezone-offset-cache-two-transitions-within-19-days.jsSource/JavaScriptCore/runtime/JSDateMath.cppSource/JavaScriptCore/runtime/JSDateMath.h
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robobun
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I re-reviewed after the force-push and didn't find bugs, but this is now a full rewrite of the DST cache (ICU ucal_getTimeZoneTransitionDate-based run boundaries replacing the bisection the PR description still refers to), so a human pass on the new design and its measured perf trade-offs is worthwhile.
What was reviewed:
offsetChange()UTC path and the ICU-failure fallback (returnsms+1/msso the cache never over-extends on error).- The LocalTime scan:
T + max(before, after)mapping underUCAL_TZ_LOCAL_FORMER, ±1-day window, and the switch fromPREVIOUS_INCLUSIVEtoPREVIOUSafter the first hit so the loop advances. localTimeOffset()merge/absorb ofm_afterand clamping tomaxECMAScriptTime/m_before->end + 1so intervals stay non-overlapping.
Extended reasoning...
Overview
Since the previous review the branch was force-pushed to a single commit that replaces the earlier bisection tweak with a different design: a new DateCache::offsetChange() queries ICU's transition table to find exact run boundaries, and DSTCache::localTimeOffset() / extendTheAfterCache() are restructured around it. The PR description still documents the old bisection-based fix and explicitly lists the change-and-change-back case (Recife etc.) as "not addressed", whereas the current code and test now cover it. The stress test grew from 5 to 10 zones.
Security risks
None identified. The change is confined to Date offset caching in runtime/JSDateMath.{h,cpp}; inputs are numeric epoch milliseconds already range-checked by callers, and the only external calls are into ICU on the VM's cached UCalendar. No parsing, allocation-size, or privilege surfaces are touched.
Level of scrutiny
High. This is correctness-critical Date semantics shared by every JIT tier, the LocalTime branch of offsetChange() encodes a non-obvious invariant about how UCAL_TZ_LOCAL_FORMER maps skipped/repeated wall-clock times, and the commit message itself reports 1.6×–2.1× slowdowns on some access patterns (weekly walk, random-over-50-years, Asia/Gaza) that a maintainer should sign off on. The description/code divergence also means a reviewer reading only the PR body would evaluate the wrong algorithm.
Other factors
The earlier inline note about the stress test's runtime vs. the JSTests 200 ms guideline is still open and the test got larger, not smaller — I'm not repeating it here. No third-party CHANGES_REQUESTED reviews are outstanding. The bug hunt exited on dry_streak with no findings and no ruled-out candidates beyond what's listed above.
…d over by the local time offset cache Move the WebKit pin to autobuild-preview-pr-618-49b7e6a6, the preview build of oven-sh/WebKit#618 as of its second revision. JSC's DateCache::DSTCache now asks ICU's transition table where a run of one offset ends instead of taking two instants at most 19 days apart with the same offset to have that offset in between. America/Recife was on DST from 2000-10-08 to 10-15 only, and lookups on either side of that week made every instant in it report -03:00. Add the Recife, Fortaleza, Tucuman, Tunis and Gaza scenarios and a daily walk order to the offset cache test.
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…d over by the local time offset cache Move the WebKit pin to autobuild-preview-pr-618-bcfd8003, the preview build of oven-sh/WebKit#618 as of its second revision. JSC's DateCache::DSTCache now asks ICU's transition table where a run of one offset ends instead of taking two instants at most 19 days apart with the same offset to have that offset in between. America/Recife was on DST from 2000-10-08 to 10-15 only, and lookups on either side of that week made every instant in it report -03:00. Add the Recife, Fortaleza, Tucuman, Tunis and Gaza scenarios and a daily walk order to the offset cache test, and make each scenario check that the tzdata in use has the transitions it is about.
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One portability note for |
…stead of assuming none changes twice in 19 days DateCache::DSTCache::localTimeOffset() caches the local time offset as intervals. It took two instants at most 19 days apart with the same offset to have that offset in between, and found the instant where the offset changes by bisecting between two cached intervals with different offsets. A zone can change its offset more than once within 19 days, so both went wrong: - A change and a change back (America/Recife was on DST from 2000-10-08 to 10-15 only, America/Fortaleza to 10-22, America/Argentina/Tucuman was at -04 from 2004-06-01 to 06-13, Asia/Gaza has predicted Ramadan breaks of 7 and 14 days in 2040 and later) was merged over. The short period got the offset around it for every instant the cache had not seen before, so which instants were wrong depended on the lookups the process had made earlier. A daily walk in steps of 19 days or less never saw the period at all. - A bisection probe between two changes to different offsets (America/Cambridge_Bay 2000-10-29 and 11-05, America/Asuncion 2024-10-06 and 10-15) matched neither interval. It was filed under the later one, so the instants from the probe up to that interval got its offset or its DST flag, and debug builds failed ASSERT(m_after->offset == offset). The cache now asks ICU's transition table where a run of one offset ends (ucal_getTimeZoneTransitionDate), which is exact. A lookup shortly past a cached interval settles that interval up to the next change, and starts an interval of its own when it lies past the change. A lookup shortly before a cached interval with the same offset extends that interval back to the start of its run. The bisection is gone. The 19 day constant only decides what counts as shortly. For the local time cache, a transition at T from offset a to offset b moves the offset at T + max(a, b), since calculateLocalTimeOffset() reads the local times that do not exist or exist twice with the former offset. Checked against ICU 78 for all 71209 transitions of all 638 zones from 1850 to 2100. Cost: a transition lookup is about 0.2 us for instants in a zone's rule era (America/New_York from 2007 on) and 2 to 13 us in its table of past transitions (Asia/Gaza lists its transitions up to 2086). It replaces the offset probes the old code made every 19 days and at every change, and there is at most one per run of one offset. A cache hit is unchanged. Release jsc shell, ns per Date, old -> new, America/New_York 1995 to 2025: daily forward walk 45 to 80 -> 60 to 80, daily backward 310 to 640 -> 65 to 85, hourly 25 to 45 -> 25 to 35, weekly 140 to 270 -> 270 to 360, random over one year 74 -> 70, random over 50 years 450 -> 750 to 800, local fields daily 130 -> 180 to 190. Asia/Gaza: daily forward 82 -> 164, random over 50 years 930 -> 1780. UTC: unchanged. JetStream2 date-format-xparb: unchanged (best 1.8 to 2.0 ms either way in New_York, UTC and Gaza).
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Problem
DateCache::DSTCache::localTimeOffset()(runtime/JSDateMath.cpp) takes two instants at most 19 days apart with the same offset to have that offset in between, and finds an offset change by bisecting between two cached intervals with different offsets. A zone can change its offset more than once within 19 days. Then a short period gets the offset around it (Date: an offset change that reverts within 19 days can be skipped by the local time offset cache (America/Recife, October 2000) bun#42248), or a bisection probe matches neither interval and is filed under the later one (Date: offset changes less than 19 days apart no longer poison the local time offset cache (WebKit bump for oven-sh/WebKit#618) bun#42205).TZ=America/Recife jsc -e "new Date('2000-10-07T12:00Z').getTimezoneOffset(); new Date('2000-10-20T12:00Z').getTimezoneOffset(); print(new Date('2000-10-10T12:00Z').getTimezoneOffset())"prints 180. A fresh process prints 120 (Recife was on DST from 2000-10-08 to 10-15 only). A daily walk from 2000-09-08 never sees the seven DST days.TZ=America/Asuncionwith lookups at 2024-10-06T03:59Z and 04:00Z hitsASSERTION FAILED: m_after->offset == offsetin Debug, and Release then prints "Paraguay Standard Time" for 2024-10-14 (summer time).Fix
ucal_getTimeZoneTransitionDate), in the newDateCache::offsetChange(). A lookup shortly past a cached interval settles that interval up to the next change and starts an interval of its own when it lies past the change. A lookup shortly before an interval with the same offset extends it back to the start of its run. The bisection is gone. At most one transition lookup per run of one offset.ucal_getTimeZoneOffsetFromLocalwithUCAL_TZ_LOCAL_FORMERgives. Checked for all 71209 transitions of all 638 ICU zones, 1850 to 2100.JSTests/stress/date-timezone-offset-cache-changes-within-19-days.js(ten zones againstIntl.DateTimeFormat, forward, daily, shuffled and primed orders). The unpatched jsc fails it, Debug and Release. Also 840,000 random lookups in 30 zones and five access patterns againstIntl.DateTimeFormat, 0 wrong, and the JSTestsdate*andintl-date*tests with unchanged results.Background
DateCache::DaylightSavingsOffset(): 32 intervals[start, end] -> {offset, isDST}, am_before/m_afterpair around the last lookup, and 19 days as the farthest it probed ahead before it bisected. V8, Node and SpiderMonkey (30 days there) share the same-offset defect, so this is the first engine to drop it. The 19 day constant now only decides what counts as shortly past or before an interval.date-format-xparb, the workload the cache serves, is unchanged (best of 200 iterations 1.8 to 2.0 ms before and after, in New_York, UTC and Gaza).Notes
getHistoricalOffset()withkFormerfor both options shifts each transition by the offset after it for a gap and by the offset before it for an overlap, so by the larger of the two.SimpleTimeZone::getOffsetFromLocal()subtracts the DST savings before it evaluates the rule, which comes out the same. Two transitions less than a day apart could put the local change points out of order, sooffsetChange()scans every transition within a day and takes the nearest change point. ICU 78's data has no two transitions less than two days apart.Intl.DateTimeFormatreports. 840,000 checks on the patched Release and Debug jsc, 0 wrong. The same script passes on the unpatched jsc too: random instants rarely land in a short period, the stress test targets them.ucal_*directly against the ICU 78.3 in the prebuilt: an offset probe is 100 to 250 ns (Asia/Gaza 800 ns),ucal_getTimeZoneTransitionDate225 ns in a rule era, 2 to 5 us in the historic table (a linear scan from its end plus two rule clones), 13 us for Asia/Gaza.localTimeOffset()is the same in WebKit/WebKitmain, and neither the patch nor the test depends onUSE(BUN_JSC_ADDITIONS). I file it at bugs.webkit.org with this patch and the numbers once it lands here, so that the fork does not carry the divergence for long.//@ runDefault): the cache is C++ behind every tier, and the test takes 23 s on a Debug+ASan jsc (0.3 s in release). It reuses oneDatefor the lookups because the cost of a zone change grows with the number ofDatecells in the heap.stress/intl-datetimeformat-default-timezone-change.jsand the twointl-datetimeformat-default-formatter-stale-across-*tests fail on the unpatched jsc here as well.localTimeToMS(). The two merge cleanly in either order.