GC: audit Proxy metadata operands across reentrant traps

Parent: #465. Related: #808 and #801/#805. Caller-side follow-up: #811.

Confirmed baseline

Engine f2340f2 reproduces nine failing logical moving-GC cases among fifteen isolated ReleaseSafe tests (five process aborts, four assertion failures). A failure in the first forced execution mode stops that logical test; do not count its second mode as executed.

The gaps are continued Proxy target/handler operands across trap lookup and revocation, descriptor values across conversion getters, and retained trap/ target descriptors across nested GetOwnProperty/IsExtensible callbacks. Additional semantic controls expose nested ownKeys invariant ordering, early descriptor-return/throw ordering, accessor validation order, and inherited setters being invoked during descriptor publication.

Implementation and evidence

• Capture and root the validated target/handler before GetMethod, including revocation during a trap getter. Root/reload result objects, target descriptors, value/getter fields, and receivers across their actual observable callbacks.
• Own continued encoded key bytes. Apply nested internal methods, and snapshot the ownKeys nonconfigurable partition once in key order. Ordinary target storage uses attributes without allocating a JavaScript descriptor per key.
• ToPropertyDescriptor validates a getter before reading setter; descriptor publication creates own data fields and never invokes inherited setters.
• 23 actual-moving cases in both forced tiers (46 verified relocations per test configuration), ten semantic controls in both tiers, shared no-GIL four-lane identity checks, and two allocation-failure sweeps pass Debug, ReleaseSafe and unsuppressed Debug+TSan (26 selected tests each).
• 33 paired local witnesses improve 21 -> 33 passing; each candidate witness explicitly passes tree-walker and required bytecode without fallback. Node v24.18.0 independently matches all 33 expected results.
• All 4,048 upstream tests across 14 Proxy/Reflect/Object/expression subtrees remain passing with byte-identical reports: 0 upstream Test262 flips.
• Full sharded Debug: 2,068 passed, one platform skip, zero failures/leaks, 541.0 seconds across four shards.

Landed on origin/main as fix(proxy): retain metadata operands across reentrant traps (#810), authored/committed by Chris without trailers. ReleaseSafe+TSan also passes all 26 focused tests (9.502s). Seeded Debug fuzz passes 400 default and 300 exact-value programs, both from seed 1, with zero failures.

Pushed integration run is queued. Closure still needs integration evidence; prior CI's separate parallel-collection completion failure is tracked in #812, not claimed fixed here. All completed local build caches were removed; reproducing binaries, source hashes, exact fixtures and logs remain available.

This is synchronous helper ownership. It does not redesign native stacks, queues/jobs, Promise/Worker, Map/Set iterator or native activation lifecycle. No pinning, stale-pointer recovery, disabled GC, relaxed invariant or sanitizer suppression was added. Exact fixtures, hashes and logs are retained locally in /Users/chris/.cache/zig-js-proxy.Hdh6eC.

Now reproduced against exact engine baseline f2340f2. Fifteen isolated ReleaseSafe moving-GC tests run both forced tiers when possible: six logical tests pass, nine fail (five process aborts and four assertion failures). Failing groups are ownKeys trap lookup/revocation, descriptor trap lookup/revocation, descriptor enumerable/writable/accessor conversion, and retained descriptors across nested target/extensibility callbacks. Each selected test must match exactly one linked test; an earlier empty linker placeholder run was rejected as non-evidence. Ten additional ordinary semantic controls also fail baseline, including a segfault on revocation during ownKeys lookup; Node matches all 25 expected control results. Implementation now targets captured handler/target ownership plus exact metadata ordering and own descriptor publication. Evidence: /Users/chris/.cache/zig-js-proxy.Hdh6eC .

Candidate core fix now passes all 26 focused Debug and ReleaseSafe tests: 23 actual-moving cases in both forced tiers (46 verified moves), ten descriptor/order controls in both tiers, shared no-GIL four-lane identity checks, and allocation-failure restoration. The 33 paired local witnesses improve 21 -> 33 passing (baseline includes three segfaults), with zero required-tier fallback. All 4,048 upstream tests across 14 Proxy/Reflect/Object/expression subtrees remain passing; this is 0 upstream Test262 flips. Unsuppressed TSan and full sharded Debug are running. Independent caller-side ownership gaps are tracked in #811; they are not claimed fixed by callee-local roots. Completed build caches were removed after preserving binary hashes and logs.

Verification update: full sharded Debug passed 2,068 tests, one platform skip, zero failures/leaks. All 26 focused tests also pass both unsuppressed Debug+TSan (814.594s) and ReleaseSafe+TSan (9.502s). Sampling confirmed Debug sanitizer time was active allocator stack-trace work; that run was allowed to finish. Seeded default/verify fuzz is the remaining local check before landing. Temporary corpus fixtures and all completed local build caches have been removed after preserving executable hashes and evidence.

Pushed fix(proxy): retain metadata operands across reentrant traps (#810) to origin/main as Chris, no trailers. All local checks are complete, including 700 seeded fuzz programs with zero failures. Integration run 33140348656 is queued; keeping this issue open pending pushed integration evidence. #811 tracks continued caller-side ownership; #812 tracks the separate Linux mid-GC completion assertions.

Post-landing CI is complete for 0ad2b5d9: run 33140348656 finished with 52 successful jobs, one intentional skip, and one failure. The sole failure was the pre-existing #722 witness cve/mc-lock-stop-vs-park.js (expected 400 but got 401), now recorded with its exact job evidence in #722. All Proxy metadata, unit, TSan, no-GIL TSan, test262-parallel, docs, WebAssembly, and threadfuzz jobs relevant to this issue passed. Closing #810; the independent caller audit continues in #811.