Root cause (found via bin/repro_jit_resume.ml, 9 surgical cases): when a
`perform` (durable kv read) fires inside a native HO-primitive callback
(map/filter/reduce/for-each/some/every?), the VmSuspended unwound through
the primitive's native OCaml loop (List.map etc.), destroying the loop's
iteration state. The remaining elements were dropped and the stack left
misaligned, so the NEXT CALL_PRIM (map/rest/drop) read wrong args —
"map: expected (fn list)", "rest: 1 list arg", "drop: list and number".
Only triggers in the http-listen + cek_run_with_io serving path (epoch
eval has no synchronous resolver, so conformance was 271/271).
(A) lib/sx_vm.ml call_closure_reuse: when a callback suspends AND a
synchronous IO resolver is installed (serving mode), resolve the
callback's IO inline and run it to completion right there, returning its
value to the native loop — so the loop is never unwound. Scoped to the
resolver-set path; the CEK-driven path (flow/reactive/async tests) keeps
its existing reuse_stack behaviour, so nothing else changes. reuse_stack
is isolated across the nested resume.
(A') lib/sx_vm.ml resume_vm: re-assert _active_vm := Some vm for the
duration of the resumed run (mirrors call_closure). call_closure restored
_active_vm to the caller when VmSuspended unwound, so HO callbacks during
a resume could land on the wrong VM. Latent-bug fix.
(B) bin/sx_server.ml register_jit_hook: the resolve_loop runs inside the
VmSuspended handler, so a non-VmSuspended exception from resume_vm escaped
to the http handler (→ 500). Catch it and fall back to CEK for THIS call
(mark jit_failed, return None → interpreter re-runs it). Self-heals on the
first hit, not a retry. Defense-in-depth; with (A) it shouldn't trigger.
Verification: repro 9/9 (incl. host shape: map[cb→interpreted-helper
perform]→drop = (7 8); reduce; nested map). Standard + --full OCaml
conformance unchanged at 4834/1110 (baseline identical — the 1110 are
pre-existing environmental: host-call-fn/browser-platform symbols,
rational display, tw/regex). Host loop to re-verify 271/271 serving and
drop its (jit-exclude! "host/*" "dream-*" "dr/*") band-aid.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
giles
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