Check for isolate termination in iterating C++ callbacks#6331
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Check for isolate termination in iterating C++ callbacks#6331
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anonrig
approved these changes
Mar 16, 2026
dcarney-cf
approved these changes
Mar 16, 2026
dcarney-cf
reviewed
Mar 16, 2026
Root cause: When JS code uses Array.from() or spread on a C++ iterator (like the SQL cursor), V8's Torque builtin loops over .next() calls without JS back-edge interrupt checks. If TerminateExecution() is called (e.g. by the near-heap-limit callback when memory is exceeded), the termination request sits unprocessed in the stack guard, and the iterator keeps running, growing the heap until it hits the 4GB pointer compression cage limit and crashes. Three changes: 1. deps/workerd/src/workerd/jsg/setup.h -- Added requestTermination() / isTerminationRequested() flag on IsolateBase, checkable from C++ callbacks without relying on V8's internal interrupt processing. 2. deps/workerd/src/workerd/jsg/iterator.h + iterator.c++ -- nextImpl() checks isTerminationRequested() after each successful row and throws a JS exception if set. V8 sees the exception on the callback return path and stops the iteration. 3. src/edgeworker/scheduling/limit-enforcer-impl.c++ -- terminate() now calls requestTermination() alongside TerminateExecution(). Bug: EW-10621
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kentonv
reviewed
Mar 18, 2026
| // Signals that execution should be terminated immediately, including during C++ iterator | ||
| // callbacks where V8's own TerminateExecution() interrupt would not be checked. | ||
| // Also calls V8's TerminateExecution(). See IsolateBase::requestTermination(). | ||
| void requestTermination(); |
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I think you need to make this method thread-safe. It looks like you've arranged for it to be called from many places where we may be terminating from another thread. So this needs to be const and probably the terminationRequested boolean needs to be an atomic.
(Though don't think there's any problem in practice since it's just a boolean...)
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Root cause: When JS code uses Array.from() or spread on a C++ iterator (like the SQL cursor), V8's Torque builtin loops over .next() calls without JS back-edge interrupt checks. If TerminateExecution() is called (e.g. by the near-heap-limit callback when memory is exceeded), the termination request sits unprocessed in the stack guard, and the iterator keeps running, growing the heap until it hits the 4GB pointer compression cage limit and crashes.
Three changes:
deps/workerd/src/workerd/jsg/setup.h -- Added requestTermination() / isTerminationRequested() flag on IsolateBase, checkable from C++ callbacks without relying on V8's internal interrupt processing.
deps/workerd/src/workerd/jsg/iterator.h + iterator.c++ -- nextImpl() checks isTerminationRequested() after each successful row and throws a JS exception if set. V8 sees the exception on the callback return path and stops the iteration.
src/edgeworker/scheduling/limit-enforcer-impl.c++ -- terminate() now calls requestTermination() alongside TerminateExecution().
Bug: EW-10621