Fix BaseException escape from host callback trampoline (#336)#337
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alexcrichton merged 1 commit intobytecodealliance:mainfrom May 7, 2026
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…nce#336) The host-callback trampoline in `wasmtime/_func.py` caught `Exception`, which let `BaseException` subclasses (`KeyboardInterrupt`, `SystemExit`, custom `BaseException` subclasses) escape into the Rust array_call trampoline with an undefined `c_void_p` return value, causing a libmalloc SIGABRT inside `HostFunc::array_call_trampoline`. Broadens the catch to `BaseException`. The existing `LAST_EXCEPTION` / `Trap("python exception")` machinery handles propagation back to the Python caller unchanged. Adds regression tests for `KeyboardInterrupt`, `SystemExit`, and a custom `BaseException` subclass raised from host callbacks. Fixes bytecodealliance#336
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This was referenced May 7, 2026
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Summary
Fixes #336.
The host-callback trampoline in
wasmtime/_func.pycaughtExceptionrather thanBaseException, soBaseExceptionsubclasses (KeyboardInterrupt,SystemExit, customBaseExceptionsubclasses) escaped the handler and caused the Python process to SIGABRT inside Rust'sHostFunc::array_call_trampoline(libmalloc "pointer being freed was not allocated") rather than propagating to the Python caller.The fix broadens the catch to
BaseException. The existingLAST_EXCEPTION/Trap("python exception")machinery already handles re-raising on the Python side, so no other code changes are needed.This is Option A from the issue — the minimum diff that closes the abort. Today, the original Python exception is re-raised at the call boundary via
maybe_raise_last_exn, so after this fixKeyboardInterruptfrom a host callback comes back asKeyboardInterruptand^Cfeels like^C. Option B (catchBaseExceptionbut explicitly preserveKeyboardInterrupt/SystemExitsemantics in some other way, or re-shape propagation) would be a behavioural decision; happy to send a follow-up PR if you'd prefer something different here. Flagging the choice for maintainer preference.Test plan
uv run pytest)test_host_base_exceptioncoversKeyboardInterrupt,SystemExit, and a customBaseExceptionsubclass from host callbacksFatal Python error: Aborted) before the fix, proving the test exercises the bugNotes
LAST_EXCEPTION: Optional[Exception]is now technically narrower than what we may store. Project mypy settings don't flag it; left untouched to keep the diff minimal, but happy to widen it if preferred.