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Unsupported externref types prevent older browsers from instantiating a WASM module even when JSPI is not used. Guard the JSPI syscall wrappers with __wasm_reference_types__ and make the interpreter's JSPI startup hook a no-op when reference types are disabled. Keep Emscripten's synchronous read and poll implementations and the unrelated syscall overrides available. Signed-off-by: Garrett Brown <themagnificentmrb@gmail.com>
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Summary
Fix Emscripten builds when WebAssembly reference types are disabled with
-mno-reference-types. Fixes #161782.CPython's optional JSPI syscall wrappers unconditionally use
externreftypes, preventing compilation and WebAssembly instantiation on older browsers such as Chromium 85, even when JSPI is unused.This change:
__wasm_reference_types__.The change is limited to
Python/emscripten_syscalls.cand includes a NEWS fragment.Motivation
Discovered while bringing up Kodi's WebAssembly build on a 2022 Samsung Q60B TV using Emscripten 7.0.0.
Chromium 85 does not support WebAssembly reference types. Runtime feature detection cannot prevent compilation or instantiation failures caused by unsupported
externreftypes in the module.This allows CPython to support targets without reference types while retaining JSPI functionality on newer runtimes.
Related downstream fix: xbmc/xbmc#30431
Testing
Validated against CPython
mainusing Emscripten 7.0.0 and Node.js 26:-mno-reference-typesand-mreference-typescompile with warnings treated as errors.externrefinstructions and pass WebAssembly validation with reference types disabled.git diff --checkpasses.A full CPython build and test suite were not run. The upstream change has not yet been tested directly on Samsung Chromium 85.
AI disclosure
Model: GPT-6 Astra
Thinking effort: Extra High