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webassembly: Fix GC tracing of object returned from top-level functions.#19141
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dpgeorge wants to merge 1 commit intomicropython:masterfrom
Open
webassembly: Fix GC tracing of object returned from top-level functions.#19141dpgeorge wants to merge 1 commit intomicropython:masterfrom
dpgeorge wants to merge 1 commit intomicropython:masterfrom
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This commit makes sure that the GC explicitly traces the Python object returned from all top-level functions. Without this, it's possible that a JsProxy that is created as the return object is freed on the Python side (and hence the underlying JavaScript object entry set to `undefined`) just prior to the top-level C function returning. So when it does return and control resumes on the JavaScript side, the JavaScript object corresponding to the JsProxy is gone. For example, the case covered by the test added here has the return value (Python object) from `proxy_c_to_js_call()` reclaimed by the Python GC via `proxy_js_free_obj()` before it can be accessed from the JavaScript side, and so the JavaScript reference in `proxy_js_ref` is `undefined`, Signed-off-by: Damien George <damien@micropython.org>
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Summary
This commit makes sure that the GC explicitly traces the Python object returned from all top-level functions. Without this, it's possible that a JsProxy that is created as the return object is freed on the Python side (and hence the underlying JavaScript object entry set to
undefined) just prior to the top-level C function returning. So when it does return and control resumes on the JavaScript side, the JavaScript object corresponding to the JsProxy is gone.For example, the case covered by the test added here has the return value (Python object) from
proxy_c_to_js_call()reclaimed by the Python GC viaproxy_js_free_obj()before it can be accessed from the JavaScript side, and so the JavaScript reference inproxy_js_refisundefined,Testing
I found this in a web app I'm building, there was some strange behaviour which was traced to the issue fixed here. That web app now behaves correctly with this fix.
Also added a test which triggers the same problem. It'll run in CI.
Trade-offs and Alternatives
There may be other ways to structure the fix, but PR #18021 already fixed a related case, and this PR is extending/improving upon that fix.
Generative AI
I did not use generative AI tools when creating this PR.