webassembly: reuse JsProxy objects where possible and fix bug with lost references#18021
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dpgeorge merged 4 commits intoSep 26, 2025
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@WebReflection FYI |
This test is not a PyProxy test, rather it's a JsProxy test. Signed-off-by: Damien George <damien@micropython.org>
Doing GC calls in the entry path (when JavaScript calls into MicroPython at the top/outer level) can lead to freeing of objects which are still in use. This is because the (JavaScript) objects are referenced in the input arguments to the C functions and they are not yet converted to full proxy objects and not yet tracked properly by the live-object tracker. Signed-off-by: Damien George <damien@micropython.org>
Commit ffa98cb improved equality for `JsProxy` objects so that, eg, `js.Object == js.Object` is true. As mentioned in micropython#17758, a further optimisation is to make identity work in that case, eg `js.Object is js.Object` should be true (on the Python side). This commit implements that, by keeping track of all `JsProxy` Python objects and reusing them where possible: where the underlying JS ref is equal, ie they point to the same JS object. That reduces memory churn and gives better identity behaviour of JS objects proxied over to Python. As part of this, a bug is fixed where JS objects can be freed while there's still a `JsProxy` referring to that JS object. A test is added for that exact scenario, and the test now passes. Signed-off-by: Damien George <damien@micropython.org>
With the parent commit implementing proper identities, this equality check option is no longer needed. Signed-off-by: Damien George <damien@micropython.org>
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Summary
PR #17758 improved equality for
JsProxyobjects so that, eg,js.Object == js.Objectis true.As mentioned in that PR, a further optimisation is to make identity work, eg
js.Object is js.Objectshould be true (on the Python side).This PR implements that, by keeping track of all
JsProxyPython objects and reusing them where possible (where the underlying JS ref is equal, ie they point to the same JS object). That reduces memory churn and gives better identity behaviour of JS objects proxied over to Python.As part of this, a bug is fixed where JS objects can be freed while there's still a
JsProxyreferring to that JS object. A test is added for that exact scenario, and the test now passes.Testing
Tested locally using the test suite. A new test is added to cover the bug (see above) and existing tests are updated to test object identity.
Trade-offs and Alternatives
This uses memory to keep track of all
JsProxyobjects, but that's probably counteracted by the fact that there are now lessJsProxyobjects lying around, due to reuse.Performance should be unchanged because the code was already checking for existing JS objects on the JS side. That information is now passed across to the Python side without any extra lookup overhead.