gh-145037: Fix Emscripten trampoline with emcc >= 4.0.19 by hoodmane · Pull Request #145038 · python/cpython · GitHub

This undoes a change made as a part of #137470. We add an emscripten_trampoline field in pycore_runtime_structs.h and initialize it from JS initialization code with the wasm-gc based trampoline if possible. Otherwise we fall back to the JS trampoline.

cc @freakboy3742 @ryanking13

Issue: Emscripten: Trampoline doesn't work with emcc 4.0.19 due to table/memory visibility #145037

This undoes a change made as a part of PR 137470. We add an `emscripten_trampoline` field in `pycore_runtime_structs.h` and initialize it from JS initialization code with the wasm-gc based trampoline if possible. Otherwise we fall back to the JS trampoline.

!buildbot emscripten

🤖 New build scheduled with the buildbot fleet by @freakboy3742 for commit 526e102 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F145038%2Fmerge

The command will test the builders whose names match following regular expression: emscripten

The builders matched are:

WASM Emscripten PR

@hoodmane Not sure what is going on here, but this has broken the Emscripten buildbot.

Looks like there are some mistakes to fix here.

!buildbot emscripten

I think it should be better now.

🤖 New build scheduled with the buildbot fleet by @hoodmane for commit f3d03d3 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F145038%2Fmerge

The command will test the builders whose names match following regular expression: emscripten

The builders matched are:

WASM Emscripten PR

Okay now it is better.

I think this makes sense; one detail flagged inline that I want to confirm before approving.

This is the only part that gives me pause - I'm not sure if the specific offsets inside this structure are something that have consequences. There's no explicit CODEOWNERS reference for this file; @ericsnowcurrently has a few close-by entries, and @markshannon has most of the git blames

For the two of you - are there any offset/ordering concerns in pyruntimestate (even if just from a code organization perspective) that we should be concern about here?

Or if CODEOWNERS/git blame has mislead me... any idea who I should be asking?

Are you suggesting I should add this at the end?

Also, #137470 changed this header without problem, so if there is someone relying on offsets in this struct they started doing so after then.

I'm not sure if the end is a better place - I was mostly checking with Eric/Mark to see if there were any conventions I wasn't (but should be) aware of. But given the history from #137470, maybe I'm overthinking it.

Thanks for the review @freakboy3742 !

Can we backport to 3.14.x?

Thanks @hoodmane for the PR, and @freakboy3742 for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

…nGH-145038) This undoes a change made as a part of PR 137470, for compatibility with EMSDK 4.0.19. It adds `emscripten_trampoline` field in `pycore_runtime_structs.h` and initializes it from JS initialization code with the wasm-gc based trampoline if possible. Otherwise we fall back to the JS trampoline. (cherry picked from commit 43fdb70) Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>

GH-145283 is a backport of this pull request to the 3.14 branch.

I wasn't sure if a backport was needed, since 3.14 is theoretically locked at 4.0.9; but I guess it doesn't hurt to allow for forwards compatibility.

…45038) (#145283) This undoes a change made as a part of PR 137470, for compatibility with EMSDK 4.0.19. It adds `emscripten_trampoline` field in `pycore_runtime_structs.h` and initializes it from JS initialization code with the wasm-gc based trampoline if possible. Otherwise we fall back to the JS trampoline. (cherry picked from commit 43fdb70) Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>

…n#145038) This undoes a change made as a part of PR 137470, for compatibility with EMSDK 4.0.19. It adds `emscripten_trampoline` field in `pycore_runtime_structs.h` and initializes it from JS initialization code with the wasm-gc based trampoline if possible. Otherwise we fall back to the JS trampoline.

hoodmane requested review from AA-Turner, corona10, emmatyping and erlend-aasland as code owners February 20, 2026 12:29

bedevere-app Bot added the awaiting review label Feb 20, 2026

bedevere-app Bot mentioned this pull request Feb 20, 2026

Emscripten: Trampoline doesn't work with emcc 4.0.19 due to table/memory visibility #145037

Closed

hoodmane added the skip news label Feb 20, 2026

hoodmane force-pushed the emscripten-trampoline-pointer branch from 0725f55 to 200d8fe Compare February 20, 2026 12:31

hoodmane force-pushed the emscripten-trampoline-pointer branch from 200d8fe to 526e102 Compare February 20, 2026 15:39

hoodmane added OS-emscripten interpreter-core (Objects, Python, Grammar, and Parser dirs) labels Feb 20, 2026

Fix

f3d03d3

freakboy3742 reviewed Feb 25, 2026

View reviewed changes

hoodmane mentioned this pull request Feb 25, 2026

Fix wasm-gc call trampoline when RELOCATABLE isn't passed pyodide/pyodide#6115

Merged

freakboy3742 merged commit 43fdb70 into python:main Feb 25, 2026
49 checks passed

bedevere-app Bot removed the awaiting review label Feb 25, 2026

hoodmane deleted the emscripten-trampoline-pointer branch February 26, 2026 07:35

freakboy3742 added the needs backport to 3.14 bugs and security fixes label Feb 26, 2026

bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Feb 26, 2026

+                                                     PyObject* self,
+                                                     PyObject* args,
+                                                     PyObject* kw);
+              #endif

gh-145037: Fix Emscripten trampoline with emcc >= 4.0.19 by hoodmane · Pull Request #145038 · python/cpython · GitHub

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gh-145037: Fix Emscripten trampoline with emcc >= 4.0.19 - #145038

gh-145037: Fix Emscripten trampoline with emcc >= 4.0.19#145038freakboy3742 merged 2 commits intopython:mainpython/cpython:mainfrom hoodmane:emscripten-trampoline-pointerhoodmane/cpython:emscripten-trampoline-pointerCopy head branch name to clipboard

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gh-145037: Fix Emscripten trampoline with emcc >= 4.0.19#145038
freakboy3742 merged 2 commits into
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hoodmane:emscripten-trampoline-pointer

hoodmane commented Feb 20, 2026 •
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