Since asm.js came on the scene, and now Web Assembly people have created CPython patches to support building CPython with emscripten. See:

• https://github.com/PeachPy/EmCPython -- Python 2.7
• https://github.com/dgym/cpython-emscripten/tree/master/3.5.2/patches -- Python 3.5.2
• https://github.com/iodide-project/pyodide/tree/master/cpython/patches -- Python 3.7.4

To ease the compiling of CPython with emscripten it would be helpful if patches that achieved these ends for the compiling to Web Assembly with emscripten were built into the upstream source repository itself.

If web assembly were to became a supported compilation target of the upstream CPython repository this would significantly reduce the friction of allowing CPython, and the latest CPython, to become a language readily usable within the browser.

Cheers,
Simon

Do you want to provide a pull request?

you can add

• https://github.com/pmp-p/pydk/tree/master/sources.em/Python-3.8.0b4.patchset -- Python 3.8.x

(wasm not asm.js, clang-10+ required)

demo https://pmp-p.github.io/python-next/test.html

CPython can already run in the browser with very little patching, but major issues are :

• asyncify'ing the whole wasm VM to have pre-emption over cPython's one to prevent blocking I/O slows down things *a lot* (10x)
  => (very?) bad user experience.

• the size of vm + stdlib ~ 30 MiB and wasm compilation time.
  => bad user experience on first load or slow connexion.

• the lack of threading in wasm MinimumViableProduct specification (but this is the browser standard for now), that leads to rewrite bits of stdlib ( like eg asyncio module )
  => adding more maintenance burden on stdlib (!)

i tested them all and my personnal opinion is : I can see no use case that would favour "stock" cPython wasm versus a blazing fast MicroPytho (or pycopy) wasm flavour or supercharged full stack pyodide.

Hi pmp-p and Serhiy,

I'd be more than happy to attempt a pull request, but I imagine a change
such as this needs to be discussed first, trying not to "rush to make a
patch" (https://www.youtube.com/watch?v=voXVTjwnn-U&feature=youtu.be&t=2546).
Also, I doubt I will do a good job of it... but I am more than happy to try.

A note regarding "supercharged full stack pyodide", potentially without
efforts such as upstreaming into CPython and emscripten the relevant
patches, that supercharged full stack may just unfortunately stagnate. See
pyodide/pyodide#635 (comment)

With respect to blocking when running Python as WASM, I have found running
the WebAssembly CPython within a webworker and signalling data back and
forth causes there to be no UI issues. It ends up being quite a neat set
up. Main down side right now however is the set up is currently going
stale, hence me believing reaching out like this is in the best interests
of Python going forward.

Cheers,
Simon

On Tue, 14 Apr 2020 at 18:58, pmp-p <report@bugs.python.org> wrote:

pmp-p <pmpp.pub@gmail.com> added the comment:

you can add
*
https://github.com/pmp-p/pydk/tree/master/sources.em/Python-3.8.0b4.patchset
-- Python 3.8.x

(wasm not asm.js, clang-10+ required)

demo https://pmp-p.github.io/python-next/test.html

CPython can already run in the browser with very little patching, but
major issues are :

• asyncify'ing the whole wasm VM to have pre-emption over cPython's one
  to prevent blocking I/O slows down things *a lot* (10x)
  => (very?) bad user experience.

• the size of vm + stdlib ~ 30 MiB and wasm compilation time.
  => bad user experience on first load or slow connexion.

• the lack of threading in wasm MinimumViableProduct specification (but
  this is the browser standard for now), that leads to rewrite bits of stdlib
  ( like eg asyncio module )
  => adding more maintenance burden on stdlib (!)

i tested them all and my personnal opinion is : I can see no use case that
would favour "stock" cPython wasm versus a blazing fast MicroPytho (or
pycopy) wasm flavour or supercharged full stack pyodide.

----------
nosy: +pmpp

---

Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue40280\>

---

I've been maintaining a Python Emscripten build for the Ren'Py (game engine) web port:
https://github.com/python-emscripten/python
https://renpy.beuc.net/

I recently tackled Python3 with a minimal/embeddable approach and checking the other ports already pointed in the discussion:
https://github.com/python-emscripten/python/tree/trunk/3.8/
(2 patches, and a short pyconfig.h fix-up)

There is also a Cython module to use the Emscripten C API.

Here's a demo at:
https://www.beuc.net/python-emscripten/demo/3/
(hello-world size: 3MB, with a few common modules: 4MB)

I can provide a pull request with a first few core changes.
Should this be done on github?

cross-compilation handling appears to follow an incorrect logic, in particular by querying 'dpkg' or parsing compiler output to detect include paths -- it is the (cross-)compiler's responsibility to provide the system paths, and detecting them manually causes conflicts. I had to patch setup.py. Let me know if I missed something.

Usually cross-compiling is triggered by non-matching build-type/host-type. Here cross-compilation logic is apparently triggered when exporting _PYTHON_HOST_PLATFORM=xxx manually (it's in the Makefile but not exported, and breaks normal build when exported). Is this the way it's meant to be used?

I think the first thing we should do is figure out whether we want to support Emscripten or WASI (or both).

Emscripten uses Javascript polyfills for some syscalls, while WASI makes direct calls the VM it is running in. They both can use WebAssembly for executing the code. This means Emscripten has wider API support, but WASI is lighter weight in many ways.

I think starting with patches to support Emscripten would be best, as it is easier to target, then add support for WASI later.

I think supporting WASI has a lot of value, because it can be run deterministically, which would be great for data science (Imagine a jupyter notebook that runs the same everywhere!)

One issue with WASI, and may be an issue with Emscripten, is threads. In 3.8 (or 3.9?) threadless builds were removed. However, WebAssembly's threading API is not really meant to emulate pthread, and SharedArrayBuffer, the primitive it is built on, is disabled in several browsers due to Spectre concerns.

Would patches to re-add a threadless build mode be accepted?

I have added wasm32/wasm64 architectures with emscripten/wasi operating system as cross-build targets. The values are based on Rust targets:

$ rustc --print target-list | grep wasm
wasm32-unknown-emscripten
wasm32-unknown-unknown
wasm32-wasi
wasm64-unknown-unknown

wasm (WebAssembly) is "native instruction set" for the JavaScript VM while wasi or emscripten provide operating system facilities like memory management and I/O.

Our config.sub is recent enough and has support for wasm32, wasm64, wasi, and emscripten:

$ grep was[mi] config.sub 
                        | wasm32 | wasm64 \
             | midnightbsd* | amdhsa* | unleashed* | emscripten* | wasi* \

LLVM considers was32-wasi an alias for wasm32-unknown-wasi. Verified on the WebAssembly Discover server at https://discord.com/channels/453584038356058112/596492540388179976/898618010221310062.

My last message had a couple of typos; should have been wasm32-wasi and "Discord", not "Discovery".

Do we need to care about our config.guess being updated as well? This is a totally ignorant question based on https://github.com/WebAssembly/wasi-sdk#notes-for-autoconf mentioning config.guess.

To help keep links up-to-date, Pyodide now lives at:

https://github.com/pyodide/pyodide/tree/main/cpython

Our config.sub agrees with LLVM:

$ ./config.sub wasm32-wasi
wasm32-unknown-wasi

The config.sub and config.guess scripts in main are recent enough for wasm. Just to be sure I created #29781 and plan to backport the changeset to 3.10 and 3.9. It's generally safe to update the files to latest version.

New changeset 1052a39 by Christian Heimes in branch 'main':
bpo-40280: Add wasm cross build targets (GH-29771)
1052a39

I have uploaded my config.site override to https://gist.github.com/tiran/5ccffa28723d3e4739db848451bd9efa . It contains overrides based on pyodide and overrides for new features.

I'm also getting this error with emscripten 2.0.13. _sys_shutdown is the syscall for shutdown(2) used by the socket module.

error: undefined symbol: __sys_shutdown (referenced by top-level compiled C/C++ code)
warning: Link with -s LLD_REPORT_UNDEFINED to get more information on undefined symbols
warning: To disable errors for undefined symbols use -s ERROR_ON_UNDEFINED_SYMBOLS=0
warning: ___sys_shutdown may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library
Error: Aborting compilation due to previous errors

emscripten-core/emscripten#13393

New changeset ee1e2c6 by Christian Heimes in branch 'main':
bpo-40280: Use Setup.stdlib static for wasm builds (GH-29784)
ee1e2c6

Thanks a lot for working on this!

_sys_shutdown is the syscall for shutdown(2) used by the socket module.

Yes, the issue with Emscripten is that a number of system calls are either not implemented or implemented but not tested. See a list we are using in https://github.com/pyodide/pyodide/blob/main/cpython/pyconfig.undefs.h (though things might have improved since it was created).

FYI, with Emscripten, the list of CPython unit tests that are currently skipped (as of Python 3.9.5) is in https://github.com/pyodide/pyodide/blob/main/src/tests/python_tests.txt some of those are due to browser VM limitations (e.g. virtual filestem by Emscripten that's not fully POSIX compliant, no processes, no sockets, async only via the browser event loop etc), others because we are not yet using threading since not all browsers support it, and some failures probably need more investigation.

Also opened pyodide/pyodide#2000 . Let us know if there is anything we can do help with this effort.

New changeset d224e76 by Christian Heimes in branch 'main':
bpo-40280: clean and ignore .wasm files (GH-29794)
d224e76

Thanks Roman, I replied on the pyodide issue tracker.

New changeset c02e860 by Christian Heimes in branch 'main':
bpo-40280: Misc fixes for wasm32-emscripten (GH-30722)
c02e860

New changeset d5fd438 by Christian Heimes in branch 'main':
bpo-40280: Get help() working and more (GH-30858)
d5fd438

New changeset 8464fbc by Christian Heimes in branch 'main':
bpo-40280: Skip subprocess-based tests on wasm32-emscripten (GH-30615)
8464fbc

New changeset 606e496 by Christian Heimes in branch 'main':
bpo-40280: Use presence of msvcrt module to detect Windows (GH-30930)
606e496

New changeset 96b344c by Christian Heimes in branch 'main':
bpo-40280: Address more test failures on Emscripten (GH-31050)
96b344c

New changeset 5081e78 by Christian Heimes in branch 'main':
bpo-40280: Block more non-working syscalls in Emscripten (GH-31757)
5081e78

I have fixed a signal-related issue in #75949 / bpo-23325

New changeset 95ba723 by Christian Heimes in branch 'main':
bpo-40280: Skip dysfunctional pipe tests on Emscripten (GH-31770)
95ba723

New changeset de554d6 by Christian Heimes in branch 'main':
bpo-40280: Skip more tests/features that don't apply to Emscripten (GH-31791)
de554d6

New changeset ecfff63 by Christian Heimes in branch 'main':
bpo-40280: Disable AF_UNIX, AF_PACKET, SO_REUSE* on Emscripten (bpo-31829)
ecfff63

New changeset f00ced8 by Christian Heimes in branch 'main':
bpo-40280: select: Use NULL for empty fdset (GH-31865)
f00ced8

New changeset b43b9b4 by Christian Heimes in branch 'main':
bpo-40280: Skip wakeup_fd pipe tests on Emscripten (GH-31909)
b43b9b4

New changeset a4674f0 by Christian Heimes in branch 'main':
bpo-40280: Detect presence of time.tzset and thread_time clock (GH-31898)
a4674f0

New changeset ef1327e by Christian Heimes in branch 'main':
bpo-40280: Skip more tests on Emscripten (GH-31947)
ef1327e

New changeset deeaac4 by Christian Heimes in branch 'main':
bpo-40280: Skip socket, fork, subprocess tests on Emscripten (GH-31986)
deeaac4

New changeset 8a0a9e5 by Christian Heimes in branch 'main':
bpo-40280: Add wasm32-emscripten and wasm32-wasi SOABI (GH-32095)
8a0a9e5

New changeset 17245c8 by Christian Heimes in branch 'main':
bpo-40280: Add debug Emscripten flavors (GH-32233)
17245c8

New changeset 082d349 by Christian Heimes in branch 'main':
bpo-40280: Emscripten fork_exec now fails early (GH-32224)
082d349

New changeset 59be9cd by Christian Heimes in branch 'main':
bpo-40280: Detect if WASM platform supports threading (GH-32243)
59be9cd

New changeset 4ed8a9a by Christian Heimes in branch 'main':
bpo-40280: Enable most file-at() and nanosleep APIs again (GH-32238)
4ed8a9a

pmp-p wrote on #76434:
when using --enable-wasm-dynamic-linking i think --enable-shared should set -fPIC and -s SIDE_MODULE=1 for libpython.so

I have not tried --enable-shared with Emscripten yet. So far I have assumed that the result is a fat Python binary that can load additional shared libraries.

New changeset c9844cb by Christian Heimes in branch 'main':
bpo-40280: Add --enable-wasm-dynamic-linking (GH-32253)
c9844cb

New changeset 96e0983 by Christian Heimes in branch 'main':
bpo-40280: Add limited Emscripten REPL (GH-32284)
96e0983

New changeset 765f6de by Christian Heimes in branch 'main':
bpo-40280: WASM defaults to no dynamic linking (GH-32360)
765f6de

New changeset 2b16a08 by Christian Heimes in branch 'main':
bpo-40280: Detect missing threading on WASM platforms (GH-32352)
2b16a08

Christian: IDLE and test_idle require threading. I once tested for threading in test_idle (not sure about IDLE itself) but removed the test when threading was made supposedly non-optional. Test_idle still tests for tkinter and idlelib. For PR 32352. did you miss IDLE's requirement or is tkinter and hence IDLE and its test not included on WASM/emscripten? Will it never be?

In #76533 I changed only tests that I was able to execute under wasm32-emscripten. As far as I know TK does not build for WASM. Somebody would have to port TCL/TK to WebGL or SDL2 to make it work in the browser.