# Python WebAssembly (WASM) build

**WARNING: WASM support is highly experimental! Lots of features are not working yet.**

This directory contains configuration and helpers to facilitate cross
compilation of CPython to WebAssembly (WASM). For now we support
*wasm32-emscripten* builds for modern browser and for *Node.js*. It's not
possible to build for *wasm32-wasi* out-of-the-box yet.

## wasm32-emscripten build

Cross compiling to wasm32-emscripten platform needs the [Emscripten](https://emscripten.org/)
tool chain and a build Python interpreter.
All commands below are relative to a repository checkout.

### Compile a build Python interpreter

```shell
mkdir -p builddir/build
pushd builddir/build
../../configure -C
make -j$(nproc)
popd
```

### Fetch and build additional emscripten ports

```shell
embuilder build zlib bzip2
```

### Cross compile to wasm32-emscripten for browser

```shell
mkdir -p builddir/emscripten-browser
pushd builddir/emscripten-browser

CONFIG_SITE=../../Tools/wasm/config.site-wasm32-emscripten \
  emconfigure ../../configure -C \
    --host=wasm32-unknown-emscripten \
    --build=$(../../config.guess) \
    --with-emscripten-target=browser \
    --with-build-python=$(pwd)/../build/python

emmake make -j$(nproc)
popd
```

Serve `python.html` with a local webserver and open the file in a browser.

```shell
emrun builddir/emscripten-browser/python.html
```

or

```shell
python3 -m http.server
```

### Cross compile to wasm32-emscripten for node

```
mkdir -p builddir/emscripten-node
pushd builddir/emscripten-node

CONFIG_SITE=../../Tools/wasm/config.site-wasm32-emscripten \
  emconfigure ../../configure -C \
    --host=wasm32-unknown-emscripten \
    --build=$(../../config.guess) \
    --with-emscripten-target=node \
    --with-build-python=$(pwd)/../build/python

emmake make -j$(nproc)
popd
```

```
node --experimental-wasm-threads --experimental-wasm-bulk-memory builddir/emscripten-node/python.js
```

## wasm32-emscripten limitations and issues

- Most stdlib modules with a dependency on external libraries are missing:
  ``ctypes``, ``readline``, ``sqlite3``, ``ssl``, and more.
- Shared extension modules are not implemented yet. All extension modules
  are statically linked into the main binary.
- Processes are not supported. System calls like fork, popen, and subprocess
  fail with ``ENOSYS`` or ``ENOSUP``.
- Blocking sockets are not available and non-blocking sockets don't work
  correctly, e.g. ``socket.accept`` crashes the runtime. ``gethostbyname``
  does not resolve to a real IP address. IPv6 is not available.
- The ``select`` module is limited. ``select.select()`` crashes the runtime
  due to lack of exectfd support.
- The ``*at`` variants of functions (e.g. ``openat``) are not available.
  The ``dir_fd`` argument of *os* module functions can't be used.
- Signal support is limited. ``signal.alarm``, ``itimer``, ``sigaction``
  are not available or do not work correctly. ``SIGTERM`` exits the runtime.
- Most user, group, and permission related function and modules are not
  supported or don't work as expected, e.g.``pwd`` module, ``grp`` module,
  ``os.setgroups``, ``os.chown``, and so on.
- Offset and iovec I/O functions (e.g. ``os.pread``, ``os.preadv``) are not
  available.
- ``os.mknod`` and ``os.mkfifo``
  [don't work](https://github.com/emscripten-core/emscripten/issues/16158)
  and are disabled.
- Large file support crashes the runtime and is disabled.
- ``mmap`` module is unstable. flush (``msync``) can crash the runtime.
- Resource-related functions like ``os.nice`` and most functions of the
  ``resource`` module are not available.
- Some time and datetime features are broken. ``strftime`` and ``strptime``
  have known bugs, e.g.
  [%% quoting](https://github.com/emscripten-core/emscripten/issues/16155),
  [%U off-by-one](https://github.com/emscripten-core/emscripten/issues/16156).
  Extended glibc formatting features are not available.
- ``locales`` module is affected by musl libc issues,
  [bpo-46390](https://bugs.python.org/issue46390).
- ``uuid`` module is affected by
  [memory leak](https://github.com/emscripten-core/emscripten/issues/16081)
  and crasher in Emscripten's ``freeaddrinfo``,
- Recursive ``glob`` leaks file descriptors.
- Python's object allocator ``obmalloc`` is disabled by default.
- ``ensurepip`` is not available.

### wasm32-emscripten in browsers

- The bundled stdlib is limited. Network-related modules,
  distutils, multiprocessing, dbm, tests and similar modules
  are not shipped. All other modules are bundled as pre-compiled
  ``pyc`` files.
- Threading is not supported.

### wasm32-emscripten in node

Node builds use ``NODERAWFS``, ``USE_PTHREADS`` and ``PROXY_TO_PTHREAD``
linker options.

- Node RawFS allows direct access to the host file system.
- pthread support requires WASM threads and SharedArrayBuffer (bulk memory).
  The runtime keeps a pool of web workers around. Each web worker uses
  several file descriptors (eventfd, epoll, pipe).