# Python WebAssembly (WASM) build

**WARNING: WASM support is work-in-progress! Lots of features are not working yet.**

This directory contains configuration and helpers to facilitate cross
compilation of CPython to WebAssembly (WASM). Python supports Emscripten
(*wasm32-emscripten*) and WASI (*wasm32-wasi*) targets. Emscripten builds
run in modern browsers and JavaScript runtimes like *Node.js*. WASI builds
use WASM runtimes such as *wasmtime*.

Users and developers are encouraged to use the script
`Tools/wasm/wasm_build.py`. The tool automates the build process and provides
assistance with installation of SDKs.

## wasm32-emscripten build

For now the build system has two target flavors. The ``Emscripten/browser``
target (``--with-emscripten-target=browser``) is optimized for browsers.
It comes with a reduced and preloaded stdlib without tests and threading
support. The ``Emscripten/node`` target has threading enabled and can
access the file system directly.

Cross compiling to the wasm32-emscripten platform needs the
[Emscripten](https://emscripten.org/) SDK and a build Python interpreter.
Emscripten 3.1.19 or newer are recommended. All commands below are relative
to a repository checkout.

Christian Heimes maintains a container image with Emscripten SDK, Python
build dependencies, WASI-SDK, wasmtime, and several additional tools.

From within your local CPython repo clone, run one of the following commands:

```
# Fedora, RHEL, CentOS
podman run --rm -ti -v $(pwd):/python-wasm/cpython:Z -w /python-wasm/cpython quay.io/tiran/cpythonbuild:emsdk3

# other
docker run --rm -ti -v $(pwd):/python-wasm/cpython -w /python-wasm/cpython quay.io/tiran/cpythonbuild:emsdk3
```

### Compile a build Python interpreter

From within the container, run the following command:

```shell
./Tools/wasm/wasm_build.py build
```

The command is roughly equivalent to:

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

### Cross-compile to wasm32-emscripten for browser

```shell
./Tools/wasm/wasm_build.py emscripten-browser
```

The command is roughly equivalent to:

```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.
Python comes with a minimal web server script that sets necessary HTTP
headers like COOP, COEP, and mimetypes. Run the script outside the container
and from the root of the CPython checkout.

```shell
./Tools/wasm/wasm_webserver.py
```

and open http://localhost:8000/builddir/emscripten-browser/python.html . This
directory structure enables the *C/C++ DevTools Support (DWARF)* to load C
and header files with debug builds.


### Cross compile to wasm32-emscripten for node

```shell
./Tools/wasm/wasm_build.py emscripten-browser-dl
```

The command is roughly equivalent to:

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

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

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

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

(``--experimental-wasm-bigint`` is not needed with recent NodeJS versions)

# wasm32-emscripten limitations and issues

Emscripten before 3.1.8 has known bugs that can cause memory corruption and
resource leaks. 3.1.8 contains several fixes for bugs in date and time
functions.

## Network stack

- Python's socket module does not work with Emscripten's emulated POSIX
  sockets yet. Network modules like ``asyncio``, ``urllib``, ``selectors``,
  etc. are not available.
- Only ``AF_INET`` and ``AF_INET6`` with ``SOCK_STREAM`` (TCP) or
  ``SOCK_DGRAM`` (UDP) are available. ``AF_UNIX`` is not supported.
- ``socketpair`` does not work.
- 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.

## processes, signals

- Processes are not supported. System calls like fork, popen, and subprocess
  fail with ``ENOSYS`` or ``ENOSUP``.
- Signal support is limited. ``signal.alarm``, ``itimer``, ``sigaction``
  are not available or do not work correctly. ``SIGTERM`` exits the runtime.
- Keyboard interrupt (CTRL+C) handling is not implemented yet.
- Resource-related functions like ``os.nice`` and most functions of the
  ``resource`` module are not available.

## threading

- Threading is disabled by default. The ``configure`` option
  ``--enable-wasm-pthreads`` adds compiler flag ``-pthread`` and
  linker flags ``-sUSE_PTHREADS -sPROXY_TO_PTHREAD``.
- pthread support requires WASM threads and SharedArrayBuffer (bulk memory).
  The Node.JS runtime keeps a pool of web workers around. Each web worker
  uses several file descriptors (eventfd, epoll, pipe).
- It's not advised to enable threading when building for browsers or with
  dynamic linking support; there are performance and stability issues.

## file system

- 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. ``lchown`` and `lchmod`` are
  not available.
- ``umask`` is a no-op.
- hard links (``os.link``) are not supported.
- 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.

## Misc

- Heap memory and stack size are limited. Recursion or extensive memory
  consumption can crash Python.
- Most stdlib modules with a dependency on external libraries are missing,
  e.g. ``ctypes``, ``readline``, ``ssl``, and more.
- Shared extension modules are not implemented yet. All extension modules
  are statically linked into the main binary. The experimental configure
  option ``--enable-wasm-dynamic-linking`` enables dynamic extensions
  supports. It's currently known to crash in combination with threading.
- glibc extensions for date and time formatting are not available.
- ``locales`` module is affected by musl libc issues,
  [bpo-46390](https://bugs.python.org/issue46390).
- Python's object allocator ``obmalloc`` is disabled by default.
- ``ensurepip`` is not available.
- Some ``ctypes`` features like ``c_longlong`` and ``c_longdouble`` may need
   NodeJS option ``--experimental-wasm-bigint``.

## wasm32-emscripten in browsers

- The interactive shell does not handle copy 'n paste and unicode support
  well.
- The bundled stdlib is limited. Network-related modules,
  multiprocessing, dbm, tests and similar modules
  are not shipped. All other modules are bundled as pre-compiled
  ``pyc`` files.
- In-memory file system (MEMFS) is not persistent and limited.
- Test modules are disabled by default. Use ``--enable-test-modules`` build
  test modules like ``_testcapi``.

## wasm32-emscripten in node

Node builds use ``NODERAWFS``.

- Node RawFS allows direct access to the host file system without need to
  perform ``FS.mount()`` call.

## wasm64-emscripten

- wasm64 requires recent NodeJS and ``--experimental-wasm-memory64``.
- ``EM_JS`` functions must return ``BigInt()``.
- ``Py_BuildValue()`` format strings must match size of types. Confusing 32
  and 64 bits types leads to memory corruption, see
  [gh-95876](https://github.com/python/cpython/issues/95876) and
  [gh-95878](https://github.com/python/cpython/issues/95878).

# Hosting Python WASM builds

The simple REPL terminal uses SharedArrayBuffer. For security reasons
browsers only provide the feature in secure environents with cross-origin
isolation. The webserver must send cross-origin headers and correct MIME types
for the JavaScript and WASM files. Otherwise the terminal will fail to load
with an error message like ``Browsers disable shared array buffer``.

## Apache HTTP .htaccess

Place a ``.htaccess`` file in the same directory as ``python.wasm``.

```
# .htaccess
Header set Cross-Origin-Opener-Policy same-origin
Header set Cross-Origin-Embedder-Policy require-corp

AddType application/javascript js
AddType application/wasm wasm

<IfModule mod_deflate.c>
    AddOutputFilterByType DEFLATE text/html application/javascript application/wasm
</IfModule>
```

# WASI (wasm32-wasi)

WASI builds require [WASI SDK](https://github.com/WebAssembly/wasi-sdk) 16.0+.

## Cross-compile to wasm32-wasi

The script ``wasi-env`` sets necessary compiler and linker flags as well as
``pkg-config`` overrides. The script assumes that WASI-SDK is installed in
``/opt/wasi-sdk`` or ``$WASI_SDK_PATH``.

```shell
./Tools/wasm/wasm_build.py wasi
```

The command is roughly equivalent to:

```shell
mkdir -p builddir/wasi
pushd builddir/wasi

CONFIG_SITE=../../Tools/wasm/config.site-wasm32-wasi \
  ../../Tools/wasm/wasi-env ../../configure -C \
    --host=wasm32-unknown-wasi \
    --build=$(../../config.guess) \
    --with-build-python=$(pwd)/../build/python

make -j$(nproc)
popd
```

## WASI limitations and issues (WASI SDK 15.0)

A lot of Emscripten limitations also apply to WASI. Noticeable restrictions
are:

- Call stack size is limited. Default recursion limit and parser stack size
  are smaller than in regular Python builds.
- ``socket(2)`` cannot create new socket file descriptors. WASI programs can
  call read/write/accept on a file descriptor that is passed into the process.
- ``socket.gethostname()`` and host name resolution APIs like
  ``socket.gethostbyname()`` are not implemented and always fail.
- ``open(2)`` checks flags more strictly. Caller must pass either
  ``O_RDONLY``, ``O_RDWR``, or ``O_WDONLY`` to ``os.open``. Directory file
  descriptors must be created with flags ``O_RDONLY | O_DIRECTORY``.
- ``chmod(2)`` is not available. It's not possible to modify file permissions,
  yet. A future version of WASI may provide a limited ``set_permissions`` API.
- User/group related features like ``os.chown()``, ``os.getuid``, etc. are
  stubs or fail with ``ENOTSUP``.
- File locking (``fcntl``) is not available.
- ``os.pipe()``, ``os.mkfifo()``, and ``os.mknod()`` are not supported.
- ``process_time`` does not work as expected because it's implemented using
  wall clock.
- ``os.umask()`` is a stub.
- ``sys.executable`` is empty.
- ``/dev/null`` / ``os.devnull`` may not be available.
- ``os.utime*()`` is buggy in WASM SDK 15.0, see
  [utimensat() with timespec=NULL sets wrong time](https://github.com/bytecodealliance/wasmtime/issues/4184)
- ``os.symlink()`` fails with ``PermissionError`` when attempting to create a
  symlink with an absolute path with wasmtime 0.36.0. The wasmtime runtime
  uses ``openat2(2)`` syscall with flag ``RESOLVE_BENEATH`` to open files.
  The flag causes the syscall to reject symlinks with absolute paths.
- ``os.curdir`` (aka ``.``) seems to behave differently, which breaks some
  ``importlib`` tests that add ``.`` to ``sys.path`` and indirectly
  ``sys.path_importer_cache``.
- WASI runtime environments may not provide a dedicated temp directory.


# Detect WebAssembly builds

## Python code

```python
import os, sys

if sys.platform == "emscripten":
    # Python on Emscripten
if sys.platform == "wasi":
    # Python on WASI

if os.name == "posix":
    # WASM platforms identify as POSIX-like.
    # Windows does not provide os.uname().
    machine = os.uname().machine
    if machine.startswith("wasm"):
        # WebAssembly (wasm32, wasm64 in the future)
```

```python
>>> import os, sys
>>> os.uname()
posix.uname_result(
    sysname='Emscripten',
    nodename='emscripten',
    release='3.1.19',
    version='#1',
    machine='wasm32'
)
>>> os.name
'posix'
>>> sys.platform
'emscripten'
>>> sys._emscripten_info
sys._emscripten_info(
    emscripten_version=(3, 1, 10),
    runtime='Mozilla/5.0 (X11; Linux x86_64; rv:104.0) Gecko/20100101 Firefox/104.0',
    pthreads=False,
    shared_memory=False
)
```

```python
>>> sys._emscripten_info
sys._emscripten_info(
    emscripten_version=(3, 1, 19),
    runtime='Node.js v14.18.2',
    pthreads=True,
    shared_memory=True
)
```

```python
>>> import os, sys
>>> os.uname()
posix.uname_result(
    sysname='wasi',
    nodename='(none)',
    release='0.0.0',
    version='0.0.0',
    machine='wasm32'
)
>>> os.name
'posix'
>>> sys.platform
'wasi'
```

## C code

Emscripten SDK and WASI SDK define several built-in macros. You can dump a
full list of built-ins with ``emcc -dM -E - < /dev/null`` and
``/path/to/wasi-sdk/bin/clang -dM -E - < /dev/null``.

```C
#ifdef __EMSCRIPTEN__
    // Python on Emscripten
#endif
```

* WebAssembly ``__wasm__`` (also ``__wasm``)
* wasm32 ``__wasm32__`` (also ``__wasm32``)
* wasm64 ``__wasm64__``
* Emscripten ``__EMSCRIPTEN__`` (also ``EMSCRIPTEN``)
* Emscripten version ``__EMSCRIPTEN_major__``, ``__EMSCRIPTEN_minor__``, ``__EMSCRIPTEN_tiny__``
* WASI ``__wasi__``

Feature detection flags:

* ``__EMSCRIPTEN_PTHREADS__``
* ``__EMSCRIPTEN_SHARED_MEMORY__``
* ``__wasm_simd128__``
* ``__wasm_sign_ext__``
* ``__wasm_bulk_memory__``
* ``__wasm_atomics__``
* ``__wasm_mutable_globals__``

## Install SDKs and dependencies manually

In some cases (e.g. build bots) you may prefer to install build dependencies
directly on the system instead of using the container image. Total disk size
of SDKs and cached libraries is about 1.6 GB.

### Install OS dependencies

```shell
# Debian/Ubuntu
apt update
apt install -y git make xz-utils bzip2 curl python3-minimal ccache
```

```shell
# Fedora
dnf install -y git make xz bzip2 which ccache
```

### Install [Emscripten SDK](https://emscripten.org/docs/getting_started/downloads.html)

**NOTE**: Follow the on-screen instructions how to add the SDK to ``PATH``.

```shell
git clone https://github.com/emscripten-core/emsdk.git /opt/emsdk
/opt/emsdk/emsdk install latest
/opt/emsdk/emsdk activate latest
```

### Optionally: enable ccache for EMSDK

The ``EM_COMPILER_WRAPPER`` must be set after the EMSDK environment is
sourced. Otherwise the source script removes the environment variable.

```
. /opt/emsdk/emsdk_env.sh
EM_COMPILER_WRAPPER=ccache
```

### Optionally: pre-build and cache static libraries

Emscripten SDK provides static builds of core libraries without PIC
(position-independent code). Python builds with ``dlopen`` support require
PIC. To populate the build cache, run:

```shell
. /opt/emsdk/emsdk_env.sh
embuilder build zlib bzip2 MINIMAL_PIC
embuilder build --pic zlib bzip2 MINIMAL_PIC
```

### Install [WASI-SDK](https://github.com/WebAssembly/wasi-sdk)

**NOTE**: WASI-SDK's clang may show a warning on Fedora:
``/lib64/libtinfo.so.6: no version information available``,
[RHBZ#1875587](https://bugzilla.redhat.com/show_bug.cgi?id=1875587). The
warning can be ignored.

```shell
export WASI_VERSION=16
export WASI_VERSION_FULL=${WASI_VERSION}.0
curl -sSf -L -O https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-${WASI_VERSION}/wasi-sdk-${WASI_VERSION_FULL}-linux.tar.gz
mkdir -p /opt/wasi-sdk
tar --strip-components=1 -C /opt/wasi-sdk -xvf wasi-sdk-${WASI_VERSION_FULL}-linux.tar.gz
rm -f wasi-sdk-${WASI_VERSION_FULL}-linux.tar.gz
```

### Install [wasmtime](https://github.com/bytecodealliance/wasmtime) WASI runtime

wasmtime 0.38 or newer is required.

```shell
curl -sSf -L -o ~/install-wasmtime.sh https://wasmtime.dev/install.sh
chmod +x ~/install-wasmtime.sh
~/install-wasmtime.sh --version v0.38.0
ln -srf -t /usr/local/bin/ ~/.wasmtime/bin/wasmtime
```


### WASI debugging

* ``wasmtime run -g`` generates debugging symbols for gdb and lldb. The
  feature is currently broken, see
  https://github.com/bytecodealliance/wasmtime/issues/4669 .
* The environment variable ``RUST_LOG=wasi_common`` enables debug and
  trace logging.