bpo-40280: Address more test failures on Emscripten (GH-31050)

Co-authored-by: Brett Cannon <brett@python.org>

2 files changed, 109 insertions, 23 deletions

diff --git a/Tools/wasm/README.md b/Tools/wasm/README.md
index f59b876..1cdaa4e 100644
--- a/Tools/wasm/README.md
+++ b/Tools/wasm/README.md
@@ -1,7 +1,11 @@
 # 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).
+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
 
@@ -22,16 +26,14 @@ popd
 ### Fetch and build additional emscripten ports
 
 ```shell
-embuilder build zlib
+embuilder build zlib bzip2
 ```
 
-### Cross compile to wasm32-emscripten
-
-For browser:
+### Cross compile to wasm32-emscripten for browser
 
 ```shell
-mkdir -p builddir/emscripten
-pushd builddir/emscripten
+mkdir -p builddir/emscripten-browser
+pushd builddir/emscripten-browser
 
 CONFIG_SITE=../../Tools/wasm/config.site-wasm32-emscripten \
   emconfigure ../../configure -C \
@@ -41,11 +43,27 @@ CONFIG_SITE=../../Tools/wasm/config.site-wasm32-emscripten \
     --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
 ```
 
-For node:
+### 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 \
@@ -54,18 +72,70 @@ CONFIG_SITE=../../Tools/wasm/config.site-wasm32-emscripten \
     --with-build-python=$(pwd)/../build/python
 
 emmake make -j$(nproc)
+popd
 ```
 
-### Test in browser
-
-Serve `python.html` with a local webserver and open the file in a browser.
-
-```shell
-emrun python.html
 ```
-
-or
-
-```shell
-python3 -m http.server
+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).
diff --git a/Tools/wasm/config.site-wasm32-emscripten b/Tools/wasm/config.site-wasm32-emscripten
index 413506b..98991b4 100644
--- a/Tools/wasm/config.site-wasm32-emscripten
+++ b/Tools/wasm/config.site-wasm32-emscripten
@@ -27,9 +27,6 @@ ac_cv_func_prlimit=no
 # unsupported syscall, https://github.com/emscripten-core/emscripten/issues/13393
 ac_cv_func_shutdown=no
 
-# breaks build, see https://github.com/ethanhs/python-wasm/issues/16
-ac_cv_lib_bz2_BZ2_bzCompress=no
-
 # clock_nanosleep() causes time.sleep() to sleep forever.
 # nanosleep() works correctly
 ac_cv_func_clock_nanosleep=no
@@ -66,6 +63,11 @@ ac_cv_func_pwritev=no
 ac_cv_func_pipe2=no
 ac_cv_func_nice=no
 ac_cv_func_setitimer=no
+# unsupported syscall: __syscall_prlimit64
+ac_cv_func_prlimit=no
+# unsupported syscall: __syscall_getrusage
+ac_cv_func_getrusage=no
+ac_cv_func_posix_fallocate=no
 
 # Syscalls that resulted in a segfault
 ac_cv_func_utimensat=no
@@ -78,6 +80,20 @@ ac_cv_header_sys_ioctl_h=no
 ac_cv_func_openpty=no
 ac_cv_func_forkpty=no
 
+# mkfifo and mknod are broken, create regular file
+ac_cv_func_mkfifo=no
+ac_cv_func_mkfifoat=no
+ac_cv_func_mknod=no
+ac_cv_func_mknodat=no
+
+# always fails with permission error
+ac_cv_func_setgroups=no
+ac_cv_func_setresuid=no
+ac_cv_func_setresgid=no
+
+# alarm signal is not delivered, may need a callback into the event loop?
+ac_cv_func_alarm=no
+
 # To use dlopen, you need to use Emscripten's linking support,
-# see https://github.com/emscripten-core/emscripten/wiki/Linking)
+# see https://emscripten.org/docs/compiling/Dynamic-Linking.html
 ac_cv_func_dlopen=no