Build and test Python in CI (WebAssembly#667)

alexcrichton · web-flow · commit d5b3d5630589 · 2025-11-12T18:33:58.000+01:00
In general wasi-libc's test suite is quite bare-bones. In an effort to
help increase test coverage this commit adds a job to CI which builds
Python with a source-built version of wasi-libc and runs its test suite.
Also in general I don't know Python, nor its build system, nor its WASI
port, nor its CI configuration. Here the build system is copied over
from Python's own CI configuration and adapted to run in this
repository.

Locally this job takes ~7 minutes. This will become the longest step in
CI, but personally I feel that it's very much worth it to get the
drastically increased test coverage.
diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
@@ -213,3 +213,43 @@ jobs:
         submodules: true
     - run: make bindings
     - run: git diff --exit-code
+
+  # Currently wasi-libc's test suite is pretty bare-bones. To try to increase
+  # test coverage build Python in CI as well with a fresh copy of wasi-libc.
+  # This involves a bit of configuration to get all the stars to align, but in
+  # the end this runs at least some Python tests against a from-source-built
+  # copy of wasi-libc.
+  python:
+    name: Test Python
+    runs-on: ubuntu-latest
+    env: ${{ matrix.env || fromJSON('{}') }}
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - env:
+              TARGET_TRIPLE: wasm32-wasip1
+    steps:
+    - uses: actions/checkout@v4.1.7
+      with:
+        submodules: true
+    - run: |
+        curl https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-24/wasi-sdk-24.0-x86_64-linux.tar.gz -L | tar xzvf -
+        echo "WASI_SDK_PATH=`pwd`/wasi-sdk-24.0-x86_64-linux" >> $GITHUB_ENV
+      if: runner.os == 'Linux'
+      shell: bash
+      working-directory: ${{ runner.tool_cache }}
+    - name: Setup `wasmtime`
+      uses: bytecodealliance/actions/wasmtime/setup@v1
+      with:
+        version: "38.0.3"
+    - run: make -j4 CC=$WASI_SDK_PATH/bin/clang AR=$WASI_SDK_PATH/bin/llvm-ar
+    - name: Setup testing
+      run: |
+        cmake -S test -B testbuild -G Ninja \
+          -DTARGET_TRIPLE=${{ env.TARGET_TRIPLE }} \
+          -DCMAKE_LINK_DEPENDS_USE_LINKER=OFF \
+          -DPYTHON_TESTS=ON \
+          -DCMAKE_C_COMPILER=clang
+    - name: Build and test python
+      run: ninja -C testbuild python
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
@@ -15,6 +15,7 @@ include(CTest)
 enable_testing()
 
 set(TARGET_TRIPLE "wasm32-wasi" CACHE STRING "WASI target to test")
+option(PYTHON_TESTS "Build Python with this wasi-libc and run its tests" OFF)
 
 # ========= Sysroot sanity check ================================
 
@@ -419,3 +420,71 @@ if (TARGET_TRIPLE MATCHES "-threads")
   # "poll_oneoff" can't be implemented in the browser
   set_tests_properties(libc_test_functional_pthread_cond.wasm PROPERTIES LABELS v8fail)
 endif()
+
+# If enabled add a copy of Python which is built against `wasi-libc` and run
+# its tests.
+if (PYTHON_TESTS)
+  find_program(PYTHON python3 python REQUIRED)
+  find_program(MAKE make REQUIRED)
+
+  set(flags "--target=${TARGET_TRIPLE} --sysroot=${SYSROOT_DIR}")
+
+  ExternalProject_Add(
+    python
+
+    # Pin the source to 3.14 for now, but this is fine to change later if
+    # tests still pass.
+    URL https://github.com/python/cpython/archive/refs/tags/v3.14.0.tar.gz
+
+    # Python as-is doesn't pass with the current wasi-libc. For example
+    # wasi-libc now provides dummy pthread symbols which tricks Python into
+    # thinking it can spawn threads, so a patch is needed for a WASI-specific
+    # clause to disable that.
+    #
+    # More generally though this is an escape hatch to apply any other
+    # changes as necessary without trying to upstream the patches to Python
+    # itself. The patch is most "easily" generated by checking out cpython
+    # at the `v3.14.0` tag, applying the existing patch, editing source,
+    # and then regenerating the patch.
+    PATCH_COMMAND
+      patch -Np1 < ${CMAKE_CURRENT_SOURCE_DIR}/scripts/cpython3.14.patch
+
+    # The WASI build of Python looks to need an in-source build, or otherwise I
+    # couldn't figure out an out-of-tree build.
+    BUILD_IN_SOURCE ON
+
+    # These steps take a long time, so stream the output to the terminal instead
+    # of capturing it by default.
+    USES_TERMINAL_CONFIGURE ON
+    USES_TERMINAL_BUILD ON
+    USES_TERMINAL_TEST ON
+
+    # The following steps are copied from Python's own CI for managing WASI.
+    # In general I don't know what they do. If Python's CI changes these
+    # should change as well.
+    CONFIGURE_COMMAND
+      ${PYTHON} <SOURCE_DIR>/Tools/wasm/wasi configure-build-python -- --config-cache --with-pydebug
+    COMMAND
+      ${PYTHON} <SOURCE_DIR>/Tools/wasm/wasi make-build-python
+
+    BUILD_COMMAND
+      ${CMAKE_COMMAND}
+        -E env CFLAGS=${flags} LDFLAGS=${flags} --
+        ${PYTHON} <SOURCE_DIR>/Tools/wasm/wasi configure-host -- --config-cache
+    COMMAND
+      ${CMAKE_COMMAND}
+        -E env CFLAGS=${flags} LDFLAGS=${flags} --
+        ${PYTHON} <SOURCE_DIR>/Tools/wasm/wasi make-host
+    COMMAND
+      ${CMAKE_COMMAND}
+        -E env CFLAGS=${flags} LDFLAGS=${flags} --
+        ${MAKE} --directory cross-build/wasm32-wasip1 pythoninfo
+
+    INSTALL_COMMAND ""
+
+    TEST_COMMAND
+      ${CMAKE_COMMAND}
+        -E env CFLAGS=${flags} LDFLAGS=${flags} --
+        ${MAKE} --directory cross-build/wasm32-wasip1 test
+  )
+endif()
diff --git a/test/scripts/cpython3.14.patch b/test/scripts/cpython3.14.patch
@@ -0,0 +1,51 @@
+diff --git a/Include/pyport.h b/Include/pyport.h
+index 3eac119bf8e..6a4df4bf75f 100644
+--- a/Include/pyport.h
++++ b/Include/pyport.h
+@@ -503,6 +503,7 @@ extern "C" {
+  * Thread support is stubbed and any attempt to create a new thread fails.
+  */
+ #if (!defined(HAVE_PTHREAD_STUBS) && \
++     !defined(__wasi__) && \
+       (!defined(__EMSCRIPTEN__) || defined(__EMSCRIPTEN_PTHREADS__)))
+ #  define Py_CAN_START_THREADS 1
+ #endif
+diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
+index e2117b9588d..1a14c758c55 100644
+--- a/Lib/test/test_sys.py
++++ b/Lib/test/test_sys.py
+@@ -737,7 +737,7 @@ def test_thread_info(self):
+         elif sys.platform == "emscripten":
+             self.assertIn(info.name, {"pthread", "pthread-stubs"})
+         elif sys.platform == "wasi":
+-            self.assertEqual(info.name, "pthread-stubs")
++            self.assertIn(info.name, {"pthread", "pthread-stubs"})
+ 
+     @unittest.skipUnless(support.is_emscripten, "only available on Emscripten")
+     def test_emscripten_info(self):
+diff --git a/Tools/wasm/wasi/__main__.py b/Tools/wasm/wasi/__main__.py
+index 54ccc95157d..e92e7419086 100644
+--- a/Tools/wasm/wasi/__main__.py
++++ b/Tools/wasm/wasi/__main__.py
+@@ -307,7 +307,7 @@ def main():
+                         # Make sure the stack size will work for a pydebug
+                         # build.
+                         # Use 16 MiB stack.
+-                        "--wasm max-wasm-stack=16777216 "
++                        "--wasm max-wasm-stack=33554432 "
+                         # Enable thread support; causes use of preview1.
+                         #"--wasm threads=y --wasi threads=y "
+                         # Map the checkout to / to load the stdlib from /Lib.
+diff --git a/Tools/wasm/wasm_build.py b/Tools/wasm/wasm_build.py
+index bcb80212362..95e4d91211e 100755
+--- a/Tools/wasm/wasm_build.py
++++ b/Tools/wasm/wasm_build.py
+@@ -329,7 +329,7 @@ def _check_wasi() -> None:
+         # workaround for https://github.com/python/cpython/issues/95952
+         "HOSTRUNNER": (
+             "wasmtime run "
+-            "--wasm max-wasm-stack=16777216 "
++            "--wasm max-wasm-stack=33554432 "
+             "--wasi preview2 "
+             "--dir {srcdir}::/ "
+             "--env PYTHONPATH=/{relbuilddir}/build/lib.wasi-wasm32-{version}:/Lib"
