Build with wasm exception handling

For now, we need to use a custom rust toolchain for this. Apparently all
alternatives to this don't work. -Zbuild-std doesn't work with panic=abort
(rust-lang/cargo#7359)
and my attempts to use a custom sysroot with either
https://github.com/RalfJung/rustc-build-sysroot/ or
https://github.com/DianaNites/cargo-sysroot/  seem to hit the same problem
as with `-Zbuild-std`. Thus, I think the only reasonable way to go is to
build the sysroot from the rust source directory. Perhaps we can eventually
approach this by copying the `lib/rustlib/wasm32-unknown-emscripten/lib/`
folder out of the build of the rust compiler on top of a nightly install of
the compiler. For now, there is the additional problem that we need this
patch to fix unwind=abort:
rust-lang/rust#135450

I got my copy of the rust compiler by checking out this commit:
hoodmane/rust@052ba16
two commits ahead of the rust main branch and running:
```
./x build --stage 2 --target x86_64-unknown-linux-gnu,wasm32-unknown-emscripten
```

Author: hoodmane

pyodide_build/build_env.py

@@ -19,8 +19,7 @@
 from pyodide_build.recipe import load_all_recipes
 
 RUST_BUILD_PRELUDE = """
-rustup toolchain install ${RUST_TOOLCHAIN} && rustup default ${RUST_TOOLCHAIN}
-rustup target add wasm32-unknown-emscripten --toolchain ${RUST_TOOLCHAIN}
+rustup default ${RUST_TOOLCHAIN}
 """
 
 

pyodide_build/buildpkg.py

@@ -460,13 +460,29 @@ def _get_helper_vars(self) -> dict[str, str]:
         }
 
 
+def _ensure_rust_toolchain():
+    return
+    pyodide_root = get_pyodide_root()
+    toolchain_version = "emscripten_2025-01-13_052ba16"
+    toolchain_path = (pyodide_root / ".rust-toolchain" / toolchain_version)
+    if toolchain_path.exists():
+        return
+    from .xbuildenv import download_and_unpack_archive
+    download_and_unpack_archive("http://pyodide-cache.s3-website-us-east-1.amazonaws.com/rustc/{toolchain_version}", toolchain_path)
+    result = subprocess.run(["rustup", "toolchain", "link", toolchain_version, toolchain_path], check = False)
+    if result.returncode != 0:
+        logger.error("ERROR: rustup toolchain install failed")
+        exit_with_stdio(result)
+
+
 class RecipeBuilderPackage(RecipeBuilder):
     """
     Recipe builder for python packages.
     """
 
     def _build_package(self, bash_runner: BashRunnerWithSharedEnvironment) -> None:
         if self.recipe.is_rust_package():
+            _ensure_rust_toolchain()
             bash_runner.run(
                 RUST_BUILD_PRELUDE,
                 script_name="rust build prelude",

pyodide_build/config.py

@@ -207,7 +207,7 @@ def to_env(self) -> dict[str, str]:
     "rustflags": "-C link-arg=-sSIDE_MODULE=2 -C link-arg=-sWASM_BIGINT -Z link-native-libraries=no",
     "cargo_build_target": "wasm32-unknown-emscripten",
     "cargo_target_wasm32_unknown_emscripten_linker": "emcc",
-    "rust_toolchain": "nightly-2024-01-29",
+    "rust_toolchain": "emscripten_2025-01-13_052ba16",
     # Other configuration
     "pyodide_jobs": "1",
     "skip_emscripten_version_check": "0",

pyodide_build/xbuildenv.py

@@ -21,6 +21,50 @@
 PYTHON_VERSION_MARKER_FILE = ".build-python-version"
 
 
+def download_and_unpack_archive(url: str, path: Path) -> None:
+    """
+    Download the cross-build environment from the given URL and extract it to the given path.
+
+    Parameters
+    ----------
+    url
+        URL to download the cross-build environment from.
+    path
+        Path to extract the cross-build environment to.
+        If the path already exists, raise an error.
+    """
+    logger.info("Downloading Pyodide cross-build environment from %s", url)
+
+    if path.exists():
+        raise FileExistsError(f"Path {path} already exists")
+
+    try:
+        resp = urlopen(url)
+        data = resp.read()
+    except Exception as e:
+        raise ValueError(
+            f"Failed to download cross-build environment from {url}"
+        ) from e
+
+    # FIXME: requests makes a verbose output (see: https://github.com/pyodide/pyodide/issues/4810)
+    # r = requests.get(url)
+
+    # if r.status_code != 200:
+    #     raise ValueError(
+    #         f"Failed to download cross-build environment from {url} (status code: {r.status_code})"
+    #     )
+
+    with NamedTemporaryFile(suffix=".tar") as f:
+        f_path = Path(f.name)
+        f_path.write_bytes(data)
+        with warnings.catch_warnings():
+            # Python 3.12-3.13 emits a DeprecationWarning when using shutil.unpack_archive without a filter,
+            # but filter doesn't work well for zip files, so we suppress the warning until we find a better solution.
+            # https://github.com/python/cpython/issues/112760
+            warnings.simplefilter("ignore")
+            shutil.unpack_archive(str(f_path), path)
+
+
 class CrossBuildEnvManager:
     """
     Manager for the cross-build environment.
@@ -194,7 +238,7 @@ def install(
                 download_path,
             )
         else:
-            self._download(download_url, download_path)
+            download_and_unpack_archive(download_url, download_path)
 
         try:
             # there is an redundant directory "xbuildenv" inside the xbuildenv archive
@@ -240,49 +284,6 @@ def _find_latest_version(self) -> str:
 
         return latest.version
 
-    def _download(self, url: str, path: Path) -> None:
-        """
-        Download the cross-build environment from the given URL and extract it to the given path.
-
-        Parameters
-        ----------
-        url
-            URL to download the cross-build environment from.
-        path
-            Path to extract the cross-build environment to.
-            If the path already exists, raise an error.
-        """
-        logger.info("Downloading Pyodide cross-build environment from %s", url)
-
-        if path.exists():
-            raise FileExistsError(f"Path {path} already exists")
-
-        try:
-            resp = urlopen(url)
-            data = resp.read()
-        except Exception as e:
-            raise ValueError(
-                f"Failed to download cross-build environment from {url}"
-            ) from e
-
-        # FIXME: requests makes a verbose output (see: https://github.com/pyodide/pyodide/issues/4810)
-        # r = requests.get(url)
-
-        # if r.status_code != 200:
-        #     raise ValueError(
-        #         f"Failed to download cross-build environment from {url} (status code: {r.status_code})"
-        #     )
-
-        with NamedTemporaryFile(suffix=".tar") as f:
-            f_path = Path(f.name)
-            f_path.write_bytes(data)
-            with warnings.catch_warnings():
-                # Python 3.12-3.13 emits a DeprecationWarning when using shutil.unpack_archive without a filter,
-                # but filter doesn't work well for zip files, so we suppress the warning until we find a better solution.
-                # https://github.com/python/cpython/issues/112760
-                warnings.simplefilter("ignore")
-                shutil.unpack_archive(str(f_path), path)
-
     def _install_cross_build_packages(
         self, xbuildenv_root: Path, xbuildenv_pyodide_root: Path
     ) -> None: