Use loky.cpu_count() instead of os.cpu_count(). <!-- Thank you for contributing to Pyodide! All improvements are welcome,
     so don't be afraid to make a PR. -->

### Description

As discussed in https://github.com/pyodide/pyodide/pull/3559#issuecomment-1435965170

### Checklists

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests

import sys
from collections.abc import Sequence
from importlib.abc import Loader, MetaPathFinder
from importlib.machinery import ModuleSpec
from importlib.util import spec_from_loader
from types import ModuleType
from typing import Any

from ._core_docs import JsProxy


class JsFinder(MetaPathFinder):
    def __init__(self) -> None:
        self.jsproxies: dict[str, Any] = {}

    def find_spec(
        self,
        fullname: str,
        path: Sequence[bytes | str] | None,
        target: ModuleType | None = None,
    ) -> ModuleSpec | None:
        [parent, _, child] = fullname.rpartition(".")
        if parent:
            parent_module = sys.modules[parent]
            if not isinstance(parent_module, JsProxy):
                # Not one of us.
                return None
            try:
                jsproxy = getattr(parent_module, child)
            except AttributeError:
                raise ModuleNotFoundError(
                    f"No module named {fullname!r}", name=fullname
                ) from None
            if not isinstance(jsproxy, JsProxy):
                raise ModuleNotFoundError(
                    f"No module named {fullname!r}", name=fullname
                )
        else:
            try:
                jsproxy = self.jsproxies[fullname]
            except KeyError:
                return None
        loader = JsLoader(jsproxy)
        return spec_from_loader(fullname, loader, origin="javascript")

    def register_js_module(self, name: str, jsproxy: Any) -> None:
        """
        Registers ``jsproxy`` as a JavaScript module named ``name``. The module
        can then be imported from Python using the standard Python import
        system. If another module by the same name has already been imported,
        this won't have much effect unless you also delete the imported module
        from :py:data:`sys.modules`. This is called by the JavaScript API
        :js:func:`pyodide.registerJsModule`.

        Parameters
        ----------
        name :
            Name of js module

        jsproxy :
            JavaScript object backing the module
        """
        assert JsProxy is not None
        if not isinstance(name, str):
            raise TypeError(
                f"Argument 'name' must be a str, not {type(name).__name__!r}"
            )
        if not isinstance(jsproxy, JsProxy):
            raise TypeError(
                f"Argument 'jsproxy' must be a JsProxy, not {type(jsproxy).__name__!r}"
            )
        self.jsproxies[name] = jsproxy

    def unregister_js_module(self, name: str) -> None:
        """
        Unregisters a JavaScript module with given name that has been previously
        registered with :js:func:`pyodide.registerJsModule` or
        :py:func:`pyodide.ffi.register_js_module`. If a JavaScript module with that name
        does not already exist, will raise an error. If the module has already
        been imported, this won't have much effect unless you also delete the
        imported module from :py:data:`sys.modules`. This is called by the JavaScript
        API :js:func:`pyodide.unregisterJsModule`.

        Parameters
        ----------
        name :
            Name of the module to unregister
        """
        try:
            del self.jsproxies[name]
        except KeyError:
            raise ValueError(
                f"Cannot unregister {name!r}: no Javascript module with that name is registered"
            ) from None


class JsLoader(Loader):
    def __init__(self, jsproxy: Any) -> None:
        self.jsproxy = jsproxy

    def create_module(self, spec: ModuleSpec) -> Any:
        return self.jsproxy

    def exec_module(self, module: ModuleType) -> None:
        pass

    # used by importlib.util.spec_from_loader
    def is_package(self, fullname: str) -> bool:
        return True


jsfinder: JsFinder = JsFinder()
register_js_module = jsfinder.register_js_module
unregister_js_module = jsfinder.unregister_js_module


def register_js_finder() -> None:
    """A bootstrap function, called near the end of Pyodide initialization.

    It is called in ``loadPyodide`` in ``pyodide.js`` once ``_pyodide_core`` is ready
    to set up the js import mechanism.

        1. Put the right value into the global variable ``JsProxy`` so that
           ``JsFinder.find_spec`` can decide whether parent module is a Js module.
        2. Add ``jsfinder`` to metapath to allow js imports.

    This needs to be a function to allow the late import from ``_pyodide_core``.
    """
    for importer in sys.meta_path:
        if isinstance(importer, JsFinder):
            raise RuntimeError("JsFinder already registered")

    sys.meta_path.append(jsfinder)


STDLIBS = sys.stdlib_module_names | {"test"}
UNVENDORED_STDLIBS_AND_TEST: set[str] = set()


from importlib import _bootstrap  # type: ignore[attr-defined]

orig_get_module_not_found_error: Any = None
REPODATA_PACKAGES_IMPORT_TO_PACKAGE_NAME: dict[str, str] = {}

SEE_PACKAGE_LOADING = (
    "\nSee https://pyodide.org/en/stable/usage/loading-packages.html for more details."
)

YOU_CAN_INSTALL_IT_BY = """
You can install it by calling:
  await micropip.install("{package_name}") in Python, or
  await pyodide.loadPackage("{package_name}") in JavaScript\
"""


def get_module_not_found_error(import_name):

    package_name = REPODATA_PACKAGES_IMPORT_TO_PACKAGE_NAME.get(import_name, "")

    if not package_name and import_name not in STDLIBS:
        return orig_get_module_not_found_error(import_name)

    if package_name in UNVENDORED_STDLIBS_AND_TEST:
        msg = "The module '{package_name}' is unvendored from the Python standard library in the Pyodide distribution."
        msg += YOU_CAN_INSTALL_IT_BY
    elif import_name in STDLIBS:
        msg = (
            "The module '{import_name}' is removed from the Python standard library in the"
            " Pyodide distribution due to browser limitations."
        )
    else:
        msg = "The module '{package_name}' is included in the Pyodide distribution, but it is not installed."
        msg += YOU_CAN_INSTALL_IT_BY

    msg += SEE_PACKAGE_LOADING
    return ModuleNotFoundError(
        msg.format(import_name=import_name, package_name=package_name)
    )


def register_module_not_found_hook(packages: Any, unvendored: Any) -> None:
    """
    A function that adds UnvendoredStdlibFinder to the end of sys.meta_path.

    Note that this finder must be placed in the end of meta_paths
    in order to prevent any unexpected side effects.
    """
    global orig_get_module_not_found_error
    global REPODATA_PACKAGES_IMPORT_TO_PACKAGE_NAME
    global UNVENDORED_STDLIBS_AND_TEST
    REPODATA_PACKAGES_IMPORT_TO_PACKAGE_NAME = packages.to_py()
    UNVENDORED_STDLIBS_AND_TEST = set(unvendored.to_py())
    orig_get_module_not_found_error = _bootstrap._get_module_not_found_error
    _bootstrap._get_module_not_found_error = get_module_not_found_error

import sys
from collections.abc import Sequence
from importlib.abc import Loader, MetaPathFinder
from importlib.machinery import ModuleSpec
from importlib.util import spec_from_loader
from types import ModuleType
from typing import Any

from ._core_docs import JsProxy


class JsFinder(MetaPathFinder):
    def __init__(self) -> None:
        self.jsproxies: dict[str, Any] = {}

    def find_spec(
        self,
        fullname: str,
        path: Sequence[bytes | str] | None,
        target: ModuleType | None = None,
    ) -> ModuleSpec | None:
        [parent, _, child] = fullname.rpartition(".")
        if parent:
            parent_module = sys.modules[parent]
            if not isinstance(parent_module, JsProxy):
                # Not one of us.
                return None
            try:
                jsproxy = getattr(parent_module, child)
            except AttributeError:
                raise ModuleNotFoundError(
                    f"No module named {fullname!r}", name=fullname
                ) from None
            if not isinstance(jsproxy, JsProxy):
                raise ModuleNotFoundError(
                    f"No module named {fullname!r}", name=fullname
                )
        else:
            try:
                jsproxy = self.jsproxies[fullname]
            except KeyError:
                return None
        loader = JsLoader(jsproxy)
        return spec_from_loader(fullname, loader, origin="javascript")

    def register_js_module(self, name: str, jsproxy: Any) -> None:
        """
        Registers ``jsproxy`` as a JavaScript module named ``name``. The module
        can then be imported from Python using the standard Python import
        system. If another module by the same name has already been imported,
        this won't have much effect unless you also delete the imported module
        from :py:data:`sys.modules`. This is called by the JavaScript API
        :js:func:`pyodide.registerJsModule`.

        Parameters
        ----------
        name :
            Name of js module

        jsproxy :
            JavaScript object backing the module
        """
        assert JsProxy is not None
        if not isinstance(name, str):
            raise TypeError(
                f"Argument 'name' must be a str, not {type(name).__name__!r}"
            )
        if not isinstance(jsproxy, JsProxy):
            raise TypeError(
                f"Argument 'jsproxy' must be a JsProxy, not {type(jsproxy).__name__!r}"
            )
        self.jsproxies[name] = jsproxy

    def unregister_js_module(self, name: str) -> None:
        """
        Unregisters a JavaScript module with given name that has been previously
        registered with :js:func:`pyodide.registerJsModule` or
        :py:func:`pyodide.ffi.register_js_module`. If a JavaScript module with that name
        does not already exist, will raise an error. If the module has already
        been imported, this won't have much effect unless you also delete the
        imported module from :py:data:`sys.modules`. This is called by the JavaScript
        API :js:func:`pyodide.unregisterJsModule`.

        Parameters
        ----------
        name :
            Name of the module to unregister
        """
        try:
            del self.jsproxies[name]
        except KeyError:
            raise ValueError(
                f"Cannot unregister {name!r}: no Javascript module with that name is registered"
            ) from None


class JsLoader(Loader):
    def __init__(self, jsproxy: Any) -> None:
        self.jsproxy = jsproxy

    def create_module(self, spec: ModuleSpec) -> Any:
        return self.jsproxy

    def exec_module(self, module: ModuleType) -> None:
        pass

    # used by importlib.util.spec_from_loader
    def is_package(self, fullname: str) -> bool:
        return True


jsfinder: JsFinder = JsFinder()
register_js_module = jsfinder.register_js_module
unregister_js_module = jsfinder.unregister_js_module


def register_js_finder() -> None:
    """A bootstrap function, called near the end of Pyodide initialization.

    It is called in ``loadPyodide`` in ``pyodide.js`` once ``_pyodide_core`` is ready
    to set up the js import mechanism.

        1. Put the right value into the global variable ``JsProxy`` so that
           ``JsFinder.find_spec`` can decide whether parent module is a Js module.
        2. Add ``jsfinder`` to metapath to allow js imports.

    This needs to be a function to allow the late import from ``_pyodide_core``.
    """
    for importer in sys.meta_path:
        if isinstance(importer, JsFinder):
            raise RuntimeError("JsFinder already registered")

    sys.meta_path.append(jsfinder)


STDLIBS = sys.stdlib_module_names | {"test"}
UNVENDORED_STDLIBS_AND_TEST: set[str] = set()


from importlib import _bootstrap  # type: ignore[attr-defined]

orig_get_module_not_found_error: Any = None
REPODATA_PACKAGES_IMPORT_TO_PACKAGE_NAME: dict[str, str] = {}

SEE_PACKAGE_LOADING = (
    "\nSee https://pyodide.org/en/stable/usage/loading-packages.html for more details."
)

YOU_CAN_INSTALL_IT_BY = """
You can install it by calling:
  await micropip.install("{package_name}") in Python, or
  await pyodide.loadPackage("{package_name}") in JavaScript\
"""


def get_module_not_found_error(import_name):

    package_name = REPODATA_PACKAGES_IMPORT_TO_PACKAGE_NAME.get(import_name, "")

    if not package_name and import_name not in STDLIBS:
        return orig_get_module_not_found_error(import_name)

    if package_name in UNVENDORED_STDLIBS_AND_TEST:
        msg = "The module '{package_name}' is unvendored from the Python standard library in the Pyodide distribution."
        msg += YOU_CAN_INSTALL_IT_BY
    elif import_name in STDLIBS:
        msg = (
            "The module '{import_name}' is removed from the Python standard library in the"
            " Pyodide distribution due to browser limitations."
        )
    else:
        msg = "The module '{package_name}' is included in the Pyodide distribution, but it is not installed."
        msg += YOU_CAN_INSTALL_IT_BY

    msg += SEE_PACKAGE_LOADING
    return ModuleNotFoundError(
        msg.format(import_name=import_name, package_name=package_name)
    )


def register_module_not_found_hook(packages: Any, unvendored: Any) -> None:
    """
    A function that adds UnvendoredStdlibFinder to the end of sys.meta_path.

    Note that this finder must be placed in the end of meta_paths
    in order to prevent any unexpected side effects.
    """
    global orig_get_module_not_found_error
    global REPODATA_PACKAGES_IMPORT_TO_PACKAGE_NAME
    global UNVENDORED_STDLIBS_AND_TEST
    REPODATA_PACKAGES_IMPORT_TO_PACKAGE_NAME = packages.to_py()
    UNVENDORED_STDLIBS_AND_TEST = set(unvendored.to_py())
    orig_get_module_not_found_error = _bootstrap._get_module_not_found_error
    _bootstrap._get_module_not_found_error = get_module_not_found_error
