Revert "feat(python): Raise an error when users try to use Polars API…

… in a fork()-without-execve() child (#19149)"

This reverts commit 48bc09b.

py-polars/polars/__init__.py

@@ -414,38 +414,3 @@ def __getattr__(name: str) -> Any:
 
     msg = f"module {__name__!r} has no attribute {name!r}"
     raise AttributeError(msg)
-
-
-# fork() breaks Polars thread pool. Instead of silently hanging when users do
-# this, e.g. by using multiprocessing's footgun default setting on Linux, warn
-# them instead:
-def __install_postfork_hook() -> None:
-    def fail(*args: Any, **kwargs: Any) -> None:
-        message = """\
-Using fork() can cause Polars to deadlock in the child process.
-In addition, using fork() with Python in general is a recipe for mysterious
-deadlocks and crashes.
-
-The most likely reason you are seeing this error is because you are using the
-multiprocessing module on Linux, which uses fork() by default. This will be
-fixed in Python 3.14. Until then, you want to use the "spawn" context instead.
-
-See https://docs.pola.rs/user-guide/misc/multiprocessing/ for details.
-"""
-        raise RuntimeError(message)
-
-    def post_hook_child() -> None:
-        # Switch most public Polars API to fail when called. This won't catch
-        # _all_ edge cases, but does make it more likely users get told they
-        # tried to do something broken.
-        for name in __all__:
-            if callable(globals()[name]):
-                globals()[name] = fail
-
-    import os
-
-    if hasattr(os, "register_at_fork"):
-        os.register_at_fork(after_in_child=post_hook_child)
-
-
-__install_postfork_hook()

py-polars/tests/unit/test_polars_import.py

@@ -1,8 +1,6 @@
 from __future__ import annotations
 
 import compileall
-import multiprocessing
-import os
 import subprocess
 import sys
 from pathlib import Path
@@ -99,23 +97,3 @@ def test_polars_import() -> None:
             import_time_ms = polars_import_time // 1_000
             msg = f"Possible import speed regression; took {import_time_ms}ms\n{df_import}"
             raise AssertionError(msg)
-
-
-def run_in_child() -> pl.Series:
-    return pl.Series([1, 2, 3])
-
-
-@pytest.mark.filterwarnings("ignore")
-@pytest.mark.skipif(not hasattr(os, "fork"), reason="Requires fork()")
-def test_fork_safety() -> None:
-    # Using fork()-based multiprocessing shouldn't work:
-    with (
-        multiprocessing.get_context("fork").Pool(1) as pool,
-        pytest.raises(RuntimeError, match=r"Using fork\(\) can cause Polars"),
-    ):
-        pool.apply(run_in_child)
-
-    # Using forkserver and spawn context should not error out:
-    for context in ["spawn", "forkserver"]:
-        with multiprocessing.get_context(context).Pool(1) as pool:
-            pool.apply(run_in_child)