Use forkserver instead of fork for multiprocessing

Polars gives warnings if using fork.
preexec_fn in create_subprocess_exec is unsafe using fork.
python 3.14 will set forkserver as default on linux/bsd.

src/ert/__main__.py

@@ -4,6 +4,7 @@
 import locale
 import logging
 import logging.config
+import multiprocessing
 import os
 import re
 import resource
@@ -706,4 +707,9 @@ def main() -> None:
 
 
 if __name__ == "__main__":
+    if (
+        sys.platform == "linux"
+        and multiprocessing.get_start_method(allow_none=True) != "forkserver"
+    ):
+        multiprocessing.set_start_method("forkserver")
     main()

tests/ert/conftest.py

@@ -1,5 +1,6 @@
 import fileinput
 import logging
+import multiprocessing
 import os
 import resource
 import shutil
@@ -485,3 +486,12 @@ def __init__(self, *args, **kwargs):
             super().__init__(*args, **kwargs)
 
     monkeypatch.setattr("ert.cli.main.EvaluatorServerConfig", MockESConfig)
+
+
+@pytest.fixture(scope="session", autouse=True)
+def set_multiprocessing_method():
+    if (
+        sys.platform == "linux"
+        and multiprocessing.get_start_method(allow_none=True) != "forkserver"
+    ):
+        multiprocessing.set_start_method("forkserver")