GH-84559: Deprecate fork being the multiprocessing default. (#100618)

This starts the process. Users who don't specify their own start method
and use the default on platforms where it is 'fork' will see a
DeprecationWarning upon multiprocessing.Pool() construction or upon
multiprocessing.Process.start() or concurrent.futures.ProcessPool use.

See the related issue and documentation within this change for details.

Lib/multiprocessing/context.py

@@ -23,6 +23,9 @@ class TimeoutError(ProcessError):
 class AuthenticationError(ProcessError):
     pass
 
+class DefaultForkDeprecationWarning(DeprecationWarning):
+    pass
+
 #
 # Base type for contexts. Bound methods of an instance of this type are included in __all__ of __init__.py
 #
@@ -258,6 +261,7 @@ class DefaultContext(BaseContext):
         return self._actual_context._name
 
     def get_all_start_methods(self):
+        """Returns a list of the supported start methods, default first."""
         if sys.platform == 'win32':
             return ['spawn']
         else:
@@ -280,6 +284,23 @@ if sys.platform != 'win32':
             from .popen_fork import Popen
             return Popen(process_obj)
 
+    _warn_package_prefixes = (os.path.dirname(__file__),)
+
+    class _DeprecatedForkProcess(ForkProcess):
+        @classmethod
+        def _Popen(cls, process_obj):
+            import warnings
+            warnings.warn(
+                "The default multiprocessing start method will change "
+                "away from 'fork' in Python >= 3.14, per GH-84559. "
+                "Use multiprocessing.get_context(X) or .set_start_method(X) to "
+                "explicitly specify it when your application requires 'fork'. "
+                "The safest start method is 'spawn'.",
+                category=DefaultForkDeprecationWarning,
+                skip_file_prefixes=_warn_package_prefixes,
+            )
+            return super()._Popen(process_obj)
+
     class SpawnProcess(process.BaseProcess):
         _start_method = 'spawn'
         @staticmethod
@@ -303,6 +324,9 @@ if sys.platform != 'win32':
         _name = 'fork'
         Process = ForkProcess
 
+    class _DefaultForkContext(ForkContext):
+        Process = _DeprecatedForkProcess
+
     class SpawnContext(BaseContext):
         _name = 'spawn'
         Process = SpawnProcess
@@ -318,13 +342,16 @@ if sys.platform != 'win32':
         'fork': ForkContext(),
         'spawn': SpawnContext(),
         'forkserver': ForkServerContext(),
+        # Remove None and _DefaultForkContext() when changing the default
+        # in 3.14 for https://github.com/python/cpython/issues/84559.
+        None: _DefaultForkContext(),
     }
     if sys.platform == 'darwin':
         # bpo-33725: running arbitrary code after fork() is no longer reliable
         # on macOS since macOS 10.14 (Mojave). Use spawn by default instead.
         _default_context = DefaultContext(_concrete_contexts['spawn'])
     else:
-        _default_context = DefaultContext(_concrete_contexts['fork'])
+        _default_context = DefaultContext(_concrete_contexts[None])
 
 else: