bitbake: Use a "fork" multiprocessing context

Python 3.14 changes the default multiprocessing context from "fork" to
"forkserver"; however bitbake heavily relies on "fork" to efficiently
pass data to the child processes. As such, make "fork" context in the bb
namespace and use it in place of the normal multiprocessing module.

Note that multiprocessing contexts were added in Python 3.4, so this
should be safe to use even before Python 3.14

[YOCTO #15858]

(Bitbake rev: 15d7448e04aa78c827d2cef9eb1a62bd6e0dd119)

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

bitbake/lib/bb/__init__.py

@@ -37,6 +37,34 @@ class BBHandledException(Exception):
 import os
 import logging
 from collections import namedtuple
+import multiprocessing as mp
+
+# Python 3.14 changes the default multiprocessing context from "fork" to
+# "forkserver". However, bitbake heavily relies on "fork" behavior to
+# efficiently pass data to the child processes. Places that need this should do:
+#   from bb import multiprocessing
+# in place of
+#   import multiprocessing
+
+class MultiprocessingContext(object):
+    """
+    Multiprocessing proxy object that uses the "fork" context for a property if
+    available, otherwise goes to the main multiprocessing module. This allows
+    it to be a drop-in replacement for the multiprocessing module, but use the
+    fork context
+    """
+    def __init__(self):
+        super().__setattr__("_ctx", mp.get_context("fork"))
+
+    def __getattr__(self, name):
+        if hasattr(self._ctx, name):
+            return getattr(self._ctx, name)
+        return getattr(mp, name)
+
+    def __setattr__(self, name, value):
+        raise AttributeError(f"Unable to set attribute {name}")
+
+multiprocessing = MultiprocessingContext()
 
 
 class NullHandler(logging.Handler):

bitbake/lib/bb/asyncrpc/serv.py

@@ -11,7 +11,7 @@ import os
 import signal
 import socket
 import sys
-import multiprocessing
+from bb import multiprocessing
 import logging
 from .connection import StreamConnection, WebsocketConnection
 from .exceptions import ClientError, ServerError, ConnectionClosedError, InvokeError

bitbake/lib/bb/cooker.py

@@ -12,7 +12,7 @@
 import sys, os, glob, os.path, re, time
 import itertools
 import logging
-import multiprocessing
+from bb import multiprocessing
 import threading
 from io import StringIO, UnsupportedOperation
 from contextlib import closing

bitbake/lib/bb/server/process.py

@@ -13,7 +13,7 @@
 import bb
 import bb.event
 import logging
-import multiprocessing
+from bb import multiprocessing
 import threading
 import array
 import os

bitbake/lib/bb/tests/support/httpserver.py

@@ -3,7 +3,7 @@
 #
 
 import http.server
-import multiprocessing
+from bb import multiprocessing
 import os
 import traceback
 import signal

bitbake/lib/bb/utils.py

@@ -14,7 +14,7 @@ import logging
 import bb
 import bb.msg
 import locale
-import multiprocessing
+from bb import multiprocessing
 import fcntl
 import importlib
 import importlib.machinery
@@ -1174,8 +1174,6 @@ def process_profilelog(fn, pout = None):
 #
 def multiprocessingpool(*args, **kwargs):
 
-    import multiprocessing.pool
-    #import multiprocessing.util
     #multiprocessing.util.log_to_stderr(10)
     # Deal with a multiprocessing bug where signals to the processes would be delayed until the work
     # completes. Putting in a timeout means the signals (like SIGINT/SIGTERM) get processed.

bitbake/lib/hashserv/tests.py

@@ -11,7 +11,7 @@ from bb.asyncrpc import InvokeError
 from .client import ClientPool
 import hashlib
 import logging
-import multiprocessing
+from bb import multiprocessing
 import os
 import sys
 import tempfile