grass.pygrass: Pin fork start method for RPC servers on Python 3.14

python/grass/pygrass/rpc/__init__.py

@@ -12,7 +12,6 @@
 
 import sys
 from ctypes import CFUNCTYPE, c_void_p
-from multiprocessing import Lock, Pipe, Process
 
 import grass.lib.gis as libgis
 from grass.exceptions import FatalError
@@ -22,7 +21,7 @@
 from grass.pygrass.vector import VectorTopo
 from grass.pygrass.vector.basic import Bbox
 
-from .base import RPCServerBase
+from .base import MP_CONTEXT, RPCServerBase
 
 ###############################################################################
 ###############################################################################
@@ -249,9 +248,9 @@ def __init__(self):
 
     def start_server(self):
         """This function must be re-implemented in the subclasses"""
-        self.client_conn, self.server_conn = Pipe(True)
-        self.lock = Lock()
-        self.server = Process(
+        self.client_conn, self.server_conn = MP_CONTEXT.Pipe(True)
+        self.lock = MP_CONTEXT.Lock()
+        self.server = MP_CONTEXT.Process(
             target=data_provider_server, args=(self.lock, self.server_conn)
         )
         self.server.daemon = True

python/grass/pygrass/rpc/base.py

@@ -16,7 +16,7 @@
 import sys
 import threading
 import time
-from multiprocessing import Lock, Pipe, Process
+import multiprocessing as mp
 from typing import TYPE_CHECKING, NoReturn
 
 from grass.exceptions import FatalError
@@ -28,6 +28,17 @@
 
 logger: logging.Logger = logging.getLogger(__name__)
 
+# Python 3.14 changed the default multiprocessing start method from "fork" to
+# "forkserver" on all platforms except macOS. The RPC worker process relies on
+# "fork" semantics: subclasses run libgis via ctypes in the worker, which must
+# inherit the initialized library state and the GRASS session environment from
+# the parent. Pin "fork" where it used to be the default; macOS and Windows
+# keep their default start method ("spawn").
+if sys.platform != "darwin" and "fork" in mp.get_all_start_methods():
+    MP_CONTEXT = mp.get_context("fork")
+else:
+    MP_CONTEXT = mp.get_context()
+
 
 ###############################################################################
 
@@ -142,9 +153,11 @@ def start_server(self):
         """This function must be re-implemented in the subclasses"""
         logger.debug("Start the libgis server")
 
-        self.client_conn, self.server_conn = Pipe(True)
-        self.lock = Lock()
-        self.server = Process(target=dummy_server, args=(self.lock, self.server_conn))
+        self.client_conn, self.server_conn = MP_CONTEXT.Pipe(True)
+        self.lock = MP_CONTEXT.Lock()
+        self.server = MP_CONTEXT.Process(
+            target=dummy_server, args=(self.lock, self.server_conn)
+        )
         self.server.daemon = True
         self.server.start()
 

python/grass/temporal/c_libraries_interface.py

@@ -16,7 +16,6 @@
 import sys
 from ctypes import CFUNCTYPE, POINTER, byref, c_int, c_void_p, cast
 from datetime import datetime
-from multiprocessing import Lock, Pipe, Process
 from typing import TYPE_CHECKING, Any, Literal
 
 import grass.lib.date as libdate
@@ -27,7 +26,7 @@
 import grass.lib.vector as libvector
 from grass.exceptions import FatalError
 from grass.pygrass.raster import RasterRow
-from grass.pygrass.rpc.base import RPCServerBase
+from grass.pygrass.rpc.base import MP_CONTEXT, RPCServerBase
 from grass.pygrass.utils import decode
 from grass.pygrass.vector import VectorTopo
 from grass.script.utils import encode
@@ -1471,9 +1470,9 @@ def __init__(self) -> None:
         RPCServerBase.__init__(self)
 
     def start_server(self) -> None:
-        self.client_conn, self.server_conn = Pipe(True)
-        self.lock = Lock()
-        self.server = Process(
+        self.client_conn, self.server_conn = MP_CONTEXT.Pipe(True)
+        self.lock = MP_CONTEXT.Lock()
+        self.server = MP_CONTEXT.Process(
             target=c_library_server, args=(self.lock, self.server_conn)
         )
         self.server.daemon = True