lib/python: Use fork start method where possible as Python 3.14 compatibility workaround

.github/workflows/osgeo4w.yml

@@ -64,7 +64,6 @@ jobs:
             libsystre
             libtre
             libwinpthread-git
-            openblas
             pcre
             pkgconf
             toolchain

mswindows/osgeo4w/build_osgeo4w.sh

@@ -37,7 +37,7 @@ CXXFLAGS="$CXXFLAGS -pipe" \
     --includedir=${OSGEO4W_ROOT_MSYS}/include \
     --libexecdir=${OSGEO4W_ROOT_MSYS}/bin \
     --prefix=${OSGEO4W_ROOT_MSYS}/apps/grass \
-    --with-blas \
+    --without-blas \
     --with-bzlib \
     --with-cairo \
     --with-cairo-includes=${OSGEO4W_ROOT_MSYS}/include/cairo \

python/grass/pygrass/messages/__init__.py

@@ -15,13 +15,15 @@
 from __future__ import annotations
 
 import sys
-from multiprocessing import Lock, Pipe, Process
+from multiprocessing import Lock, Pipe
 from typing import TYPE_CHECKING, Literal, NoReturn
 
 import grass.lib.gis as libgis
 from grass.exceptions import FatalError
+from grass.script.utils import get_fork_context
 
 if TYPE_CHECKING:
+    from multiprocessing import Process
     from multiprocessing.connection import Connection
     from multiprocessing.context import _LockLike
 
@@ -181,15 +183,19 @@ def __init__(self, raise_on_error: bool = False) -> None:
         self.raise_on_error = raise_on_error
         self.client_conn, self.server_conn = Pipe()
         self.lock = Lock()
-        self.server = Process(target=message_server, args=(self.lock, self.server_conn))
+        self.server = get_fork_context().Process(
+            target=message_server, args=(self.lock, self.server_conn)
+        )
         self.server.daemon = True
         self.server.start()
 
     def start_server(self) -> None:
         """Start the messenger server and open the pipe"""
         self.client_conn, self.server_conn = Pipe()
         self.lock = Lock()
-        self.server = Process(target=message_server, args=(self.lock, self.server_conn))
+        self.server = get_fork_context().Process(
+            target=message_server, args=(self.lock, self.server_conn)
+        )
         self.server.daemon = True
         self.server.start()
 

python/grass/pygrass/modules/interface/module.py

@@ -912,7 +912,7 @@ class MultiModule:
 
         >>> from grass.pygrass.modules import Module
         >>> from grass.pygrass.modules import MultiModule
-        >>> from multiprocessing import Process
+        >>> from multiprocessing.process import BaseProcess
         >>> import copy
 
     Synchronous module run
@@ -950,7 +950,7 @@ class MultiModule:
         ...     sync=False,
         ... )
         >>> t = mm.run()
-        >>> isinstance(t, Process)
+        >>> isinstance(t, BaseProcess)
         True
         >>> m_list = mm.wait()
         >>> m_list[0].returncode
@@ -976,7 +976,7 @@ class MultiModule:
         >>> str(mm)
         'g.region format=plain -p ; g.region format=plain -p ; g.region format=plain -p ; g.region format=plain -p ; g.region format=plain -p'
         >>> t = mm.run()
-        >>> isinstance(t, Process)
+        >>> isinstance(t, BaseProcess)
         True
         >>> m_list = mm.wait()
         >>> m_list[0].returncode

python/grass/pygrass/rpc/base.py

@@ -14,10 +14,11 @@
 import sys
 import threading
 import time
-from multiprocessing import Lock, Pipe, Process
+from multiprocessing import Lock, Pipe
 from typing import TYPE_CHECKING, NoReturn
 
 from grass.exceptions import FatalError
+from grass.script.utils import get_fork_context
 
 if TYPE_CHECKING:
     from multiprocessing.connection import Connection
@@ -142,7 +143,9 @@ def start_server(self):
 
         self.client_conn, self.server_conn = Pipe(True)
         self.lock = Lock()
-        self.server = Process(target=dummy_server, args=(self.lock, self.server_conn))
+        self.server = get_fork_context().Process(
+            target=dummy_server, args=(self.lock, self.server_conn)
+        )
         self.server.daemon = True
         self.server.start()
 

python/grass/script/__init__.py

@@ -93,6 +93,7 @@
     diff_files,
     encode,
     float_or_dms,
+    get_fork_context,
     get_lib_path,
     get_num_suffix,
     legalize_vector_name,
@@ -159,6 +160,7 @@
     "float_or_dms",
     "get_capture_stderr",
     "get_commands",
+    "get_fork_context",
     "get_lib_path",
     "get_num_suffix",
     "get_raise_on_error",

python/grass/script/utils.py

@@ -27,6 +27,7 @@
 import uuid
 import random
 import string
+import sys
 
 from pathlib import Path
 from typing import TYPE_CHECKING, AnyStr, TypeVar, cast, overload
@@ -125,6 +126,24 @@ def resolve_nprocs(nprocs: int | str) -> int:
     return max(1, available + n)
 
 
+def get_fork_context():
+    """Return a suitable :mod:`multiprocessing` fork context.
+
+    The GRASS Python APIs does not yet support `spawn` and `forkserver`
+    start-methods for multiprocessing, which became new default in
+    Python 3.14. This function provides a temporary workaround that
+    switches back to `fork` on Linux until the new default methods are
+    supported.
+    The function should be removed as soon as the GRASS Python APIs support
+    the new default start-methods.
+    """
+    import multiprocessing
+
+    if sys.platform != "darwin" and "fork" in multiprocessing.get_all_start_methods():
+        return multiprocessing.get_context("fork")
+    return multiprocessing.get_context()
+
+
 def diff_files(
     filename_a: FileDescriptorOrPath, filename_b: FileDescriptorOrPath
 ) -> list[str]:

python/grass/temporal/c_libraries_interface.py

@@ -14,7 +14,7 @@
 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 multiprocessing import Lock, Pipe
 from typing import TYPE_CHECKING, Any, Literal
 
 import grass.lib.date as libdate
@@ -1471,7 +1471,7 @@ def __init__(self) -> None:
     def start_server(self) -> None:
         self.client_conn, self.server_conn = Pipe(True)
         self.lock = Lock()
-        self.server = Process(
+        self.server = gs.get_fork_context().Process(
             target=c_library_server, args=(self.lock, self.server_conn)
         )
         self.server.daemon = True

python/grass/temporal/extract.py

@@ -10,7 +10,6 @@
 
 import re
 import sys
-from multiprocessing import Process
 
 import grass.script as gs
 from grass.exceptions import CalledModuleError
@@ -207,6 +206,7 @@ def extract_dataset(
 
     # Run the mapcalc expression
     if expression:
+        mp_context = gs.get_fork_context()
         proc_count = 0
         proc_list = []
 
@@ -262,15 +262,15 @@ def extract_dataset(
             # Add process to the process list
             if type == "raster":
                 msgr.verbose(_('Applying r.mapcalc expression: "%s"') % expr)
-                proc_list.append(Process(target=run_mapcalc2d, args=(expr,)))
+                proc_list.append(mp_context.Process(target=run_mapcalc2d, args=(expr,)))
             elif type == "raster3d":
                 msgr.verbose(_('Applying r3.mapcalc expression: "%s"') % expr)
-                proc_list.append(Process(target=run_mapcalc3d, args=(expr,)))
+                proc_list.append(mp_context.Process(target=run_mapcalc3d, args=(expr,)))
             elif type == "vector":
                 msgr.verbose(_('Applying v.extract where statement: "%s"') % expression)
                 if row["layer"]:
                     proc_list.append(
-                        Process(
+                        mp_context.Process(
                             target=run_vector_extraction,
                             args=(
                                 row["name"] + "@" + row["mapset"],
@@ -283,7 +283,7 @@ def extract_dataset(
                     )
                 else:
                     proc_list.append(
-                        Process(
+                        mp_context.Process(
                             target=run_vector_extraction,
                             args=(
                                 row["name"] + "@" + row["mapset"],

python/grass/temporal/mapcalc.py

@@ -10,7 +10,6 @@
 import copy
 import sys
 from datetime import datetime
-from multiprocessing import Process
 
 import grass.script as gs
 from grass.exceptions import CalledModuleError
@@ -234,6 +233,7 @@ def dataset_mapcalculator(
         num = len(map_matrix[0])
 
         # Parallel processing
+        mp_context = gs.get_fork_context()
         proc_list = []
         proc_count = 0
 
@@ -309,9 +309,13 @@ def dataset_mapcalculator(
 
             # Start the parallel r.mapcalc computation
             if type == "raster":
-                proc_list.append(Process(target=_run_mapcalc2d, args=(expr,)))
+                proc_list.append(
+                    mp_context.Process(target=_run_mapcalc2d, args=(expr,))
+                )
             else:
-                proc_list.append(Process(target=_run_mapcalc3d, args=(expr,)))
+                proc_list.append(
+                    mp_context.Process(target=_run_mapcalc3d, args=(expr,))
+                )
             proc_list[proc_count].start()
             proc_count += 1