gh-77377: Ensure multiprocessing SemLock is valid for spawn-based Process before serializing it (#107275)

Ensure multiprocessing SemLock is valid for spawn Process before serializing it.

Creating a multiprocessing SemLock with a fork context, and then trying to pass it to a spawn-created Process, would segfault if not detected early.

---------

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>

1 files changed, 7 insertions, 2 deletions

diff --git a/Lib/multiprocessing/synchronize.py b/Lib/multiprocessing/synchronize.py
index 42624b5..2328d33 100644
--- a/Lib/multiprocessing/synchronize.py
+++ b/Lib/multiprocessing/synchronize.py
@@ -50,8 +50,8 @@ class SemLock(object):
     def __init__(self, kind, value, maxvalue, *, ctx):
         if ctx is None:
             ctx = context._default_context.get_context()
-        name = ctx.get_start_method()
-        unlink_now = sys.platform == 'win32' or name == 'fork'
+        self.is_fork_ctx = ctx.get_start_method() == 'fork'
+        unlink_now = sys.platform == 'win32' or self.is_fork_ctx
         for i in range(100):
             try:
                 sl = self._semlock = _multiprocessing.SemLock(
@@ -103,6 +103,11 @@ class SemLock(object):
         if sys.platform == 'win32':
             h = context.get_spawning_popen().duplicate_for_child(sl.handle)
         else:
+            if self.is_fork_ctx:
+                raise RuntimeError('A SemLock created in a fork context is being '
+                                   'shared with a process in a spawn context. This is '
+                                   'not supported. Please use the same context to create '
+                                   'multiprocessing objects and Process.')
             h = sl.handle
         return (h, sl.kind, sl.maxvalue, sl.name)