Make the test suite work with 3.15

Author: A5rocks

.github/workflows/ci.yml

@@ -168,7 +168,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python: ['3.10', '3.11', '3.12', '3.13', '3.14', '3.14t']
+        python: ['3.10', '3.11', '3.12', '3.13', '3.14', '3.14t', '3.15-dev']
         arch: ['x86', 'x64']
         lsp: ['']
         lsp_extract_file: ['']
@@ -242,7 +242,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python: ['pypy-3.11', '3.10', '3.11', '3.12', '3.13', '3.14', '3.14t']
+        python: ['pypy-3.11', '3.10', '3.11', '3.12', '3.13', '3.14', '3.14t', '3.15-dev']
         check_formatting: ['0']
         no_test_requirements: ['0']
         extra_name: ['']
@@ -320,7 +320,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python: ['pypy-3.11', '3.10', '3.11', '3.12', '3.13', '3.14', '3.14t']
+        python: ['pypy-3.11', '3.10', '3.11', '3.12', '3.13', '3.14', '3.14t', '3.15-dev']
     continue-on-error: >-
       ${{
         (

src/trio/_core/_tests/test_thread_cache.py

@@ -1,8 +1,10 @@
 from __future__ import annotations
 
 import os
+import sys
 import threading
 import time
+import warnings
 from contextlib import contextmanager
 from queue import Queue
 from typing import TYPE_CHECKING, NoReturn
@@ -210,7 +212,17 @@ def foo() -> None:
     start_thread_soon(foo, lambda _: done.set())
     done.wait()
 
-    child_pid = os.fork()
+    with warnings.catch_warnings(record=True) as emitted:
+        # TODO: remove this extra filter once we drop 3.14. See
+        # https://github.com/python-trio/trio/issues/3355.
+        warnings.simplefilter("default")
+        child_pid = os.fork()
+
+    if child_pid != 0 and sys.version_info < (3, 12):
+        # the warning is only emitted in the parent
+        assert len(emitted) == 1
+        assert isinstance(emitted[0].message, DeprecationWarning)
+        assert "fork() may lead to" in str(emitted[0].message)
 
     # try using it
     done = threading.Event()