Use a private fork context for the live server on POSIX

docs/changelog.rst

@@ -6,6 +6,10 @@ Changelog
 UNRELEASED
 ----------
 
+* Fix live server startup on POSIX with non-fork multiprocessing defaults,
+  including Python 3.14, without changing the application's start method
+  (`#206 <https://github.com/pytest-dev/pytest-flask/issues/206>`_).
+
 * Add support for Python 3.10, 3.11, and 3.12.
 * Drop support for EOL Python 3.7.
 * Add type hints.

src/pytest_flask/live_server.py

@@ -1,7 +1,6 @@
 import logging
 import multiprocessing
 import os
-import platform
 import signal
 import socket
 import time
@@ -25,8 +24,9 @@ def run(
     ) -> None: ...
 
 
-# force 'fork' on macOS
-if platform.system() == "Darwin":
+# Flask applications and local worker functions cannot be pickled.
+# Use a private context without changing the application's start method.
+if "fork" in multiprocessing.get_all_start_methods():
     multiprocessing = multiprocessing.get_context("fork")  # type: ignore[assignment]
 
 

tests/test_live_server.py

@@ -1,3 +1,4 @@
+import multiprocessing
 import os
 
 import pytest
@@ -77,6 +78,38 @@ def test_a(live_server):
         result.stdout.fnmatch_lines(["*passed*"])
         assert result.ret == 0
 
+    def test_non_fork_start_method(self, appdir):
+        appdir.create_test_module(
+            """
+            import multiprocessing
+            from urllib.request import urlopen
+
+            import pytest
+
+            @pytest.mark.parametrize("start_method", [
+                method for method in multiprocessing.get_all_start_methods()
+                if method != "fork"
+            ])
+            def test_a(live_server, start_method):
+                multiprocessing.set_start_method(start_method, force=True)
+                message = "response from a local route"
+
+                @live_server.app.route("/" + start_method, endpoint=start_method)
+                def closure():
+                    return message
+
+                live_server.start()
+                url = live_server.url("/" + start_method)
+                with urlopen(url, timeout=5) as response:
+                    assert response.read() == message.encode()
+                assert multiprocessing.get_start_method() == start_method
+            """
+        )
+        result = appdir.runpytest_subprocess(
+            "--no-start-live-server", "-o", "live_server_scope=function"
+        )
+        result.assert_outcomes(passed=len(multiprocessing.get_all_start_methods()) - 1)
+
     def test_stop_cleanly_join_exception(self, appdir, live_server, caplog):
         # timeout = 'a' here to force an exception when
         # attempting to self._process.join()