make ACK win32 specific

easier than bugfixing py3.11/macos

_trio_parallel_workers/__init__.py

@@ -61,11 +61,11 @@ def worker_behavior(recv_pipe, send_pipe, idle_timeout, init, retire):
     # between processes. (Trio will take charge via cancellation.)
     signal.signal(signal.SIGINT, signal.SIG_IGN)
     try:
-        if isinstance(init, bytes):  # true except on "fork"
-            # Signal successful startup to spawn/forkserver parents.
+        if sys.platform == "win32":
+            # Signal successful startup.
             send_pipe.send_bytes(ACK)
+        if isinstance(init, bytes):  # true except on "fork"
             init = loads(init)
-        if isinstance(retire, bytes):  # true except on "fork"
             retire = loads(retire)
         init()
         while safe_poll(recv_pipe, idle_timeout):

trio_parallel/_proc.py

@@ -69,7 +69,10 @@ async def start(self):
         self._child_send_pipe.close()
         self._child_recv_pipe.close()
 
-        # The following is mainly needed in the case of accidental recursive spawn
+        if sys.platform != "win32":
+            return
+
+        # Give a nice error on accidental recursive spawn instead of hanging
         async def wait_for_ack():
             try:
                 code = await self._receive_chan.receive()

trio_parallel/_tests/test_defaults.py

@@ -214,6 +214,8 @@ def test_startup_failure_doesnt_hang(tmp_path):
     )
     assert not result.stdout
     assert b"An attempt has been made to start a new process" in result.stderr
+    assert b"ExceptionGroup" not in result.stderr
+    assert b"MultiError" not in result.stderr
     assert result.returncode