Give the Windows IO backend the error messages its siblings have

newsfragments/3509.bugfix.rst

@@ -0,0 +1,4 @@
+On Windows, :func:`trio.lowlevel.wait_readable`, :func:`trio.lowlevel.wait_writable`
+and :func:`trio.lowlevel.notify_closing` raised ``BusyResourceError`` and
+``ClosedResourceError`` without any message. They now explain themselves, as they
+already did on the epoll and kqueue backends.

src/trio/_core/_io_windows.py

@@ -740,7 +740,9 @@ async def _afd_poll(self, sock: _HasFileNo | int, mode: str) -> None:
             waiters = AFDWaiters()
             self._afd_waiters[base_handle] = waiters
         if getattr(waiters, mode) is not None:
-            raise _core.BusyResourceError
+            raise _core.BusyResourceError(
+                "another task is already reading / writing this socket",
+            )
         setattr(waiters, mode, _core.current_task())
         # Could potentially raise if the handle is somehow invalid; that's OK,
         # we let it escape.
@@ -822,7 +824,10 @@ def notify_closing(self, handle: Handle | int | _HasFileNo) -> None:
         handle = _get_base_socket(handle)
         waiters = self._afd_waiters.get(handle)
         if waiters is not None:
-            wake_all(waiters, _core.ClosedResourceError())
+            wake_all(
+                waiters,
+                _core.ClosedResourceError("another task closed this socket"),
+            )
             self._refresh_afd(handle)
 
     ################################################################

src/trio/_core/_tests/test_io.py

@@ -162,6 +162,12 @@ async def block_on_write() -> None:
     assert record == ["cancelled"]
 
 
+# The two real BusyResourceError texts from the three backends: epoll and
+# windows word it one way, kqueue another. Matching both asserts the message is
+# one of them rather than merely non-empty.
+BUSY_MESSAGE = r"already reading / writing this|multiple listeners for same ident"
+
+
 @read_socket_test
 async def test_double_read(socketpair: SocketPair, wait_readable: WaitSocket) -> None:
     a, _b = socketpair
@@ -170,7 +176,8 @@ async def test_double_read(socketpair: SocketPair, wait_readable: WaitSocket) ->
     async with _core.open_nursery() as nursery:
         nursery.start_soon(wait_readable, a)
         await wait_all_tasks_blocked()
-        with pytest.raises(_core.BusyResourceError):
+        # epoll and windows say one thing here, kqueue another, so match both.
+        with pytest.raises(_core.BusyResourceError, match=BUSY_MESSAGE):
             await wait_readable(a)
         nursery.cancel_scope.cancel()
 
@@ -184,7 +191,8 @@ async def test_double_write(socketpair: SocketPair, wait_writable: WaitSocket) -
     async with _core.open_nursery() as nursery:
         nursery.start_soon(wait_writable, a)
         await wait_all_tasks_blocked()
-        with pytest.raises(_core.BusyResourceError):
+        # epoll and windows say one thing here, kqueue another, so match both.
+        with pytest.raises(_core.BusyResourceError, match=BUSY_MESSAGE):
             await wait_writable(a)
         nursery.cancel_scope.cancel()
 
@@ -201,11 +209,15 @@ async def test_interrupted_by_close(
     a, _b = socketpair
 
     async def reader() -> None:
-        with pytest.raises(_core.ClosedResourceError):
+        with pytest.raises(
+            _core.ClosedResourceError, match="another task closed this "
+        ):
             await wait_readable(a)
 
     async def writer() -> None:
-        with pytest.raises(_core.ClosedResourceError):
+        with pytest.raises(
+            _core.ClosedResourceError, match="another task closed this "
+        ):
             await wait_writable(a)
 
     fill_socket(a)