Use settings from Trio for Selector waker socketpair (#836)

docs/versionhistory.rst

@@ -3,6 +3,13 @@ Version history
 
 This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
+**UNRELEASED**
+
+- Configure ``SO_RCVBUF``, ``SO_SNDBUF`` and ``TCP_NODELAY`` on the selector
+  thread waker socket pair. This should improve the performance of ``wait_readable()``
+  and ``wait_writable()`` when using the ``ProactorEventLoop``
+  (`#836 <https://github.com/agronholm/anyio/pull/836>`_; PR by @graingert)
+
 **4.7.0**
 
 - Updated ``TaskGroup`` to work with asyncio's eager task factories

src/anyio/_core/_asyncio_selector_thread.py

@@ -21,6 +21,23 @@ def __init__(self) -> None:
         self._send, self._receive = socket.socketpair()
         self._send.setblocking(False)
         self._receive.setblocking(False)
+        # This somewhat reduces the amount of memory wasted queueing up data
+        # for wakeups. With these settings, maximum number of 1-byte sends
+        # before getting BlockingIOError:
+        #   Linux 4.8: 6
+        #   macOS (darwin 15.5): 1
+        #   Windows 10: 525347
+        # Windows you're weird. (And on Windows setting SNDBUF to 0 makes send
+        # blocking, even on non-blocking sockets, so don't do that.)
+        self._receive.setsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF, 1)
+        self._send.setsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF, 1)
+        # On Windows this is a TCP socket so this might matter. On other
+        # platforms this fails b/c AF_UNIX sockets aren't actually TCP.
+        try:
+            self._send.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)
+        except OSError:
+            pass
+
         self._selector.register(self._receive, EVENT_READ)
         self._closed = False