Use selectors instead of select.poll in sync.WebSocket Server for mul…

…ti-platform support (#1349)

* use multiplatform selector instead of poll

* don't use os.pipe with the I/O multiplexing selector on win32

On the Win32 platform, only sockets can be used with I/O
multiplexing (such as that performed by selectors.DefaultSelector);
the pipe cannot be added to the selector. However, on the win32
platform, simply closing the listener socket is enough to cause the
call to select to return -- the additional pipe is redundant.

On Mac OS X (and possibly other BSD derivatives), closing the listener
socket isn't enough. In the interest of maximum compatibility, we
simply disable the use of os.pipe on the Win32 platform.

* exclude platform checks for win32 from coverage testing

pyproject.toml

@@ -61,6 +61,7 @@ source = [
 exclude_lines = [
     "except ImportError:",
     "if self.debug:",
+    "if sys.platform != \"win32\":",
     "pragma: no cover",
     "raise AssertionError",
     "raise NotImplementedError",

src/websockets/sync/server.py

@@ -3,9 +3,10 @@
 import http
 import logging
 import os
-import select
+import selectors
 import socket
 import ssl
+import sys
 import threading
 from types import TracebackType
 from typing import Any, Callable, Optional, Sequence, Type
@@ -199,7 +200,8 @@ def __init__(
         if logger is None:
             logger = logging.getLogger("websockets.server")
         self.logger = logger
-        self.shutdown_watcher, self.shutdown_notifier = os.pipe()
+        if sys.platform != "win32":
+            self.shutdown_watcher, self.shutdown_notifier = os.pipe()
 
     def serve_forever(self) -> None:
         """
@@ -214,15 +216,16 @@ def serve_forever(self) -> None:
                 server.serve_forever()
 
         """
-        poller = select.poll()
-        poller.register(self.socket)
-        poller.register(self.shutdown_watcher)
+        poller = selectors.DefaultSelector()
+        poller.register(self.socket, selectors.EVENT_READ)
+        if sys.platform != "win32":
+            poller.register(self.shutdown_watcher, selectors.EVENT_READ)
 
         while True:
-            poller.poll()
+            poller.select()
             try:
                 # If the socket is closed, this will raise an exception and exit
-                # the loop. So we don't need to check the return value of poll().
+                # the loop. So we don't need to check the return value of select().
                 sock, addr = self.socket.accept()
             except OSError:
                 break
@@ -235,7 +238,8 @@ def shutdown(self) -> None:
 
         """
         self.socket.close()
-        os.write(self.shutdown_notifier, b"x")
+        if sys.platform != "win32":
+            os.write(self.shutdown_notifier, b"x")
 
     def fileno(self) -> int:
         """