add a new is_readable method to SocketType (fix #760)

Author: ziirish

docs/source/reference-io.rst

@@ -403,6 +403,10 @@ Socket objects
          left in an unknown state – possibly open, and possibly
          closed. The only reasonable thing to do is to close it.
 
+   .. method:: is_readable
+
+      Check whether the socket is readable or not.
+
    .. method:: sendfile
 
       `Not implemented yet! <https://github.com/python-trio/trio/issues/45>`__

newsfragments/760.feature.rst

@@ -0,0 +1,3 @@
+We added a new ``is_readable`` method to the ``trio.socket.SocketType``
+object that allows you to check whether a socket is ready to be read
+or not.

trio/_socket.py

@@ -1,5 +1,6 @@
 import os as _os
 import sys as _sys
+import select
 import socket as _stdlib_socket
 from functools import wraps as _wraps
 
@@ -289,7 +290,7 @@ def socket(
 
 def _sniff_sockopts_for_fileno(family, type, proto, fileno):
     """Correct SOCKOPTS for given fileno, falling back to provided values.
-    
+
     """
     # Wrap the raw fileno into a Python socket object
     # This object might have the wrong metadata, but it lets us easily call getsockopt
@@ -478,6 +479,15 @@ def shutdown(self, flag):
         if flag in [_stdlib_socket.SHUT_WR, _stdlib_socket.SHUT_RDWR]:
             self._did_shutdown_SHUT_WR = True
 
+    def is_readable(self):
+        # use select.select on Windows, and select.poll everywhere else
+        if _sys.platform == "win32":
+            rready, _, _ = select.select([self._sock], [], [], 0)
+            return bool(rready)
+        p = select.poll()
+        p.register(self._sock, select.POLLIN)
+        return bool(p.poll(0))
+
     async def wait_writable(self):
         await _core.wait_writable(self._sock)
 

trio/tests/test_socket.py

@@ -404,6 +404,23 @@ async def test_SocketType_simple_server(address, socket_type):
             assert await client.recv(1) == b"x"
 
 
+async def test_SocketTupe_is_readable():
+    listener = tsocket.socket(tsocket.AF_INET)
+    client = tsocket.socket(tsocket.AF_INET)
+    with listener, client:
+        await listener.bind(('127.0.0.1', 0))
+        listener.listen(20)
+        addr = listener.getsockname()[:2]
+        async with _core.open_nursery() as nursery:
+            nursery.start_soon(client.connect, addr)
+            server, client_addr = await listener.accept()
+        with server:
+            assert not client.is_readable()
+            await server.send(b"x")
+            assert client.is_readable()
+            assert await client.recv(1) == b"x"
+
+
 # On some macOS systems, getaddrinfo likes to return V4-mapped addresses even
 # when we *don't* pass AI_V4MAPPED.
 # https://github.com/python-trio/trio/issues/580