Issue 3848: document the fact that epoll register raises an IOError if

an fd is registered twice, and add some additional epoll tests.  Patch
by Christian Heimes.

Doc/library/select.rst

@@ -160,6 +160,11 @@ Edge and Level Trigger Polling (epoll) Objects
 
    Register a fd descriptor with the epoll object.
 
+   .. note::
+
+     Registering a file descriptor that's already registered raises an
+     IOError -- contrary to :ref:`poll-objects`'s register.
+
 
 .. method:: epoll.modify(fd, eventmask)
 

Lib/test/test_epoll.py

@@ -95,6 +95,34 @@ class TestEPoll(unittest.TestCase):
         finally:
             ep.close()
 
+        # adding by object w/ fileno works, too.
+        ep = select.epoll(2)
+        try:
+            ep.register(server, select.EPOLLIN | select.EPOLLOUT)
+            ep.register(client, select.EPOLLIN | select.EPOLLOUT)
+        finally:
+            ep.close()
+
+        ep = select.epoll(2)
+        try:
+            # TypeError: argument must be an int, or have a fileno() method.
+            self.assertRaises(TypeError, ep.register, object(),
+                select.EPOLLIN | select.EPOLLOUT)
+            self.assertRaises(TypeError, ep.register, None,
+                select.EPOLLIN | select.EPOLLOUT)
+            # ValueError: file descriptor cannot be a negative integer (-1)
+            self.assertRaises(ValueError, ep.register, -1,
+                select.EPOLLIN | select.EPOLLOUT)
+            # IOError: [Errno 9] Bad file descriptor
+            self.assertRaises(IOError, ep.register, 10000,
+                select.EPOLLIN | select.EPOLLOUT)
+            # registering twice also raises an exception
+            ep.register(server, select.EPOLLIN | select.EPOLLOUT)
+            self.assertRaises(IOError, ep.register, server,
+                select.EPOLLIN | select.EPOLLOUT)
+        finally:
+            ep.close()
+
     def test_fromfd(self):
         server, client = self._connected_pair()