Accept empty addr in UDP on_receive

* Fixes #304

tests/test_udp.py

@@ -284,6 +284,50 @@ async def run():
 
         self.loop.run_until_complete(run())
 
+    def test_socketpair(self):
+        peername = asyncio.Future(loop=self.loop)
+
+        class Proto(MyDatagramProto):
+            def datagram_received(self, data, addr):
+                super().datagram_received(data, addr)
+                peername.set_result(addr)
+
+        s1, s2 = socket.socketpair(socket.AF_UNIX, socket.SOCK_DGRAM, 0)
+
+        with s1, s2:
+            try:
+                f = self.loop.create_datagram_endpoint(
+                    lambda: Proto(loop=self.loop), sock=s1)
+            except TypeError as ex:
+                # asyncio in 3.5.0 doesn't have the 'sock' argument
+                if 'got an unexpected keyword argument' not in ex.args[0]:
+                    raise
+            else:
+                tr, pr = self.loop.run_until_complete(f)
+                self.assertIsInstance(pr, Proto)
+
+                s2.send(b'hello, socketpair')
+                addr = self.loop.run_until_complete(
+                    asyncio.wait_for(peername, 1, loop=self.loop))
+                if sys.platform.startswith('linux'):
+                    self.assertEqual(addr, None)
+                else:
+                    self.assertEqual(addr, '')
+                self.assertEqual(pr.nbytes, 17)
+
+                if not self.is_asyncio_loop():
+                    # asyncio doesn't support sendto(xx) on UDP sockets
+                    # https://git.io/Jfqbw
+                    data = b'from uvloop'
+                    tr.sendto(data)
+                    result = self.loop.run_until_complete(asyncio.wait_for(
+                        self.loop.run_in_executor(None, s2.recv, 1024),
+                        1, loop=self.loop))
+                    self.assertEqual(data, result)
+
+                tr.close()
+                self.loop.run_until_complete(pr.done)
+
 
 class Test_UV_UDP(_TestUDP, tb.UVTestCase):
 

uvloop/handles/udp.pyx

@@ -344,6 +344,12 @@ cdef void __uv_udp_on_receive(uv.uv_udp_t* handle,
 
     if addr is NULL:
         pyaddr = None
+    elif addr.sa_family == uv.AF_UNSPEC:
+        # https://github.com/MagicStack/uvloop/issues/304
+        IF UNAME_SYSNAME == "Linux":
+            pyaddr = None
+        ELSE:
+            pyaddr = ''
     else:
         try:
             pyaddr = __convert_sockaddr_to_pyaddr(addr)
@@ -356,13 +362,6 @@ cdef void __uv_udp_on_receive(uv.uv_udp_t* handle,
         udp._on_receive(None, exc, pyaddr)
         return
 
-    if pyaddr is None:
-        udp._fatal_error(
-            RuntimeError(
-                'uv_udp.receive callback: addr is NULL and nread >= 0'),
-            False)
-        return
-
     if nread == 0:
         data = b''
     else: