Give up on measuring TCP socket backlog

trio/tests/test_highlevel_open_tcp_listeners.py

@@ -52,65 +52,6 @@ async def test_open_tcp_listeners_specific_port_specific_host():
         assert listener.socket.getsockname() == (host, port)
 
 
-# Warning: this sleeps, and needs to use a real sleep -- MockClock won't
-# work.
-#
-# Also, this measurement technique often works, but not always: sometimes SYN
-# cookies get triggered, and then the backlog measured this way effectively
-# becomes infinite. (In particular, this has been observed happening on
-# Travis-CI.) To avoid this blowing up and eating all FDs / ephemeral ports,
-# we put an upper limit on the number of connections we attempt, and if we hit
-# it then we return the magic string "lots". Then
-# test_open_tcp_listeners_backlog uses a special path to handle this, treating
-# it as a success -- but at least we'll see in coverage if none of our test
-# runs are actually running the test properly.
-async def measure_backlog(listener, limit):
-    client_streams = []
-    try:
-        while True:
-            # Generally the response to the listen buffer being full is that
-            # the SYN gets dropped, and the client retries after 1 second. So
-            # we assume that any connect() call to localhost that takes >0.5
-            # seconds indicates a dropped SYN.
-            #
-            # Exception: on FreeBSD when the backlog is exhausted, connect
-            # has been observed to sometimes raise ConnectionResetError.
-            with trio.move_on_after(0.5) as cancel_scope:
-                try:
-                    client_stream = await open_stream_to_socket_listener(listener)
-                except ConnectionResetError:  # pragma: no cover
-                    break
-                client_streams.append(client_stream)
-            if cancel_scope.cancelled_caught:
-                break
-            if len(client_streams) >= limit:  # pragma: no cover
-                return "lots"
-    finally:
-        # The need for "no cover" here is subtle: see
-        # https://github.com/python-trio/trio/issues/522
-        for client_stream in client_streams:  # pragma: no cover
-            await client_stream.aclose()
-
-    return len(client_streams)
-
-
-@slow
-async def test_open_tcp_listeners_backlog():
-    # Operating systems don't necessarily use the exact backlog you pass
-    async def check_backlog(nominal, required_min, required_max):
-        listeners = await open_tcp_listeners(0, backlog=nominal)
-        actual = await measure_backlog(listeners[0], required_max + 10)
-        for listener in listeners:
-            await listener.aclose()
-        print("nominal", nominal, "actual", actual)
-        if actual == "lots":  # pragma: no cover
-            return
-        assert required_min <= actual <= required_max
-
-    await check_backlog(nominal=1, required_min=1, required_max=10)
-    await check_backlog(nominal=11, required_min=11, required_max=20)
-
-
 @binds_ipv6
 async def test_open_tcp_listeners_ipv6_v6only():
     # Check IPV6_V6ONLY is working properly
@@ -175,6 +116,7 @@ class FakeSocket(tsocket.SocketType):
 
     closed = attr.ib(default=False)
     poison_listen = attr.ib(default=False)
+    backlog = attr.ib(default=None)
 
     def getsockopt(self, level, option):
         if (level, option) == (tsocket.SOL_SOCKET, tsocket.SO_ACCEPTCONN):
@@ -188,6 +130,9 @@ async def bind(self, sockaddr):
         pass
 
     def listen(self, backlog):
+        assert self.backlog is None
+        assert backlog is not None
+        self.backlog = backlog
         if self.poison_listen:
             raise FakeOSError("whoops")
 
@@ -325,3 +270,26 @@ async def test_open_tcp_listeners_socket_fails_not_afnosupport():
     assert exc_info.value.errno == errno.EINVAL
     assert exc_info.value.__cause__ is None
     assert "nope" in str(exc_info.value)
+
+
+# We used to have an elaborate test that opened a real TCP listening socket
+# and then tried to measure its backlog by making connections to it. And most
+# of the time, it worked. But no matter what we tried, it was always fragile,
+# because it had to do things like use timeouts to guess when the listening
+# queue was full, sometimes the CI hosts go into SYN-cookie mode (where there
+# effectively is no backlog), sometimes the host might not be enough resources
+# to give us the full requested backlog... it was a mess. So now we just check
+# that the backlog argument is passed through correctly.
+async def test_open_tcp_listeners_backlog():
+    fsf = FakeSocketFactory(99)
+    tsocket.set_custom_socket_factory(fsf)
+    for (given, expected) in [
+        (None, 0xFFFF),
+        (99999999, 0xFFFF),
+        (10, 10),
+        (1, 1),
+    ]:
+        listeners = await open_tcp_listeners(0, backlog=given)
+        assert listeners
+        for listener in listeners:
+            assert listener.socket.backlog == expected