Allow setting port=0 to use OS-assigned port.

aiosmtpd/controller.py

@@ -440,6 +440,10 @@ def _trigger_server(self):
         # At this point, if self.hostname is Falsy, it most likely is "" (bind to all
         # addresses). In such case, it should be safe to connect to localhost)
         hostname = self.hostname or self._localhost
+        # If port is 0, we need to get the port that the OS assigned so that we
+        # can connect.
+        if self.port == 0:
+            self.port = self.server.sockets[0].getsockname()[1]
         with ExitStack() as stk:
             s = stk.enter_context(create_connection((hostname, self.port), 1.0))
             if self.ssl_context:

aiosmtpd/docs/NEWS.rst

@@ -14,6 +14,7 @@ Fixed/Improved
 --------------
 * All Controllers now have more rationale design, as they are now composited from a Base + a Mixin
 * A whole bunch of annotations
+* Allow using port=0 with TCP controllers to use an OS-assigned port (Closes #276).
 
 
 1.4.4.post2 (2023-01-19)

aiosmtpd/tests/test_server.py

@@ -297,6 +297,15 @@ def test_hostname_none(self):
         finally:
             cont.stop()
 
+    def test_port_zero(self):
+        cont = Controller(Sink(), port=0)
+        try:
+            cont.start()
+            # Ensure port on controller has been populated with the OS-assigned port
+            assert cont.port != 0
+        finally:
+            cont.stop()
+
     def test_testconn_raises(self, mocker: MockFixture):
         mocker.patch("socket.socket.recv", side_effect=RuntimeError("MockError"))
         cont = Controller(Sink(), hostname="")