fix: add a helpful hint for when EADDRINUSE happens during startup (#1526)

bdraco · web-flow · commit 48dbb7190a4f · 2025-02-14T12:21:31.000-06:00
diff --git a/src/zeroconf/_utils/net.py b/src/zeroconf/_utils/net.py
@@ -262,6 +262,25 @@ def new_socket(
                 bind_tup,
             )
             return None
+        if ex.errno == errno.EADDRINUSE:
+            if sys.platform.startswith("darwin") or sys.platform.startswith("freebsd"):
+                log.error(
+                    "Address in use when binding to %s; "
+                    "On BSD based systems sharing the same port with another "
+                    "stack may require processes to run with the same UID; "
+                    "When using avahi, make sure disallow-other-stacks is set"
+                    " to no in avahi-daemon.conf",
+                    bind_tup,
+                )
+            else:
+                log.error(
+                    "Address in use when binding to %s; "
+                    "When using avahi, make sure disallow-other-stacks is set"
+                    " to no in avahi-daemon.conf",
+                    bind_tup,
+                )
+            # This is still a fatal error as its not going to work
+            # if we can't hear the traffic coming in.
         raise
     log.debug("Created socket %s", s)
     return s
diff --git a/tests/utils/test_net.py b/tests/utils/test_net.py
@@ -260,6 +260,40 @@ def _mock_socket(*args, **kwargs):
         netutils.new_socket(("0.0.0.0", 0))  # type: ignore[arg-type]
 
 
+def test_bind_raises_address_in_use(caplog: pytest.LogCaptureFixture) -> None:
+    """Test bind failing in new_socket returns None on EADDRINUSE."""
+
+    def _mock_socket(*args, **kwargs):
+        sock = MagicMock()
+        sock.bind = MagicMock(side_effect=OSError(errno.EADDRINUSE, f"Error: {errno.EADDRINUSE}"))
+        return sock
+
+    with (
+        pytest.raises(OSError),
+        patch.object(sys, "platform", "darwin"),
+        patch("socket.socket", _mock_socket),
+    ):
+        netutils.new_socket(("0.0.0.0", 0))  # type: ignore[arg-type]
+    assert (
+        "On BSD based systems sharing the same port with "
+        "another stack may require processes to run with the same UID"
+    ) in caplog.text
+    assert (
+        "When using avahi, make sure disallow-other-stacks is set to no in avahi-daemon.conf" in caplog.text
+    )
+
+    caplog.clear()
+    with pytest.raises(OSError), patch.object(sys, "platform", "linux"), patch("socket.socket", _mock_socket):
+        netutils.new_socket(("0.0.0.0", 0))  # type: ignore[arg-type]
+    assert (
+        "On BSD based systems sharing the same port with "
+        "another stack may require processes to run with the same UID"
+    ) not in caplog.text
+    assert (
+        "When using avahi, make sure disallow-other-stacks is set to no in avahi-daemon.conf" in caplog.text
+    )
+
+
 def test_new_respond_socket_new_socket_returns_none():
     """Test new_respond_socket returns None if new_socket returns None."""
     with patch.object(netutils, "new_socket", return_value=None):
