[PortList] plugged() failure shouldn't dirty the ports

masque/ports.py

@@ -413,6 +413,10 @@ class PortList(metaclass=ABCMeta):
         if missing_b:
             raise PortError(f'Connection destination ports were not found: {missing_b}')
         a_names, b_names = list(zip(*connections.items(), strict=True))
+        used_names = list(chain(a_names, b_names))
+        duplicate_names = {name for name in used_names if used_names.count(name) > 1}
+        if duplicate_names:
+            raise PortError(f'Each port may appear in at most one connection: {duplicate_names}')
         a_ports = [self.ports[pp] for pp in a_names]
         b_ports = [self.ports[pp] for pp in b_names]
 

masque/test/test_ports.py

@@ -182,9 +182,30 @@ def test_port_list_plugged_missing_port_raises() -> None:
         pl.plugged({"missing": "B"})
     assert set(pl.ports) == {"A", "B"}
 
+
+def test_port_list_plugged_reused_port_raises_atomically() -> None:
+    class MyPorts(PortList):
+        def __init__(self) -> None:
+            self._ports = {"A": Port((0, 0), None), "B": Port((0, 0), None), "C": Port((0, 0), None)}
+
+        @property
+        def ports(self) -> dict[str, Port]:
+            return self._ports
+
+        @ports.setter
+        def ports(self, val: dict[str, Port]) -> None:
+            self._ports = val
+
+    for connections in ({"A": "A"}, {"A": "B", "C": "B"}):
+        pl = MyPorts()
+        with pytest.raises(PortError, match="Each port may appear in at most one connection"):
+            pl.plugged(connections)
+        assert set(pl.ports) == {"A", "B", "C"}
+
+    pl = MyPorts()
     with pytest.raises(PortError, match="Connection destination ports were not found"):
         pl.plugged({"A": "missing"})
-    assert set(pl.ports) == {"A", "B"}
+    assert set(pl.ports) == {"A", "B", "C"}
 
 
 def test_port_list_plugged_mismatch() -> None: