Ensure we can't connect to occupy port

Ref https://github.com/GNS3/gns3-gui/issues/1759

gns3server/controller/link.py

@@ -58,6 +58,9 @@ class Link:
         """
 
         port = node.get_port(adapter_number, port_number)
+        if port.link is not None:
+            raise aiohttp.web.HTTPConflict(text="Port is already used")
+
         self._link_type = port.link_type
 
         for other_node in self._nodes:
@@ -86,6 +89,7 @@ class Link:
             "node": node,
             "adapter_number": adapter_number,
             "port_number": port_number,
+            "port": port,
             "label": label
         })
 
@@ -93,6 +97,7 @@ class Link:
             yield from self.create()
             for n in self._nodes:
                 n["node"].add_link(self)
+                n["port"].link = self
             self._created = True
             self._project.controller.notification.emit("link.created", self.__json__())
 
@@ -123,8 +128,11 @@ class Link:
         """
         Delete the link
         """
-        for port in self._nodes:
-            port["node"].remove_link(self)
+        for n in self._nodes:
+            # It could be different of self if we rollback an already existing link
+            if n["port"].link == self:
+                n["port"].link = None
+                n["node"].remove_link(self)
 
     @asyncio.coroutine
     def start_capture(self, data_link_type="DLT_EN10MB", capture_file_name=None):

gns3server/controller/ports/port.py

@@ -26,6 +26,18 @@ class Port:
         self._adapter_number = adapter_number
         self._port_number = port_number
         self._name = name
+        self._link = None
+
+    @property
+    def link(self):
+        """
+        Link connected to the port
+        """
+        return self._link
+
+    @link.setter
+    def link(self, val):
+        self._link = val
 
     @property
     def adapter_number(self):

tests/controller/test_link.py

@@ -74,6 +74,7 @@ def test_add_node(async_run, project, compute):
     assert link._nodes == [
         {
             "node": node1,
+            "port": node1._ports[0],
             "adapter_number": 0,
             "port_number": 4,
             'label': {
@@ -100,6 +101,30 @@ def test_add_node(async_run, project, compute):
     assert link in node2.link
 
 
+def test_add_node_already_connected(async_run, project, compute):
+    """
+    Raise an error if we try to use an already connected port
+    """
+    project.dump = AsyncioMagicMock()
+
+    node1 = Node(project, compute, "node1", node_type="qemu")
+    node1._ports = [EthernetPort("E0", 0, 0, 4)]
+
+    link = Link(project)
+    link.create = AsyncioMagicMock()
+    link._project.controller.notification.emit = MagicMock()
+    async_run(link.add_node(node1, 0, 4))
+    node2 = Node(project, compute, "node2", node_type="qemu")
+    node2._ports = [EthernetPort("E0", 0, 0, 4)]
+    async_run(link.add_node(node2, 0, 4))
+
+    assert link.create.called
+    link2 = Link(project)
+    link2.create = AsyncioMagicMock()
+    with pytest.raises(aiohttp.web.HTTPConflict):
+        async_run(link2.add_node(node1, 0, 4))
+
+
 def test_add_node_cloud(async_run, project, compute):
     node1 = Node(project, compute, "node1", node_type="qemu")
     node1._ports = [EthernetPort("E0", 0, 0, 4)]