fix(compute): refuse a duplicate port NIO and wire before bookkeeping

The compute-side backstop of the controller's duplicate-port guard
(keep-port-links + live-port bind made the controller guard reliable;
this is the layer under it). IOU and Dynamips bound the adapter NIO
before the uBridge/hypervisor attach: a failed attach left the port's
binding replaced — the link that still owned the port lost its handle
and its host state leaked (the IOU EBUSY incident's 8 orphan TAPs).
Dynamips was worse: its wiring idempotence check silently skipped the
attach on a second binding and rebound only the bookkeeping. A port
that already carries an NIO is now refused, and both types bind the
NIO only after the wiring succeeded.

gns3server/compute/dynamips/nodes/router.py

@@ -1312,12 +1312,26 @@ class Router(KernelDatapathMixin, BaseNode):
         if not adapter.port_exists(port_number):
             raise DynamipsError(f"Port {port_number} does not exist on adapter {adapter}")
 
+        # The compute-side backstop of the controller's duplicate-port guard:
+        # a port carries at most one NIO, and overwriting the one a link
+        # still owns orphans that link's teardown (its host state leaks).
+        # Refusing also keeps the kernel wiring's own idempotence check from
+        # silently skipping the attach while rebinding the bookkeeping.
+        if adapter.get_nio(port_number) is not None:
+            raise DynamipsError(
+                "Port {port_number} on slot {slot_number} of router '{name}' already has a link".format(
+                    name=self._name, slot_number=slot_number, port_number=port_number
+                )
+            )
+
         if isinstance(nio, NIOBridge):
             # Kernel datapath: the port's anchor TAP is opened in the
             # hypervisor and enslaved into the NIO's per-link bridge (the
             # wiring is deferred to the node's start when no anchors exist).
-            adapter.add_nio(port_number, nio)
+            # Wire before bookkeeping: an attach that fails mid-way leaves
+            # the port unbound instead of half-wired.
             await self._attach_kernel_nio(slot_number, port_number, nio)
+            adapter.add_nio(port_number, nio)
             log.debug(
                 'Router "{name}" [{id}]: {nio} bound to port {slot_number}/{port_number}'.format(
                     name=self._name, id=self._id, nio=nio, slot_number=slot_number, port_number=port_number