feat(esw): cascade two switches through a veth pair

Switch-to-switch links were the last shape left on the relay: the switch
fast path works by absorbing the *peer's* anchor, and a switch has none —
its ports are its own bridge, and a kernel interface belongs to exactly
one bridge, so two bridges cannot be joined by enslaving anything.

Join them with a veth pair instead: one end per bridge, each side
enslaving its own end and applying its own port mode to it — an access
port on one side and a trunk on the other behaves like the real cable it
models. uBridge's docker create_veth/delete_veth (the primitive every
Docker adapter is already born from) creates and destroys the pair; the
names derive from the link id (utils.kernel_anchor.kernel_cascade_names),
so each side computes its own end and the names survive a restart.

The pair belongs to the link, and the two ends are wired by two
independent uBridge processes — the project-open batch drives different
nodes concurrently — so the protocol is order-free: both sides create the
pair when their own end is missing (one create wins; the loser's failure
is re-checked against the kernel and "the pair is there" is not an
error), and either side's teardown destroys it (deleting one veth end
takes the other with it, so both converge on the same gone pair and the
second delif answers ENOENT). A pre-existing end — this switch restarted,
or a leftover from a previous life of the link id — is reused rather than
recreated, because the other end may still be a live port of the peer
switch; a stale tc qdisc is all a reused end can carry.

No new wire format was needed: a cascade is an anchor NIO on both ends,
the absorbed-anchor case plus the peer end's name. That also corrects the
filter routing — the single-sided rule exists because an absorbed anchor
is one interface, while a cascade has one per switch, so impairments go
to both ends like any other kernel link.

Tests: the wire format, the eligibility flip and the both-ends re-push on
node start (controller); the naming contract (utils); the switch-side
lifecycle — create, reuse, lost create race, delete destroys the pair
(compute routes).

Drive-by: the relay filter test's compute mock carried no awaitable
`put`, so tests/controller/test_kernel_datapath_link.py failed when run
alone while an argument-less full run passed; callers now get one.

tests/controller/test_kernel_datapath_link.py

@@ -53,19 +53,30 @@ async def _kernel_link(project, **kwargs):
 
 
 def _mock_compute_http(compute):
-    """Wire the async compute HTTP callbacks the relay _prepare path needs."""
+    """Wire the async compute HTTP callbacks the relay paths need: POST
+    (port reservation, NIO creation) and PUT (filter updates — without an
+    awaitable put, a relay filter update blows up on the sync MagicMock,
+    which is what made this file fail when run alone: some other module's
+    fixtures happened to mask it in a full run)."""
 
     async def subnet_callback(other):
         return ("192.168.1.1", "192.168.1.2")
 
     compute.get_ip_on_same_subnet.side_effect = subnet_callback
 
-    async def port_callback(path, data=None, **kwargs):
+    async def post_callback(path, data=None, **kwargs):
         response = MagicMock()
         response.json = {"udp_port": 1024}
         return response
 
-    compute.post.side_effect = port_callback
+    compute.post.side_effect = post_callback
+
+    async def put_callback(path, data=None, **kwargs):
+        response = MagicMock()
+        response.json = {}
+        return response
+
+    compute.put.side_effect = put_callback
 
 
 async def _relay_link(project):
@@ -855,9 +866,10 @@ async def test_switch_links_select_the_kernel_fast_path(project):
     """
     A link with an Ethernet switch endpoint rides the kernel fast path when
     the *peer* can anchor — the switch side needs no anchoring capability
-    (its bridge exists by construction). Switch-to-switch links never
-    qualify (a kernel interface belongs to exactly one bridge), nor do
-    anchor-less peers.
+    (its bridge exists by construction). A switch peer cascades through a
+    veth pair (one end per bridge — a kernel interface belongs to exactly
+    one bridge, so the pair is the only cable shape); anchor-less peers
+    stay on the relay.
     """
 
     compute = MagicMock()
@@ -874,10 +886,10 @@ async def test_switch_links_select_the_kernel_fast_path(project):
         link = await _link(project, switch, peer)
         assert link._kernel_datapath_eligible(switch, peer) is True, peer.node_type
 
-    # a switch peer: cascades need a veth pair between two bridges — relay
+    # a switch peer: cascade — the veth pair joins the two kernel bridges
     switch2 = _node(project, compute, "sw2", node_type="ethernet_switch")
     link = await _link(project, switch, switch2)
-    assert link._kernel_datapath_eligible(switch, switch2) is False
+    assert link._kernel_datapath_eligible(switch, switch2) is True
 
     # an anchor-less peer
     vpcs = _node(project, compute, "pc1", node_type="vpcs")
@@ -969,3 +981,78 @@ async def test_switch_link_repushes_the_switch_nio_on_node_start(project):
     relay_switch.put = AsyncioMagicMock()
     await relay_link.node_started(vpcs)
     assert not relay_switch.put.called
+
+
+# ---------------------------------------------------------------------------
+# Switch-to-switch cascade: one veth pair joins the two kernel bridges
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_prepare_cascade_link_emits_paired_anchor_nios(project):
+    """
+    A link between two Ethernet switches rides the kernel datapath through
+    one veth pair: each side gets an anchor NIO naming its own end and
+    carrying the other end's name (both sides create the pair when their
+    end is missing, so no ordering between the two NIO posts is needed).
+    Unlike the absorbed-anchor fast path, filters are two-sided — each end
+    is its own interface, so each direction of the link is impaired exactly
+    once like any other kernel link.
+    """
+
+    from gns3server.utils.kernel_anchor import kernel_cascade_names
+
+    compute = MagicMock()
+    compute.id = "compute-1"
+    sw1 = _node(project, compute, "sw1", node_type="ethernet_switch")
+    sw2 = _node(project, compute, "sw2", node_type="ethernet_switch")
+    link = await _link(project, sw1, sw2)
+    link._filters = {"delay": [10]}
+
+    entries = await link._prepare()
+    end0, end1 = kernel_cascade_names(link.id)
+
+    assert entries[0][3] == {
+        "type": "nio_anchor",
+        "anchor": end0,
+        "peer": end1,
+        "filters": {"delay": [10]},
+        "markers": {},
+        "suspend": False,
+    }
+    assert entries[1][3] == {
+        "type": "nio_anchor",
+        "anchor": end1,
+        "peer": end0,
+        "filters": {"delay": [10]},
+        "markers": {},
+        "suspend": False,
+    }
+    assert link.kernel_datapath is True
+
+
+@pytest.mark.asyncio
+async def test_node_started_repushes_both_cascade_ends(project):
+    """
+    A cascade link has a switch on both ends and each end is its own veth
+    half: whichever switch starts (recreating its bridge and losing its
+    half's membership), both ends are re-pushed — each switch's update
+    re-checks and re-joins its own half.
+    """
+
+    from tests.utils import AsyncioMagicMock
+
+    compute = MagicMock()
+    compute.id = "compute-1"
+    sw1 = _node(project, compute, "sw1", node_type="ethernet_switch")
+    sw2 = _node(project, compute, "sw2", node_type="ethernet_switch")
+    link = await _link(project, sw1, sw2)
+    entries = await link._prepare()
+    link._created = True
+    sw1.put = AsyncioMagicMock()
+    sw2.put = AsyncioMagicMock()
+
+    await link.node_started(sw1)
+
+    sw1.put.assert_called_once_with("/adapters/0/ports/0/nio", data=entries[0][3], timeout=120)
+    sw2.put.assert_called_once_with("/adapters/0/ports/0/nio", data=entries[1][3], timeout=120)