From 95b44713c193779ddd2ce61cbfaa39739752d410 Mon Sep 17 00:00:00 2001
From: Kristoffer Dalby <kristoffer@dalby.cc>
Date: Sat, 17 Oct 2026 12:33:37 +0000
Subject: [PATCH] state: keep probing a lone advertiser while it is unhealthy

---
 hscontrol/servertest/ha_health_test.go | 43 ++++++++++++++++++++++++++
 hscontrol/state/node_store.go          | 10 ++++--
 hscontrol/state/primaries_test.go      | 23 ++++++++++++++
 3 files changed, 73 insertions(+), 3 deletions(-)

diff --git a/hscontrol/servertest/ha_health_test.go b/hscontrol/servertest/ha_health_test.go
index c1904724b..223845694 100644
--- a/hscontrol/servertest/ha_health_test.go
+++ b/hscontrol/servertest/ha_health_test.go
@@ -392,3 +392,46 @@ func TestHAHealthProbe_NoHARoutes(t *testing.T) {
 	})
 	assert.False(t, dispatched, "no HA routes should produce no changes")
 }
+
+func TestHAHealthProbe_LastAdvertiserRecovers(t *testing.T) {
+	t.Parallel()
+
+	srv := servertest.NewServer(t)
+	user := srv.CreateUser(t, "ha-last")
+	route := netip.MustParsePrefix("10.107.0.0/24")
+	c1 := servertest.NewClient(t, srv, "ha-la-r1", servertest.WithUser(user))
+	c2 := servertest.NewClient(t, srv, "ha-la-r2", servertest.WithUser(user))
+
+	c1.WaitForPeers(t, 1, 10*time.Second)
+	c2.WaitForPeers(t, 1, 10*time.Second)
+
+	nodeID1 := advertiseAndApproveRoute(t, srv, c1, route)
+	nodeID2 := advertiseAndApproveRoute(t, srv, c2, route)
+
+	srv.State().SetNodeHealth(nodeID1, false)
+	require.Contains(t, srv.State().GetNodePrimaryRoutes(nodeID2), route)
+
+	// The remaining advertiser needs a probe before its route can return.
+	_, rc, err := srv.State().SetApprovedRoutes(nodeID2, nil)
+	require.NoError(t, err)
+	srv.App.Change(rc)
+	require.Empty(t, srv.State().GetNodePrimaryRoutes(nodeID1))
+
+	prober := state.NewHAHealthProber(
+		srv.State(),
+		types.HARouteConfig{
+			ProbeInterval: 30 * time.Second,
+			ProbeTimeout:  5 * time.Second,
+		},
+		srv.URL,
+		srv.App.MapBatcher().IsConnected,
+	)
+
+	ctx, cancel := context.WithTimeout(t.Context(), 10*time.Second)
+	defer cancel()
+
+	prober.ProbeOnce(ctx, srv.App.Change)
+
+	assert.True(t, srv.State().IsNodeHealthy(nodeID1))
+	assert.Contains(t, srv.State().GetNodePrimaryRoutes(nodeID1), route)
+}
diff --git a/hscontrol/state/node_store.go b/hscontrol/state/node_store.go
index 81b08ff7a..ca4656750 100644
--- a/hscontrol/state/node_store.go
+++ b/hscontrol/state/node_store.go
@@ -1046,8 +1046,8 @@ func (s *NodeStore) PrimaryRoutesForNode(id types.NodeID) []netip.Prefix {
 	return out
 }
 
-// HANodes returns the prefixes with two or more online advertisers, the
-// candidate set the HA prober needs to monitor.
+// HANodes returns prefixes with multiple online advertisers and lone unhealthy
+// advertisers that need another probe before their route can return.
 func (s *NodeStore) HANodes() map[netip.Prefix][]types.NodeID {
 	snap := s.data.Load()
 
@@ -1056,10 +1056,14 @@ func (s *NodeStore) HANodes() map[netip.Prefix][]types.NodeID {
 		slices.Sorted(maps.Keys(snap.nodesByID)),
 	)
 
+	unhealthy := func(id types.NodeID) bool {
+		return snap.nodesByID[id].Unhealthy
+	}
+
 	out := make(map[netip.Prefix][]types.NodeID)
 
 	for p, ids := range advertisers {
-		if len(ids) < 2 {
+		if len(ids) < 2 && !slices.ContainsFunc(ids, unhealthy) {
 			continue
 		}
 
diff --git a/hscontrol/state/primaries_test.go b/hscontrol/state/primaries_test.go
index 1ad13d156..8b17e8bd3 100644
--- a/hscontrol/state/primaries_test.go
+++ b/hscontrol/state/primaries_test.go
@@ -281,6 +281,29 @@ func TestPrimaries_HANodes(t *testing.T) {
 			},
 			want: map[netip.Prefix][]types.NodeID{},
 		},
+		{
+			name: "unhealthy-node-left-alone-is-still-monitored",
+			setup: func(f *primariesFixture) {
+				f.advertise(1, mp("192.168.1.0/24"))
+				f.advertise(2, mp("192.168.1.0/24"))
+				f.unhealthy(1)
+				f.approveRoutes(2)
+			},
+			want: map[netip.Prefix][]types.NodeID{
+				mp("192.168.1.0/24"): {1},
+			},
+		},
+		{
+			name: "node-left-alone-not-ha-once-healthy",
+			setup: func(f *primariesFixture) {
+				f.advertise(1, mp("192.168.1.0/24"))
+				f.advertise(2, mp("192.168.1.0/24"))
+				f.unhealthy(1)
+				f.approveRoutes(2)
+				f.healthy(1)
+			},
+			want: map[netip.Prefix][]types.NodeID{},
+		},
 		{
 			name: "two-nodes-same-prefix-is-ha",
 			setup: func(f *primariesFixture) {