state: preserve previous primary when all HA advertisers unhealthy

electPrimaryRoutes' all-unhealthy fallback used to pick
candidates[0] (lowest NodeID) regardless of who was prev primary.
Under cable-pull semantics IsOnline lags reality (long-poll TCP
half-open), so both routers stay in candidates and both go
Unhealthy via the prober — the fallback then churned primary to
a node that was itself unreachable.

Prefer prev when still in candidates; fall through to
candidates[0] only when prev is gone. Anti-blackhole property is
preserved (peers still see *some* primary), but no flap to a
known-bad node.

Update the property test's reference model to mirror the new
algorithm and split the unit test into existence
(KeepsAPrimary) and identity (PreservesPrevious) cases — the
old "lowest-ID wins" assertion encoded the bug.

Fixes #3203

hscontrol/state/node_store.go

@@ -620,9 +620,12 @@ func snapshotFromNodes(
 // electPrimaryRoutes picks the primary advertiser for each non-exit
 // prefix. The previous primary is preserved when it is still online
 // and healthy (anti-flap); otherwise the lowest-NodeID healthy
-// advertiser wins, falling back to the lowest-NodeID candidate when
-// every advertiser is unhealthy so peers see *some* primary instead
-// of none.
+// advertiser wins. When every advertiser is unhealthy the previous
+// primary is preserved if still a candidate, falling back to the
+// lowest-NodeID candidate so peers see *some* primary instead of
+// none. Anti-flap in the all-unhealthy case matters under cable-pull
+// where IsOnline lags reality and a naive lowest-ID fallback churns
+// primaries to a node that is itself unreachable (issue #3203).
 func electPrimaryRoutes(
 	nodes map[types.NodeID]types.Node,
 	prev map[netip.Prefix]types.NodeID,
@@ -675,7 +678,12 @@ func electPrimaryRoutes(
 		}
 
 		if !found && len(candidates) >= 1 {
-			selected = candidates[0]
+			if cur, ok := prev[prefix]; ok && slices.Contains(candidates, cur) {
+				selected = cur
+			} else {
+				selected = candidates[0]
+			}
+
 			found = true
 		}
 

hscontrol/state/primaries_property_test.go

@@ -98,7 +98,12 @@ func (m *primariesModel) updatePrimaries() {
 		}
 
 		if !found && len(nodes) >= 1 {
-			selected = nodes[0]
+			if cur, ok := m.primary[p]; ok && slices.Contains(nodes, cur) {
+				selected = cur
+			} else {
+				selected = nodes[0]
+			}
+
 			found = true
 		}
 

hscontrol/state/primaries_test.go

@@ -200,18 +200,41 @@ func TestPrimaries_RecoveryFromUnhealthyNoFlap(t *testing.T) {
 	f.requirePrimary(mp("192.168.1.0/24"), 2)
 }
 
-func TestPrimaries_AllUnhealthyKeepsLowestIDPrimary(t *testing.T) {
-	// When every advertiser is unhealthy the algorithm degrades to
-	// the lowest-ID advertiser rather than going dark — peers should
-	// still see *some* primary so connectivity can recover when one
-	// of them flips healthy.
+func TestPrimaries_AllUnhealthyKeepsAPrimary(t *testing.T) {
+	// Anti-blackhole: when every advertiser is unhealthy the
+	// algorithm keeps *some* primary so peers can recover once one
+	// flips healthy. The specific node is the prev primary when
+	// reachable (see PreservesPrevious); this test only pins the
+	// existence invariant.
+	prefix := mp("192.168.1.0/24")
 	f := newPrimariesFixture(t, 1, 2)
-	f.advertise(1, mp("192.168.1.0/24"))
-	f.advertise(2, mp("192.168.1.0/24"))
+	f.advertise(1, prefix)
+	f.advertise(2, prefix)
 	f.unhealthy(1)
 	f.unhealthy(2)
 
-	f.requirePrimary(mp("192.168.1.0/24"), 1)
+	_, ok := f.ns.PrimaryRouteFor(prefix)
+	require.True(t, ok, "all-unhealthy must still produce some primary")
+}
+
+func TestPrimaries_AllUnhealthyPreservesPrevious(t *testing.T) {
+	// Issue #3203: once a failover has moved primary to a higher-ID
+	// node, a subsequent all-unhealthy state must NOT churn primary
+	// back to the lowest-ID candidate. Under cable-pull semantics
+	// both nodes can linger as IsOnline=true (half-open TCP) and
+	// both go Unhealthy — naive `candidates[0]` would flap the
+	// primary to a node that is itself unreachable.
+	prefix := mp("10.0.0.0/24")
+	f := newPrimariesFixture(t, 1, 2)
+	f.advertise(1, prefix)
+	f.advertise(2, prefix)
+	f.requirePrimary(prefix, 1)
+
+	f.unhealthy(1)
+	f.requirePrimary(prefix, 2)
+
+	f.unhealthy(2)
+	f.requirePrimary(prefix, 2)
 }
 
 func TestPrimaries_ExitRouteNotElected(t *testing.T) {