servertest: require via compat netmaps to hold across ticks

Accepting the first matching tick let a netmap that matched once and
then drifted pass. Require several consecutive passing ticks; a netmap
that settles still passes, so no assertion loosens.

hscontrol/servertest/via_compat_test.go

@@ -250,16 +250,66 @@ func runViaMapCompat(t *testing.T, c *testcapture.Capture) {
 		}
 
 		t.Run(viewerName, func(t *testing.T) {
-			require.EventuallyWithT(t, func(collect *assert.CollectT) {
+			requireNetmapHolds(t, func(tt require.TestingT) {
 				nm := cl.Netmap()
-				require.NotNil(collect, nm, "netmap is nil")
+				require.NotNil(tt, nm, "netmap is nil")
 
-				compareNetmap(collect, nm, capture, clients, saasAddrs)
-			}, 30*time.Second, 100*time.Millisecond)
+				compareNetmap(tt, nm, capture, clients, saasAddrs)
+			})
 		})
 	}
 }
 
+// requireNetmapHolds retries check until it passes on several consecutive
+// ticks. One passing tick is not enough: a netmap that matches and then
+// drifts, such as a primary flipping away, is the bug these oracles guard.
+func requireNetmapHolds(t *testing.T, check func(require.TestingT)) {
+	t.Helper()
+
+	// Long enough for a batch still in flight to land.
+	const holdTicks = 5
+
+	held := 0
+
+	require.EventuallyWithT(t, func(collect *assert.CollectT) {
+		tick := &failRecorder{CollectT: collect}
+
+		// Deferred because FailNow exits the goroutine.
+		defer func() {
+			if tick.failed {
+				held = 0
+
+				return
+			}
+
+			held++
+			if held < holdTicks {
+				collect.Errorf("netmap held for %d of %d ticks", held, holdTicks)
+			}
+		}()
+
+		check(tick)
+	}, 30*time.Second, 100*time.Millisecond)
+}
+
+// failRecorder notes whether a tick failed, which [assert.CollectT] does
+// not expose.
+type failRecorder struct {
+	*assert.CollectT
+
+	failed bool
+}
+
+func (r *failRecorder) Errorf(format string, args ...any) {
+	r.failed = true
+	r.CollectT.Errorf(format, args...)
+}
+
+func (r *failRecorder) FailNow() {
+	r.failed = true
+	r.CollectT.FailNow()
+}
+
 // compareNetmap compares the headscale [tailcfg.MapResponse] against the
 // captured [netmap.NetworkMap] data in an IP-independent way. It validates:
 //   - Peer visibility (which peers are present, by hostname)
@@ -268,7 +318,7 @@ func runViaMapCompat(t *testing.T, c *testcapture.Capture) {
 //   - PrimaryRoutes per peer
 //   - PacketFilter (source, destination, ports) triples
 //
-// It takes [require.TestingT] so it can run inside [require.EventuallyWithT].
+// It takes [require.TestingT] so it can run inside [requireNetmapHolds].
 func compareNetmap(
 	t require.TestingT,
 	got *netmap.NetworkMap,

hscontrol/servertest/via_ha_compat_test.go

@@ -196,9 +196,9 @@ func runViaHACompat(t *testing.T, c *testcapture.Capture) {
 		}
 
 		t.Run(viewerName, func(t *testing.T) {
-			require.EventuallyWithT(t, func(collect *assert.CollectT) {
-				compareCaptureNetmap(collect, cl, capture, clients)
-			}, 30*time.Second, 100*time.Millisecond)
+			requireNetmapHolds(t, func(tt require.TestingT) {
+				compareCaptureNetmap(tt, cl, capture, clients)
+			})
 		})
 	}
 }
@@ -206,7 +206,7 @@ func runViaHACompat(t *testing.T, c *testcapture.Capture) {
 // compareCaptureNetmap compares headscale's [tailcfg.MapResponse] against a
 // [testcapture.Node]'s [netmap.NetworkMap] data. Same logic as [compareNetmap] but
 // reads from typed [testcapture] fields instead of goldenFile strings.
-// It takes [require.TestingT] so it can run inside [require.EventuallyWithT].
+// It takes [require.TestingT] so it can run inside [requireNetmapHolds].
 func compareCaptureNetmap(
 	t require.TestingT,
 	viewer *servertest.TestClient,