state: ensure netinfo is preserved and not removed

the client will send a lot of fields as `nil` if they have
not changed. NetInfo, which is inside Hostinfo, is one of those
fields and we often would override the whole hostinfo meaning that
we would remove netinfo if it hadnt changed.

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

integration/auth_key_test.go

@@ -66,6 +66,9 @@ func TestAuthKeyLogoutAndReloginSameUser(t *testing.T) {
 			}
 			requireAllClientsOnline(t, headscale, expectedNodes, true, "all clients should be connected", 30*time.Second)
 
+			// Validate that all nodes have NetInfo and DERP servers before logout
+			requireAllClientsNetInfoAndDERP(t, headscale, expectedNodes, "all clients should have NetInfo and DERP before logout", 1*time.Minute)
+
 			// assertClientsState(t, allClients)
 
 			clientIPs := make(map[TailscaleClient][]netip.Addr)
@@ -149,6 +152,9 @@ func TestAuthKeyLogoutAndReloginSameUser(t *testing.T) {
 
 			requireAllClientsOnline(t, headscale, expectedNodes, true, "all clients should be connected to batcher", 120*time.Second)
 
+			// Validate that all nodes have NetInfo and DERP servers after reconnection
+			requireAllClientsNetInfoAndDERP(t, headscale, expectedNodes, "all clients should have NetInfo and DERP after reconnection", 1*time.Minute)
+
 			err = scenario.WaitForTailscaleSync()
 			assertNoErrSync(t, err)
 
@@ -191,6 +197,60 @@ func TestAuthKeyLogoutAndReloginSameUser(t *testing.T) {
 	}
 }
 
+// requireAllClientsNetInfoAndDERP validates that all nodes have NetInfo in the database
+// and a valid DERP server based on the NetInfo. This function follows the pattern of
+// requireAllClientsOnline by using hsic.DebugNodeStore to get the database state.
+func requireAllClientsNetInfoAndDERP(t *testing.T, headscale ControlServer, expectedNodes []types.NodeID, message string, timeout time.Duration) {
+	t.Helper()
+
+	startTime := time.Now()
+	t.Logf("requireAllClientsNetInfoAndDERP: Starting validation at %s - %s", startTime.Format(TimestampFormat), message)
+
+	require.EventuallyWithT(t, func(c *assert.CollectT) {
+		// Get nodestore state
+		nodeStore, err := headscale.DebugNodeStore()
+		assert.NoError(c, err, "Failed to get nodestore debug info")
+		if err != nil {
+			return
+		}
+
+		// Validate node counts first
+		expectedCount := len(expectedNodes)
+		assert.Equal(c, expectedCount, len(nodeStore), "NodeStore total nodes mismatch")
+
+		// Check each expected node
+		for _, nodeID := range expectedNodes {
+			node, exists := nodeStore[nodeID]
+			assert.True(c, exists, "Node %d not found in nodestore", nodeID)
+			if !exists {
+				continue
+			}
+
+			// Validate that the node has Hostinfo
+			assert.NotNil(c, node.Hostinfo, "Node %d (%s) should have Hostinfo", nodeID, node.Hostname)
+			if node.Hostinfo == nil {
+				continue
+			}
+
+			// Validate that the node has NetInfo
+			assert.NotNil(c, node.Hostinfo.NetInfo, "Node %d (%s) should have NetInfo in Hostinfo", nodeID, node.Hostname)
+			if node.Hostinfo.NetInfo == nil {
+				continue
+			}
+
+			// Validate that the node has a valid DERP server (PreferredDERP should be > 0)
+			preferredDERP := node.Hostinfo.NetInfo.PreferredDERP
+			assert.Greater(c, preferredDERP, 0, "Node %d (%s) should have a valid DERP server (PreferredDERP > 0), got %d", nodeID, node.Hostname, preferredDERP)
+
+			t.Logf("Node %d (%s) has valid NetInfo with DERP server %d", nodeID, node.Hostname, preferredDERP)
+		}
+	}, timeout, 2*time.Second, message)
+
+	endTime := time.Now()
+	duration := endTime.Sub(startTime)
+	t.Logf("requireAllClientsNetInfoAndDERP: Completed validation at %s - Duration: %v - %s", endTime.Format(TimestampFormat), duration, message)
+}
+
 func assertLastSeenSet(t *testing.T, node *v1.Node) {
 	assert.NotNil(t, node)
 	assert.NotNil(t, node.GetLastSeen())