mapper: guard nil Hostinfo in addNextDNSMetadata

addNextDNSMetadata dereferenced node.Hostinfo().OS() without the .Valid() guard its siblings (RequestTags, TailNode) apply, so building the DNS config for a node with nil Hostinfo (a legacy NULL host_info row) panicked whenever a NextDNS resolver was configured. Guard the OS() dereference.

hscontrol/mapper/mapper.go

@@ -256,7 +256,13 @@ func addNextDNSMetadata(resolvers []*dnstype.Resolver, node types.NodeView) {
 
 		q := u.Query()
 		q.Set("device_name", node.Hostname())
-		q.Set("device_model", node.Hostinfo().OS())
+
+		// Guard Hostinfo().Valid() before dereferencing OS(): a node loaded
+		// from a legacy NULL host_info row has a nil Hostinfo, and OS() would
+		// panic. Mirrors the .Valid() guard in RequestTags/TailNode.
+		if node.Hostinfo().Valid() {
+			q.Set("device_model", node.Hostinfo().OS())
+		}
 
 		if ips := node.IPs(); len(ips) > 0 {
 			q.Set("device_ip", ips[0].String())

hscontrol/mapper/mapper_test.go

@@ -350,3 +350,27 @@ func TestBuildFromChangeFiltersUserProfilesByVisibility(t *testing.T) {
 			"n1 must not receive user2's profile; n2 is not ACL-visible to n1")
 	}
 }
+
+// TestGenerateDNSConfigNilHostinfoNoPanic proves generateDNSConfig does not
+// panic when a node's Hostinfo is nil (e.g. a legacy DB row with a NULL
+// host_info column). addNextDNSMetadata dereferenced node.Hostinfo().OS()
+// without the .Valid() guard its siblings (RequestTags, TailNode) apply, so
+// building such a node's map crashed the server whenever a NextDNS resolver
+// was configured.
+func TestGenerateDNSConfigNilHostinfoNoPanic(t *testing.T) {
+	node := (&types.Node{
+		Hostname: "legacy-node",
+		IPv4:     iap("100.64.0.1"),
+		// Hostinfo intentionally nil, as a legacy NULL host_info row loads.
+	}).View()
+
+	cfg := &types.Config{
+		TailcfgDNSConfig: &tailcfg.DNSConfig{
+			Resolvers: []*dnstype.Resolver{{Addr: "https://dns.nextdns.io/abc"}},
+		},
+	}
+
+	require.NotPanics(t, func() {
+		generateDNSConfig(cfg, node, nil)
+	}, "generateDNSConfig must not panic when a node has nil Hostinfo")
+}