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>

hscontrol/state/maprequest_test.go

@@ -0,0 +1,134 @@
+package state
+
+import (
+	"net/netip"
+	"testing"
+
+	"github.com/juanfont/headscale/hscontrol/types"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"tailscale.com/tailcfg"
+	"tailscale.com/types/key"
+)
+
+func TestNetInfoFromMapRequest(t *testing.T) {
+	nodeID := types.NodeID(1)
+
+	tests := []struct {
+		name            string
+		currentHostinfo *tailcfg.Hostinfo
+		reqHostinfo     *tailcfg.Hostinfo
+		expectNetInfo   *tailcfg.NetInfo
+	}{
+		{
+			name:            "no current NetInfo - return nil",
+			currentHostinfo: nil,
+			reqHostinfo: &tailcfg.Hostinfo{
+				Hostname: "test-node",
+			},
+			expectNetInfo: nil,
+		},
+		{
+			name: "current has NetInfo, request has NetInfo - use request",
+			currentHostinfo: &tailcfg.Hostinfo{
+				NetInfo: &tailcfg.NetInfo{PreferredDERP: 1},
+			},
+			reqHostinfo: &tailcfg.Hostinfo{
+				Hostname: "test-node",
+				NetInfo:  &tailcfg.NetInfo{PreferredDERP: 2},
+			},
+			expectNetInfo: &tailcfg.NetInfo{PreferredDERP: 2},
+		},
+		{
+			name: "current has NetInfo, request has no NetInfo - use current",
+			currentHostinfo: &tailcfg.Hostinfo{
+				NetInfo: &tailcfg.NetInfo{PreferredDERP: 3},
+			},
+			reqHostinfo: &tailcfg.Hostinfo{
+				Hostname: "test-node",
+			},
+			expectNetInfo: &tailcfg.NetInfo{PreferredDERP: 3},
+		},
+		{
+			name: "current has NetInfo, no request Hostinfo - use current",
+			currentHostinfo: &tailcfg.Hostinfo{
+				NetInfo: &tailcfg.NetInfo{PreferredDERP: 4},
+			},
+			reqHostinfo:   nil,
+			expectNetInfo: &tailcfg.NetInfo{PreferredDERP: 4},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := NetInfoFromMapRequest(nodeID, tt.currentHostinfo, tt.reqHostinfo)
+
+			if tt.expectNetInfo == nil {
+				assert.Nil(t, result, "expected nil NetInfo")
+			} else {
+				require.NotNil(t, result, "expected non-nil NetInfo")
+				assert.Equal(t, tt.expectNetInfo.PreferredDERP, result.PreferredDERP, "DERP mismatch")
+			}
+		})
+	}
+}
+
+func TestNetInfoPreservationInRegistrationFlow(t *testing.T) {
+	nodeID := types.NodeID(1)
+
+	// This test reproduces the bug in registration flows where NetInfo was lost
+	// because we used the wrong hostinfo reference when calling NetInfoFromMapRequest
+	t.Run("registration_flow_bug_reproduction", func(t *testing.T) {
+		// Simulate existing node with NetInfo (before re-registration)
+		existingNodeHostinfo := &tailcfg.Hostinfo{
+			Hostname: "test-node",
+			NetInfo:  &tailcfg.NetInfo{PreferredDERP: 5},
+		}
+
+		// Simulate new registration request (no NetInfo)
+		newRegistrationHostinfo := &tailcfg.Hostinfo{
+			Hostname: "test-node",
+			OS:       "linux",
+			// NetInfo is nil - this is what comes from the registration request
+		}
+
+		// Simulate what was happening in the bug: we passed the "current node being modified"
+		// hostinfo (which has no NetInfo) instead of the existing node's hostinfo
+		nodeBeingModifiedHostinfo := &tailcfg.Hostinfo{
+			Hostname: "test-node",
+			// NetInfo is nil because this node is being modified/reset
+		}
+
+		// BUG: Using the node being modified (no NetInfo) instead of existing node (has NetInfo)
+		buggyResult := NetInfoFromMapRequest(nodeID, nodeBeingModifiedHostinfo, newRegistrationHostinfo)
+		assert.Nil(t, buggyResult, "Bug: Should return nil when using wrong hostinfo reference")
+
+		// CORRECT: Using the existing node's hostinfo (has NetInfo)
+		correctResult := NetInfoFromMapRequest(nodeID, existingNodeHostinfo, newRegistrationHostinfo)
+		assert.NotNil(t, correctResult, "Fix: Should preserve NetInfo when using correct hostinfo reference")
+		assert.Equal(t, 5, correctResult.PreferredDERP, "Should preserve the DERP region from existing node")
+	})
+}
+
+// Simple helper function for tests
+func createTestNodeSimple(id types.NodeID) *types.Node {
+	user := types.User{
+		Name: "test-user",
+	}
+
+	machineKey := key.NewMachine()
+	nodeKey := key.NewNode()
+
+	node := &types.Node{
+		ID:         id,
+		Hostname:   "test-node",
+		UserID:     uint(id),
+		User:       user,
+		MachineKey: machineKey.Public(),
+		NodeKey:    nodeKey.Public(),
+		IPv4:       &netip.Addr{},
+		IPv6:       &netip.Addr{},
+	}
+
+	return node
+}