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.go

@@ -0,0 +1,50 @@
+// Package state provides pure functions for processing MapRequest data.
+// These functions are extracted from UpdateNodeFromMapRequest to improve
+// testability and maintainability.
+
+package state
+
+import (
+	"github.com/juanfont/headscale/hscontrol/types"
+	"github.com/rs/zerolog/log"
+	"tailscale.com/tailcfg"
+)
+
+// NetInfoFromMapRequest determines the correct NetInfo to use.
+// Returns the NetInfo that should be used for this request.
+func NetInfoFromMapRequest(
+	nodeID types.NodeID,
+	currentHostinfo *tailcfg.Hostinfo,
+	reqHostinfo *tailcfg.Hostinfo,
+) *tailcfg.NetInfo {
+	// If request has NetInfo, use it
+	if reqHostinfo != nil && reqHostinfo.NetInfo != nil {
+		return reqHostinfo.NetInfo
+	}
+
+	// Otherwise, use current NetInfo if available
+	if currentHostinfo != nil && currentHostinfo.NetInfo != nil {
+		log.Debug().
+			Caller().
+			Uint64("node.id", nodeID.Uint64()).
+			Int("preferredDERP", currentHostinfo.NetInfo.PreferredDERP).
+			Msg("using NetInfo from previous Hostinfo in MapRequest")
+		return currentHostinfo.NetInfo
+	}
+
+	// No NetInfo available anywhere - log for debugging
+	var hostname string
+	if reqHostinfo != nil {
+		hostname = reqHostinfo.Hostname
+	} else if currentHostinfo != nil {
+		hostname = currentHostinfo.Hostname
+	}
+
+	log.Debug().
+		Caller().
+		Uint64("node.id", nodeID.Uint64()).
+		Str("node.hostname", hostname).
+		Msg("node sent update but has no NetInfo in request or database")
+
+	return nil
+}