control/controlclient,health,tailcfg: refactor control health messages (#15839)

* control/controlclient,health,tailcfg: refactor control health messages

Updates tailscale/corp#27759

Signed-off-by: James Sanderson <jsanderson@tailscale.com>
Signed-off-by: Paul Scott <408401+icio@users.noreply.github.com>
Co-authored-by: Paul Scott <408401+icio@users.noreply.github.com>

control/controlclient/map.go

@@ -6,7 +6,10 @@ package controlclient
 import (
 	"cmp"
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
+	"io"
 	"maps"
 	"net"
 	"reflect"
@@ -828,6 +831,16 @@ func (ms *mapSession) sortedPeers() []tailcfg.NodeView {
 func (ms *mapSession) netmap() *netmap.NetworkMap {
 	peerViews := ms.sortedPeers()
 
+	// Convert all ms.lastHealth to the new [netmap.NetworkMap.DisplayMessages].
+	var msgs map[tailcfg.DisplayMessageID]tailcfg.DisplayMessage
+	for _, h := range ms.lastHealth {
+		mak.Set(&msgs, tailcfg.DisplayMessageID("control-health-"+strhash(h)), tailcfg.DisplayMessage{
+			Title:    "Coordination server reports an issue",
+			Severity: tailcfg.SeverityMedium,
+			Text:     "The coordination server is reporting a health issue: " + h,
+		})
+	}
+
 	nm := &netmap.NetworkMap{
 		NodeKey:           ms.publicNodeKey,
 		PrivateKey:        ms.privateNodeKey,
@@ -842,7 +855,7 @@ func (ms *mapSession) netmap() *netmap.NetworkMap {
 		SSHPolicy:         ms.lastSSHPolicy,
 		CollectServices:   ms.collectServices,
 		DERPMap:           ms.lastDERPMap,
-		ControlHealth:     ms.lastHealth,
+		DisplayMessages:   msgs,
 		TKAEnabled:        ms.lastTKAInfo != nil && !ms.lastTKAInfo.Disabled,
 	}
 
@@ -868,5 +881,12 @@ func (ms *mapSession) netmap() *netmap.NetworkMap {
 	if DevKnob.ForceProxyDNS() {
 		nm.DNS.Proxied = true
 	}
+
 	return nm
 }
+
+func strhash(h string) string {
+	s := sha256.New()
+	io.WriteString(s, h)
+	return hex.EncodeToString(s.Sum(nil))
+}