client/tailscale,ipn/{ipnlocal,localapi}: add a pre-shutdown localAPI endpoint that terminates control connections. (#14028)

Adds a /disconnect-control local API endpoint that just shuts down control client.
This can be run before shutting down an HA subnet router/app connector replica - it will ensure
that all connection to control are dropped and control thus considers this node inactive and tells
peers to switch over to another replica. Meanwhile the existing connections keep working (assuming
that the replica is given some graceful shutdown period).

Updates tailscale/tailscale#14020

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

ipn/localapi/localapi.go

@@ -100,6 +100,7 @@ var handler = map[string]localAPIHandler{
 	"derpmap":                     (*Handler).serveDERPMap,
 	"dev-set-state-store":         (*Handler).serveDevSetStateStore,
 	"dial":                        (*Handler).serveDial,
+	"disconnect-control":          (*Handler).disconnectControl,
 	"dns-osconfig":                (*Handler).serveDNSOSConfig,
 	"dns-query":                   (*Handler).serveDNSQuery,
 	"drive/fileserver-address":    (*Handler).serveDriveServerAddr,
@@ -952,6 +953,22 @@ func (h *Handler) servePprof(w http.ResponseWriter, r *http.Request) {
 	servePprofFunc(w, r)
 }
 
+// disconnectControl is the handler for local API /disconnect-control endpoint that shuts down control client, so that
+// node no longer communicates with control. Doing this makes control consider this node inactive. This can be used
+// before shutting down a replica of HA subnet  router or app connector deployments to ensure that control tells the
+// peers to switch over to another replica whilst still maintaining th existing peer connections.
+func (h *Handler) disconnectControl(w http.ResponseWriter, r *http.Request) {
+	if !h.PermitWrite {
+		http.Error(w, "access denied", http.StatusForbidden)
+		return
+	}
+	if r.Method != httpm.POST {
+		http.Error(w, "use POST", http.StatusMethodNotAllowed)
+		return
+	}
+	h.b.DisconnectControl()
+}
+
 func (h *Handler) reloadConfig(w http.ResponseWriter, r *http.Request) {
 	if !h.PermitWrite {
 		http.Error(w, "access denied", http.StatusForbidden)