ipn/{ipnlocal,localapi}: add debug verb to force spam IPN bus NetMap

To force the problem in its worst case scenario before fixing it.

Updates tailscale/corp#17859

Change-Id: I2c8b8e5f15c7801e1ab093feeafac52ec175a763
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -2367,6 +2367,20 @@ func (b *LocalBackend) DebugNotify(n ipn.Notify) {
 	b.send(n)
 }
 
+// DebugNotifyLastNetMap injects a fake notify message to clients,
+// repeating whatever the last netmap was.
+//
+// It should only be used via the LocalAPI's debug handler.
+func (b *LocalBackend) DebugNotifyLastNetMap() {
+	b.mu.Lock()
+	nm := b.netMap
+	b.mu.Unlock()
+
+	if nm != nil {
+		b.send(ipn.Notify{NetMap: nm})
+	}
+}
+
 // DebugForceNetmapUpdate forces a full no-op netmap update of the current
 // netmap in all the various subsystems (wireguard, magicsock, LocalBackend).
 //

ipn/localapi/localapi.go

@@ -599,6 +599,8 @@ func (h *Handler) serveDebug(w http.ResponseWriter, r *http.Request) {
 			break
 		}
 		h.b.DebugNotify(n)
+	case "notify-last-netmap":
+		h.b.DebugNotifyLastNetMap()
 	case "break-tcp-conns":
 		err = h.b.DebugBreakTCPConns()
 	case "break-derp-conns":