[client] abort Start if context cancelled while waiting for signal stream

receiveSignalEvents blocks in WaitStreamConnected until the signal stream
connects or the context is cancelled. If Stop cancelled e.ctx while Start
was parked there, Start kept going: it started the remaining subsystems on
a cancelled context and marked a shutting-down engine as started. Return
the context error from receiveSignalEvents and propagate it from Start, so
the deferred cleanup runs and the cancellation reaches the caller.

client/internal/engine.go

@@ -601,7 +601,9 @@ func (e *Engine) Start(netbirdConfig *mgmProto.NetbirdConfig, mgmtURL *url.URL)
 	e.srWatcher = guard.NewSRWatcher(e.signal, e.relayManager, e.mobileDep.IFaceDiscover, iceCfg)
 	e.srWatcher.Start(peer.IsForceRelayed())
 
-	e.receiveSignalEvents()
+	if err = e.receiveSignalEvents(); err != nil {
+		return err
+	}
 	e.receiveManagementEvents()
 	e.receiveJobEvents()
 
@@ -1712,7 +1714,7 @@ func (e *Engine) createPeerConn(pubKey string, allowedIPs []netip.Prefix, agentV
 }
 
 // receiveSignalEvents connects to the Signal Service event stream to negotiate connection with remote peers
-func (e *Engine) receiveSignalEvents() {
+func (e *Engine) receiveSignalEvents() error {
 	e.shutdownWg.Add(1)
 	go func() {
 		defer e.shutdownWg.Done()
@@ -1778,6 +1780,10 @@ func (e *Engine) receiveSignalEvents() {
 
 	// todo: consider to remove this blocker. I do not see benefit to block the Start operations
 	e.signal.WaitStreamConnected(e.ctx)
+	if err := e.ctx.Err(); err != nil {
+		return fmt.Errorf("wait for signal stream: %w", err)
+	}
+	return nil
 }
 
 func (e *Engine) parseNATExternalIPMappings() []string {