[relay] Refactor initial Relay connection (#2800)

Can support firewalls with restricted WS rules

allow to run engine without Relay servers
keep up to date Relay address changes

relay/client/guard.go

@@ -4,65 +4,120 @@ import (
 	"context"
 	"time"
 
+	"github.com/cenkalti/backoff/v4"
 	log "github.com/sirupsen/logrus"
 )
 
 var (
-	reconnectingTimeout = 5 * time.Second
+	reconnectingTimeout = 60 * time.Second
 )
 
 // Guard manage the reconnection tries to the Relay server in case of disconnection event.
 type Guard struct {
-	ctx         context.Context
-	relayClient *Client
+	// OnNewRelayClient is a channel that is used to notify the relay client about a new relay client instance.
+	OnNewRelayClient chan *Client
+	serverPicker     *ServerPicker
 }
 
 // NewGuard creates a new guard for the relay client.
-func NewGuard(context context.Context, relayClient *Client) *Guard {
+func NewGuard(sp *ServerPicker) *Guard {
 	g := &Guard{
-		ctx:         context,
-		relayClient: relayClient,
+		OnNewRelayClient: make(chan *Client, 1),
+		serverPicker:     sp,
 	}
 	return g
 }
 
-// OnDisconnected is called when the relay client is disconnected from the relay server. It will trigger the reconnection
+// StartReconnectTrys is called when the relay client is disconnected from the relay server.
+// It attempts to reconnect to the relay server. The function first tries a quick reconnect
+// to the same server that was used before, if the server URL is still valid. If the quick
+// reconnect fails, it starts a ticker to periodically attempt server picking until it
+// succeeds or the context is done.
+//
+// Parameters:
+// - ctx: The context to control the lifecycle of the reconnection attempts.
+// - relayClient: The relay client instance that was disconnected.
 // todo prevent multiple reconnection instances. In the current usage it should not happen, but it is better to prevent
-func (g *Guard) OnDisconnected() {
-	if g.quickReconnect() {
+func (g *Guard) StartReconnectTrys(ctx context.Context, relayClient *Client) {
+	if relayClient == nil {
+		goto RETRY
+	}
+	if g.isServerURLStillValid(relayClient) && g.quickReconnect(ctx, relayClient) {
 		return
 	}
 
-	ticker := time.NewTicker(reconnectingTimeout)
+RETRY:
+	ticker := exponentTicker(ctx)
 	defer ticker.Stop()
 
 	for {
 		select {
 		case <-ticker.C:
-			err := g.relayClient.Connect()
-			if err != nil {
-				log.Errorf("failed to reconnect to relay server: %s", err)
+			if err := g.retry(ctx); err != nil {
+				log.Errorf("failed to pick new Relay server: %s", err)
 				continue
 			}
 			return
-		case <-g.ctx.Done():
+		case <-ctx.Done():
 			return
 		}
 	}
 }
 
-func (g *Guard) quickReconnect() bool {
-	ctx, cancel := context.WithTimeout(g.ctx, 1500*time.Millisecond)
+func (g *Guard) retry(ctx context.Context) error {
+	log.Infof("try to pick up a new Relay server")
+	relayClient, err := g.serverPicker.PickServer(ctx)
+	if err != nil {
+		return err
+	}
+
+	// prevent to work with a deprecated Relay client instance
+	g.drainRelayClientChan()
+
+	g.OnNewRelayClient <- relayClient
+	return nil
+}
+
+func (g *Guard) quickReconnect(parentCtx context.Context, rc *Client) bool {
+	ctx, cancel := context.WithTimeout(parentCtx, 1500*time.Millisecond)
 	defer cancel()
 	<-ctx.Done()
 
-	if g.ctx.Err() != nil {
+	if parentCtx.Err() != nil {
 		return false
 	}
+	log.Infof("try to reconnect to Relay server: %s", rc.connectionURL)
 
-	if err := g.relayClient.Connect(); err != nil {
+	if err := rc.Connect(); err != nil {
 		log.Errorf("failed to reconnect to relay server: %s", err)
 		return false
 	}
 	return true
 }
+
+func (g *Guard) drainRelayClientChan() {
+	select {
+	case <-g.OnNewRelayClient:
+	default:
+	}
+}
+
+func (g *Guard) isServerURLStillValid(rc *Client) bool {
+	for _, url := range g.serverPicker.ServerURLs.Load().([]string) {
+		if url == rc.connectionURL {
+			return true
+		}
+	}
+	return false
+}
+
+func exponentTicker(ctx context.Context) *backoff.Ticker {
+	bo := backoff.WithContext(&backoff.ExponentialBackOff{
+		InitialInterval: 2 * time.Second,
+		Multiplier:      2,
+		MaxInterval:     reconnectingTimeout,
+		Clock:           backoff.SystemClock,
+	}, ctx)
+
+	return backoff.NewTicker(bo)
+}