[relay] evict foreign client cache on disconnect (#6015)

* [relay] evict foreign client cache on disconnect

When a foreign relay's TCP connection drops, the manager's
onServerDisconnected handler only triggered reconnect logic for the
home server; the disconnected foreign entry stayed in the relayClients
cache. Subsequent OpenConn calls reused the closed client until the
60-second cleanup tick evicted it, breaking peer connectivity through
that relay for up to a minute.

Evict the foreign entry from the cache on disconnect so the next
OpenConn dials a fresh client.

Also:
- Make the reconnect backoff cap configurable via WithMaxBackoffInterval
  ManagerOption; the previous hard-coded 60s constant forced
  TestAutoReconnect to sleep ~61s. Test now polls Ready() and finishes
  in ~2s.
- Add NB_HOME_RELAY_SERVERS env var that overrides the relay URL list
  received from management, so a peer can be pinned to a specific home
  relay (used by the netbird-conn-lab Edge 4 reproducer).

* [client] treat empty NB_HOME_RELAY_SERVERS as unset

Returning (urls=[], ok=true) when the env var contained only separators or
whitespace caused callers to wipe the mgmt-provided relay list, leaving the
peer with no relays. Treat a parsed-empty result the same as an unset env.

client/internal/connect.go

@@ -333,6 +333,10 @@ func (c *ConnectClient) run(mobileDependency MobileDependency, runningChan chan
 		c.statusRecorder.MarkSignalConnected()
 
 		relayURLs, token := parseRelayInfo(loginResp)
+		if override, ok := peer.OverrideRelayURLs(); ok {
+			log.Infof("overriding relay URLs from %s: %v", peer.EnvKeyNBHomeRelayServers, override)
+			relayURLs = override
+		}
 		peerConfig := loginResp.GetPeerConfig()
 
 		engineConfig, err := createEngineConfig(myPrivateKey, c.config, peerConfig, logPath)

client/internal/engine.go

@@ -944,7 +944,12 @@ func (e *Engine) handleRelayUpdate(update *mgmProto.RelayConfig) error {
 			return fmt.Errorf("update relay token: %w", err)
 		}
 
-		e.relayManager.UpdateServerURLs(update.Urls)
+		urls := update.Urls
+		if override, ok := peer.OverrideRelayURLs(); ok {
+			log.Infof("overriding relay URLs from %s: %v", peer.EnvKeyNBHomeRelayServers, override)
+			urls = override
+		}
+		e.relayManager.UpdateServerURLs(urls)
 
 		// Just in case the agent started with an MGM server where the relay was disabled but was later enabled.
 		// We can ignore all errors because the guard will manage the reconnection retries.

client/internal/peer/env.go

@@ -7,7 +7,8 @@ import (
 )
 
 const (
-	EnvKeyNBForceRelay = "NB_FORCE_RELAY"
+	EnvKeyNBForceRelay       = "NB_FORCE_RELAY"
+	EnvKeyNBHomeRelayServers = "NB_HOME_RELAY_SERVERS"
 )
 
 func IsForceRelayed() bool {
@@ -16,3 +17,28 @@ func IsForceRelayed() bool {
 	}
 	return strings.EqualFold(os.Getenv(EnvKeyNBForceRelay), "true")
 }
+
+// OverrideRelayURLs returns the relay server URL list set in
+// NB_HOME_RELAY_SERVERS (comma-separated) and a boolean indicating whether
+// the override is active. When the env var is unset, the boolean is false
+// and the caller should keep the list received from the management server.
+// Intended for lab/debug scenarios where a peer must pin to a specific home
+// relay regardless of what management offers.
+func OverrideRelayURLs() ([]string, bool) {
+	raw := os.Getenv(EnvKeyNBHomeRelayServers)
+	if raw == "" {
+		return nil, false
+	}
+	parts := strings.Split(raw, ",")
+	urls := make([]string, 0, len(parts))
+	for _, p := range parts {
+		p = strings.TrimSpace(p)
+		if p != "" {
+			urls = append(urls, p)
+		}
+	}
+	if len(urls) == 0 {
+		return nil, false
+	}
+	return urls, true
+}