authmailbox: pass fallback service URLs into MultiSubscriptionConfig

Add support for passing fallback authmailbox service URLs into
MultiSubscriptionConfig. This prepares the config structure to handle
server-defined fallback behavior across multiple subscriptions.

Author: ffranr

authmailbox/multi_subscription.go

@@ -53,6 +53,9 @@ type MultiSubscriptionConfig struct {
 	// clients. All fields except the ServerAddress are used to create
 	// new mailbox clients when needed.
 	BaseClientConfig ClientConfig
+
+	// FallbackMboxURLs are fallback proof courier AuthMailbox services.
+	FallbackMboxURLs []url.URL
 }
 
 // NewMultiSubscription creates a new MultiSubscription instance.

tapgarden/custodian.go

@@ -244,6 +244,7 @@ func NewCustodian(cfg *CustodianConfig) *Custodian {
 					MinBackoff: backoffCfg.InitialBackoff,
 					MaxBackoff: backoffCfg.MaxBackoff,
 				},
+				FallbackMboxURLs: cfg.FallbackMboxURLs,
 			},
 		),
 		ContextGuard: &fn.ContextGuard{