try deferred resolve for exit handler

llarp/handlers/exit.cpp

@@ -171,10 +171,20 @@ namespace llarp
           }
           else if(m_SNodeKeys.find(pubKey) == m_SNodeKeys.end())
           {
-            // we do not have it mapped
+            // we do not have it mapped, async obtain it
-            // map it
+            ObtainSNodeSession(
-            ip = ObtainServiceNodeIP(r);
+                r, [&](std::shared_ptr< exit::BaseSession > session) {
-            msg.AddINReply(ip, isV6);
+                  if(session && session->IsReady())
+                  {
+                    msg.AddINReply(m_KeyToIP[pubKey], isV6);
+                  }
+                  else
+                  {
+                    msg.AddNXReply();
+                  }
+                  reply(msg);
+                });
+            return;
           }
           else
           {
@@ -196,6 +206,14 @@ namespace llarp
       return true;
     }
 
+    void
+    ExitEndpoint::ObtainSNodeSession(const RouterID &router,
+                                     exit::SessionReadyFunc obtainCb)
+    {
+      ObtainServiceNodeIP(router);
+      m_SNodeSessions[router]->AddReadyHook(obtainCb);
+    }
+
     llarp_time_t
     ExitEndpoint::Now() const
     {

llarp/handlers/exit.hpp

@@ -120,6 +120,11 @@ namespace llarp
       huint128_t
       ObtainServiceNodeIP(const RouterID& router);
 
+      /// async obtain snode session and call callback when it's ready to send
+      void
+      ObtainSNodeSession(const RouterID& router,
+                         exit::SessionReadyFunc obtainCb);
+
       bool
       QueueSNodePacket(const llarp_buffer_t& buf, huint128_t from);