Avoid uneeded queries in session stores if sid is not given.

actionpack/lib/action_dispatch/middleware/session/abstract_store.rb

@@ -191,8 +191,11 @@ def set_cookie(request, options)
 
         def load_session(env)
           stale_session_check! do
-            sid = current_session_id(env)
-            sid, session = get_session(env, sid)
+            if sid = current_session_id(env)
+              sid, session = get_session(env, sid)
+            else
+              sid, session = generate_sid, {}
+            end
             [sid, session]
           end
         end

actionpack/lib/action_dispatch/middleware/session/mem_cache_store.rb

@@ -25,7 +25,6 @@ def initialize(app, options = {})
 
       private
         def get_session(env, sid)
-          sid ||= generate_sid
           begin
             session = @pool.get(sid) || {}
           rescue MemCache::MemCacheError, Errno::ECONNREFUSED

activerecord/lib/active_record/session_store.rb

@@ -294,7 +294,6 @@ def destroy
     private
       def get_session(env, sid)
         Base.silence do
-          sid ||= generate_sid
           session = find_session(sid)
           env[SESSION_RECORD_KEY] = session
           [sid, session.data]