LMail: destruct cgroup_xattr with the global Lua state

When the current Lua thread is gone already, it must not be used to
destruct the `cgroup_xattr` field.

debian/changelog

@@ -1,6 +1,6 @@
 cm4all-qrelay (0.25) unstable; urgency=low
 
-  * 
+  * fix use-after-free crash bug
 
  --   
 

src/Connection.cxx

@@ -78,7 +78,7 @@ QmqpRelayConnection::OnRequest(AllocatedArray<std::byte> &&payload)
 
 	handler->Push(L);
 
-	NewLuaMail(L, std::move(mail), peer_cred);
+	NewLuaMail(L, GetMainState(), std::move(mail), peer_cred);
 
 	Resume(L, 1);
 }

src/LMail.cxx

@@ -389,9 +389,10 @@ RegisterLuaMail(lua_State *L)
 }
 
 MutableMail *
-NewLuaMail(lua_State *L, MutableMail &&src, const struct ucred &peer_cred)
+NewLuaMail(lua_State *L, lua_State *main_L,
+	   MutableMail &&src, const struct ucred &peer_cred)
 {
-	return LuaMail::New(L, L, std::move(src), peer_cred);
+	return LuaMail::New(L, main_L, std::move(src), peer_cred);
 }
 
 MutableMail &

src/LMail.hxx

@@ -12,8 +12,14 @@ struct MutableMail;
 void
 RegisterLuaMail(lua_State *L);
 
+/**
+ * @param L the lua_State on whose stack the new object will be pushed
+ * @param main_L the lua_State which is used to destruct Lua values
+ * owned by the new object
+ */
 MutableMail *
-NewLuaMail(lua_State *L, MutableMail &&src, const struct ucred &peer_cred);
+NewLuaMail(lua_State *L, lua_State *main_L,
+	   MutableMail &&src, const struct ucred &peer_cred);
 
 MutableMail &
 CastLuaMail(lua_State *L, int idx);