For any commands that use concatenate_with_db, we'll pass the tenantId, if it exists on the opMsgRequest, to the NamespaceString constructor. In order to do this, we'll need to pass tenantId to gen_namespace_check() (we'll need to pass it through _gen_command_deserializer())- it can default to None for both functions. In CommandWithNamespaceTypeInfo::gen_namespace_check(), pass this tenantId to IDLParserErrorContext::parseNSCollectionRequired. We'll have to add tenantId as an optional parameter to IDLParserErrorContext::parseNSCollectionRequired.

Once this is done, we should be able to successfully run some commands on a standalone mongod (or perhaps a single node RS) using either a securityToken or the $tenant field to pass the tenantId. Only commands which are IDL defined will work at this point, let's write a simple workload that runs create collection, some CRUD ops, and find commands using both the securityToken and $tenant.

Here are the example idl generated cpp code:

-  for constructing the "to" field of "renameCollection"

if (MONGO_likely(ctxt.checkAndAssertType(commandElement, String))) {
           _commandParameter = NamespaceString(request.getValidatedTenantId(), commandElement.valueStringData());
       }
       else {
           ctxt.throwMissingField(kCommandParameterFieldName);
       }
   } 

-  for constructing the nss from command feild of "renameCollection" cmd. Its cmd namespace is "namespace: type".

   {
        if (MONGO_likely(ctxt.checkAndAssertType(commandElement, String))) {
            _commandParameter = NamespaceString(request.getValidatedTenantId(), commandElement.valueStringData());
        }
        else {
            ctxt.throwMissingField(kCommandParameterFieldName);
        }
    }

-  for constructing the nss from db name and command field of "insert" cmd. Its cmd namespace is "namespace: concatenate_with_db".

    invariant(_nss.isEmpty());
    DatabaseName dbName(request.getValidatedTenantId(), _dbName);
    _nss = ctxt.parseNSCollectionRequired(dbName, commandElement, false);