From HELP-92917.

Background

The field waiting was added with version 2.4 of mongod (released 2013). If a customer is running a pre-2.4 version of mongod, then documents will likely exist without the waiting field.

The customer in the linked HELP-92917 ticket sees an error doing a default initialSync when adding a new RS member. Their workaround was to do the file-based initialSync, which unblocked them and allowed them to add the RS member (although without compaction, which was desired by the customer).

The issue is that this field is parsed as required in type_mongos.cpp#L83, and this causes a uassertStatusOK to be thrown, which crashes the server process with a std::terminate.

Proposal

This field should be parsed passively, as other customers who upgraded from pre-2.4 will likely run into this issue.

The only class that writes this document is router_uptime_reporter.cpp#L114, and it is hard-coded to true with comment:

// balancer is never active in the router. Here for backwards compatibility only.

Therefore, if the field does not exist, we can simply return a default true value.