The code around ttl indexes is lacking good coverage.. This ticket will look into adding more unit coverage where possible/appropriate. More information on the existing coverage files can be found by following this guide. File of interest:

mongo/db/ttl.cpp

Failpoints to also look at:

• hangTTLMonitorWithLock
• hangTTLMonitorBetweenPasses
• skipTTLIndexValidationOnCreateIndex