A hang was observed in the DeleteOpIsIdBased unittest in repltests.cpp. The test performs several deletes (which create delete oplog entries) and immediately queries the oplog, triggering a call to waitForAllEarlierOplogWritesToBeVisible. The stack trace is approximately:

Thread 1: "testsuite" (Thread 0x7fdf3e0f7ac0 (LWP 67799))
 .
#10 0x000055f6e9f94225 in mongo::Interruptible::waitForConditionOrInterrupt
#11 mongo::WiredTigerOplogManager::waitForAllEarlierOplogWritesToBeVisible
 .
#16 0x000055f6eb3e54fb in mongo::(anonymous namespace)::FindCmd::Invocation::run
 .
#28 0x000055f6e97c822d in ReplTests::Base::applyAllOperations
#29 0x000055f6e9824b59 in ReplTests::DeleteOpIsIdBased::run 

This hang was observed approximately once in Evergreen. It seems likely to be a race involving the WTOplogJournalThread and the main thread, where the main thread is expecting the WTOplogJournalThread to call _setOplogReadTimestamp but it already has / never does. As lingzhi.deng showed me, it may be because waitForAllEarlierOplogWritesToBeVisible increments _opsWaitingForVisibility tell this thread that someone is waiting for it, but the thread checks a different member, _opsWaitingForJournal, to determine if there are any waiters.