1.  ShardRegistry::_periodicReload causes a reload to occur. ShardRegistry::_getDataAsync advances the ReadThroughCache's timeInStore to some t1 with non-zero topologyTime. ReadThroughCache::acquireAsync creates an inProgressLookup with t1, and add a promise for it to  inProgressLookup._outstanding . 2. ShardRegistry::_lookup starts running, and meanwhile the test runs setFCV from 4.9 to 4.4. 3. ShardRegistryData::createFromCatalogClient returns. useActualTopologyTime() is false so it returns the cached data's topology time (i.e. Timestamp(0,0) since this is the first reload) as result.t.  4. Inside ReadThroughCache::_doLookupWhileNotValid, inProgressLookup.getPromisesLessThanTime returns nothing because the first promise in _outstanding is the promise for t1 which has non-zero topologyTime (i.e.  t1 > result.t ) so the for loop breaks early here . 5. The promisesToSet is empty so mustDoAnotherLoop is true. The _inProgressLookup for t1 remains in the cache, and another round of lookup starts, again no promises can be fulfilled because of 4. 6. Future reloads join this infinitely looping inProgressLookup. (That's why in the hang analyzer output, there are multiple mongo::ShardRegistry::_periodicReload threads).