There are a number of index build-related files that are lacking good unit 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. Some files of interest include:

mongo/db/catalog/index_builds_manager.cpp

mongo/db/index_builds_coordinator.cpp

mongo/db/repl_index_build_state.cpp

mongo/db/index_builds_coordinator_mongod.cpp

mongo/db/active_index_builds.cpp

mongo/db/index/index_build_interceptor.cpp

mongo/db/catalog/index_build_oplog_entry.cpp

mongo/db/index_build_entry_helpers.cpp

Failpoints to look at:

hangBeforeGettingIndexBuildEntry
hangAfterAcquiringIndexBuildSlot
hangAfterRegisteringIndexBuild
hangBeforeInitializingIndexBuild
hangIndexBuildAfterSignalPrimaryForCommitReadiness
hangBeforeRunningIndexBuild
hangIndexBuildBeforeSignalingPrimaryForAbort
hangIndexBuildBeforeTransitioningReplStateTokAwaitPrimaryAbort
hangBeforeVoteCommitIndexBuild
hangAfterIndexBuildFirstDrain
hangAfterIndexBuildSecondDrain
hangAfterIndexBuildDumpsInsertsFromBulk
hangAfterIndexBuildDumpsInsertsFromBulkLock
hangAfterInitializingIndexBuild
hangBeforeCompletingAbort
failIndexBuildOnCommit
hangIndexBuildBeforeAbortCleanUp
hangIndexBuildOnStepUp
hangAfterSettingUpResumableIndexBuild
hangIndexBuildBeforeCommit
hangBeforeBuildingIndex
hangBeforeBuildingIndexSecond
hangIndexBuildBeforeWaitingUntilMajorityOpTime
hangBeforeUnregisteringAfterCommit
failSetUpResumeIndexBuild
failIndexBuildWithError
failIndexBuildWithErrorInSecondDrain
hangInRemoveIndexBuildEntryAfterCommitOrAbort
hangIndexBuildOnSetupBeforeTakingLocks
hangAbortIndexBuildByBuildUUIDAfterLocks
hangOnStepUpAsyncTaskBeforeCheckingCommitQuorum
hangDuringIndexBuildDrainYield
hangDuringIndexBuildDrainYieldSecond
hangIndexBuildDuringDrainWritesPhase
hangIndexBuildDuringDrainWritesPhaseSecond
constrainMemoryForBulkBuild
hangAfterSettingUpIndexBuild
hangAfterSettingUpIndexBuildUnlocked
hangAfterStartingIndexBuild
hangAfterStartingIndexBuildUnlocked
hangIndexBuildDuringCollectionScanPhaseBeforeInsertion
hangIndexBuildDuringCollectionScanPhaseAfterInsertion
leaveIndexBuildUnfinishedForShutdown
fixedCursorDataSizeOf512KBForDataThrottle
fixedCursorDataSizeOf2MBForDataThrottle
failRecordStoreTraversal
hangDuringValidationInitialization
createIndexesWriteConflict
hangBeforeCreateIndexesCollectionCreate
hangBeforeIndexBuildAbortOnInterrupt
hangAfterIndexBuildAbort
hangCreateIndexesBeforeStartingIndexBuild
hangIndexBuildDuringBulkLoadPhase
hangIndexBuildDuringBulkLoadPhaseSecond
hangDuringIndexBuildBulkLoadYield
hangDuringIndexBuildBulkLoadYieldSecond