When secondaries receive exceptions during index builds, they rethrow immediately. This results in the index build also getting torn down immediately, because we call abortIndexBuildWithoutCleanup() while tearing down the index build.

Once the index build thread tears down, it unregisters itself. If the oplog applier is concurrently processing of the commitIndexBuild oplog entry, the call to abortIndexBuild will throw a NoSuchKey error here. We ignore NoSuchKey, so the commitIndexBuild gets processed and the stable timestamp is allowed to advance.

Upon restarting, the unfinished index build will never be completed because the "commitIndexBuild" has already been processed without successfully committing the index.