SERVER-136668 keeps a per-node count of each unfinished two-phase index build's attempts after unclean shutdowns in local.system.indexBuilds, and a node votes to abort a build once its count exceeds maxIndexBuildAttemptsAfterUncleanShutdown. The count is deleted in the same write as the build's commit or abort (SERVER-137566). Writes to local aren't rolled back, so the counts can be inaccurate in some cases when rollback undoes part of a build:

• Rollback commit/abortIndexBuild - the build restarts, but its ledger entry was deleted at the time of the node's commit/abort, so it resets to 0. This gives a crashing node up to the limit's worth of extra crashes before it can vote for an abort again.