Delivery 2 deleted the move and chain_moved, so the layout of the builds under their modules is made by the controller's start (LayBuildsOut), recorded by the key layout.done in the builds bucket, retried every half minute with an urgent condition while it fails; the chain on the bus is refused until it exists. Its source readers use the module and pointer reads.
A found ended or pending key is skipped and a pulled-in key is created only where no key exists. RecordSentWith and RecordSentUnder set sent_builds = null in their own update of the node row, so a machine delivery 2 has sent is never copied afterwards. These two updates and the copy's own null are the only writes to the column.
Delivery 1 wrote no commit on the key of a pulled-in module, so after the upgrade those modules fell out of SentBuilds. At the start of the lease holder a copy fills every key that names no commit (creating a standing pulled-in key where none stands), overwrites nothing, empties each column it copied, and raises an urgent condition while it fails. Migration 0105 no longer drops node.sent_builds (issue 1112 does).
jschoubben