Shopware 6 upgrade wizard rollback limits when migrations fail midway

Context

The Shopware 6 upgrade wizard performs incremental schema migrations but provides no automated rollback mechanism. If a migration fails partway through, the database may contain a mix of old and new schema elements, and obsolete tables from prior versions are not purged automatically.

Goal

Determine the practical boundaries of recovery after a failed upgrade without restoring from a full backup, specifically whether re-running migrations or clearing caches can reliably revert schema changes that were committed before the failure point.

Constraints and uncertainty

The wizard logs actions to var/log/upgrade.log and checks plugin compatibility, yet it does not revert incompatible plugins or undo database alterations made prior to the error. Documentation recommends clearing cache and re-running migrations, but it is unclear whether this approach can safely unwind partial schema changes or whether orphaned tables and columns remain.

What is the documented behavior of the migration runner when re-executed after a failure? Does it detect and reverse already-applied migrations, or only attempt pending ones? Are there supported procedures to identify and remove orphaned schema objects left by an interrupted upgrade?