The file a run of a source that reads one (MishkaCmsMigration.Source.file?/0) is handed, kept from the run's creation until it ends (see "The source's file" in MishkaCmsMigration.MigrationRun).

create :create do
  argument :file, {:array, :map}
  change {MishkaCmsMigration.Resources.Changes.SourceFile, step: :take}
  change MishkaCmsMigration.Resources.Changes.CheckConnection
end

update :cancel do
  change {MishkaCmsMigration.Resources.Changes.SourceFile, step: :drop}
end

step: :take drops a "file" the connection was typed with, and for a source that reads a file takes the first entry of the file argument, Gervaz's upload entry, when it names a file Gervaz staged (MishkaCmsCore.Runtime.Media.staged_upload/2; any other path, a symbolic link or a folder, is not taken). The staged file's path is the connection's "file" while the run is checked, so CheckConnection reads the source's host from it, and a before_transaction hook, which runs before CheckConnection's connect when it comes first in the action, copies it into files_dir/0 as <uuid>.xml, readable by the CMS's own user alone, and points "file" there. An after_transaction hook deletes the copy when the run is not saved. A form only validating what is typed has no upload, so nothing is copied until the run is created.

step: :drop deletes the run's file once the action has saved the run, and only a file straight inside files_dir/0.