Connects to a new run's source before the run is saved (MishkaCmsMigration.Source.connect/4, with the run's connection and its sealed secret opened), so a run is only kept for a source it can reach; a refusal is also told on telemetry. What the source says of itself (MishkaCmsMigration.Source.extra/1) becomes the run's extra.

A run that names no source_host takes the host its source reads from the connection (MishkaCmsMigration.Source.host/1), so an admin types the address once. The action lists source_host in allow_nil_input, so a connection with no host is refused once, on the connection, rather than again as a missing host.

A refusal is an error on the run's connection, in words for the reasons MishkaCmsMigration.Source names and as the reason itself for any other; a connection the source reads no host from is refused as :invalid_address. For a source that reads a file (MishkaCmsMigration.Source.file?/0) it is an error on the run's file, the form's upload, instead: :no_file when the run was handed none and :unreadable_file when the source reads no host from it, both put in words as well. A run of a source that is not installed is left to MishkaCmsMigration.Resources.Validations.SourceInstalled.

create :create do
  allow_nil_input [:source_host]
  change MishkaCmsMigration.Resources.Changes.CheckConnection
  validate present(:source_host), only_when_valid?: true
  change &MishkaCmsMigration.MigrationRun.seal_secret/2
end

The source is handed the connection as the run keeps it, as JSON with string keys, which is how a job reads it back; guarded_struct has turned the keys it knows into atoms by then.

It connects in a before_transaction hook: once, when the action runs, outside its transaction, and never while a form only validates what is typed. The hook opens the secret seal_secret/2 sealed, so it needs that change in the same action, and for a source that reads a file it needs MishkaCmsMigration.Resources.Changes.SourceFile before it, which keeps the file in its own hook first.