CSV Resumable Importer for Tainacan

This add-on registers an extra CSV importer in Tainacan whose attachment processing is resumable: the files listed in the special_attachments column are imported one per background iteration, instead of all at once.

It does not modify Tainacan core — it registers an additional importer through the tainacan-register-importers hook, so it keeps working across Tainacan updates.

This is an independent add-on developed by Bauhaus Tech. It is not affiliated with, endorsed by, or part of the Tainacan project.

The problem it solves

A CSV import in which a single item has many attachments (e.g. 150 images in special_attachments) fails with:

…and then restarts from the beginning, duplicating items.

The stock Tainacan CSV importer imports all of an item's attachments inside a single after_inserted_item() call. Each file triggers wp_generate_attachment_metadata(), which generates thumbnails through Imagick — an expensive operation. The background process only checks its soft time limit (~20s) between task() iterations; since the 150 attachments run inside a single iteration, that limit is never checked mid-loop. The item alone exceeds PHP's hard max_execution_time, the process dies mid-task without persisting progress, and the cron healthcheck re-dispatches the batch from the last checkpoint — recreating already-imported items.

How it works

Instead of importing attachments inline, this importer:

1. Queues the item's attachment paths/URLs (stored in a transient serialized together with the process state).
2. Processes one attachment per iteration, staying on the current item until the queue is empty.

Because each attachment becomes its own iteration, the soft 20s limit applies between files: the batch checkpoints regularly and the import becomes resumable. Idempotency is reinforced by persisting the already-created item id, so a resume continues the attachments instead of recreating the item.

Known limitations

Because this plugin cannot modify Tainacan core:

• A single file whose processing alone exceeds max_execution_time still fails (pathological case).
• Progress is still persisted at the core's ~20s handle() checkpoint, not per iteration (forcing a per-iteration checkpoint would require changing Background_Importer::task() in core). A fatal within the same 20s window may reprocess what was done since the last checkpoint. The definitive fix is the equivalent change upstream in Tainacan.