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docs: add description how BackgroundCleanupJob.getNewPreviewLocations works

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Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>
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Daniel Kesselberg 2022-12-20 16:05:51 +01:00
parent 06da8adcd3
commit 71a6030706
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1 changed files with 15 additions and 0 deletions

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lib/private/Preview/BackgroundCleanupJob.php
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@ -128,6 +128,21 @@ class BackgroundCleanupJob extends TimedJob {
*/
$like = $this->connection->escapeLikeParameter($data['path']) . '/_/_/_/_/_/_/_/%';
/*
* Deleting a file will not delete related previews right away.
*
* A delete request is usually an HTTP request.
* The preview deleting is done by a background job to avoid timeouts.
*
* Previews for a file are stored within a folder in appdata_/preview using the fileid as folder name.
* Preview folders in oc_filecache are identified by a.storage, a.path (cf. $like) and a.mimetype.
*
* To find preview folders to delete, we query oc_filecache for a preview folder in app data, matching the preview folder structure
* and use the name to left join oc_filecache on a.name = b.fileid. A left join returns all rows from the left table (a),
* even if there are no matches in the right table (b).
*
* If the related file is deleted, b.fileid will be null and the preview folder can be deleted.
*/
$qb = $this->connection->getQueryBuilder();
$qb->select('a.name')
->from('filecache', 'a')