From c69b7ff7e90cfe3dae66f4d1d5aeacfe224ebe4f Mon Sep 17 00:00:00 2001
From: Carl Schwan <carlschwan@kde.org>
Date: Thu, 9 Jul 2026 17:03:50 +0200
Subject: [PATCH] fix(preview): Handle unique constraints violation exception

Delete the preview from storage and fetch the actual one from the DB.

This should not happen often.

Signed-off-by: Carl Schwan <carl@carlschwan.eu>
---
 lib/private/Preview/Db/PreviewMapper.php | 16 +++++++++++++++
 lib/private/Preview/Generator.php        | 26 +++++++++++++++++++++++-
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/lib/private/Preview/Db/PreviewMapper.php b/lib/private/Preview/Db/PreviewMapper.php
index e66e2c2ef0a4d..d6508c64afe52 100644
--- a/lib/private/Preview/Db/PreviewMapper.php
+++ b/lib/private/Preview/Db/PreviewMapper.php
@@ -9,6 +9,7 @@
 
 namespace OC\Preview\Db;
 
+use OCP\AppFramework\Db\DoesNotExistException;
 use OCP\AppFramework\Db\Entity;
 use OCP\AppFramework\Db\QBMapper;
 use OCP\DB\Exception;
@@ -227,4 +228,19 @@ public function getPreviewsForMimeTypes(array $mimeTypes): \Generator {
 			));
 		return $this->yieldEntities($qb);
 	}
+
+	public function getPreviewForSpecification(array $parameters): ?Preview {
+		$qb = $this->db->getQueryBuilder();
+		$this->joinLocation($qb);
+
+		foreach ($parameters as $key => $value) {
+			$qb->andWhere($qb->expr()->eq($key, $qb->createNamedParameter($value)));
+		}
+
+		try {
+			return $this->findEntity($qb);
+		} catch (DoesNotExistException) {
+			return null;
+		}
+	}
 }
diff --git a/lib/private/Preview/Generator.php b/lib/private/Preview/Generator.php
index 2549d40213a90..f93fbd4e0ec27 100644
--- a/lib/private/Preview/Generator.php
+++ b/lib/private/Preview/Generator.php
@@ -590,6 +590,30 @@ public function savePreview(Preview $previewEntry, IImage $preview): Preview {
 		}
 		$previewEntry->setSize($size);
 		$previewEntry->generateId();
-		return $this->previewMapper->insert($previewEntry);
+		try {
+			return $this->previewMapper->insert($previewEntry);
+		} catch (DBException $e) {
+			if ($e->getReason() !== DBException::REASON_UNIQUE_CONSTRAINT_VIOLATION) {
+				throw $e;
+			}
+
+			$this->storageFactory->deletePreview($previewEntry);
+
+			$this->logger->debug('Generating a preview but one already exists.', ['exception' => $e]);
+
+			$preview = $this->previewMapper->getPreviewForSpecification([
+				'file_id' => $previewEntry->getFileId(),
+				'width' => $previewEntry->getWidth(),
+				'height' => $previewEntry->getHeight(),
+				'mimetype_id' => $previewEntry->getMimetypeId(),
+				'cropped' => $previewEntry->isCropped(),
+				'version_id' => $previewEntry->getVersion() ?? '-1',
+			]);
+
+			if ($preview === null) {
+				throw new \RuntimeException('Unable to save a preview because it already exists but fetching it again yield nothing');
+			}
+			return $preview;
+		}
 	}
 }