[Bug]: Preview generation fails when DB entry exists, but local file does not

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

• Follow-up to https://github.com/nextcloud/server/issues/63229
• Ref https://github.com/nextcloud/server/issues/63348

If we end up in the situation that the DB entry exists, but the file does not, we are unable to re-generate the preview via occ, we end up with:

root@nextcloud:/var/www/nextcloud# occ preview:generate 1411721

In LocalPreviewStorage.php line 59:
                                                                                                                            
  Unable to open preview stream at /var/nextcloud/data/appdata_oc4r8rpp6vq8/preview/d/2/c/e/d/1/3/1411721/1024-768-max.jpg  
                                                                                                                            

preview:generate [-s|--size SIZE] [-c|--crop] [-m|--mode MODE] [--] <file>

root@nextcloud:/var/www/nextcloud# vi lib/private/Preview/Storage/LocalPreviewStorage.php 
root@nextcloud:/var/www/nextcloud# occ preview:generate 1411721
#0 /var/www/nextcloud/lib/private/Preview/Storage/StorageFactory.php(32): OC\Preview\Storage\LocalPreviewStorage->readPreview(Object(OC\Preview\Db\Preview))
#1 /var/www/nextcloud/lib/private/Preview/Storage/PreviewFile.php(48): OC\Preview\Storage\StorageFactory->readPreview(Object(OC\Preview\Db\Preview))
#2 /var/www/nextcloud/lib/private/Preview/GeneratorHelper.php(30): OC\Preview\Storage\PreviewFile->getContent()
#3 /var/www/nextcloud/lib/private/Preview/Generator.php(180): OC\Preview\GeneratorHelper->getImage(Object(OC\Preview\Storage\PreviewFile))
#4 /var/www/nextcloud/lib/private/PreviewManager.php(203): OC\Preview\Generator->generatePreviews(Object(OC\Files\Node\File), Array, '...')
#5 /var/www/nextcloud/core/Command/Preview/Generate.php(94): OC\PreviewManager->generatePreviews(Object(OC\Files\Node\File), Array)
#6 /var/www/nextcloud/3rdparty/symfony/console/Command/Command.php(326): OC\Core\Command\Preview\Generate->execute(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#7 /var/www/nextcloud/3rdparty/symfony/console/Application.php(1098): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#8 /var/www/nextcloud/3rdparty/symfony/console/Application.php(324): Symfony\Component\Console\Application->doRunCommand(Object(OC\Core\Command\Preview\Generate), Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#9 /var/www/nextcloud/3rdparty/symfony/console/Application.php(175): Symfony\Component\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#10 /var/www/nextcloud/lib/private/Console/Application.php(209): Symfony\Component\Console\Application->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#11 /var/www/nextcloud/console.php(92): OC\Console\Application->run(Object(Symfony\Component\Console\Input\ArgvInput))
#12 /var/www/nextcloud/occ(33): require_once('...')

Best case would probably even be automatic handling for that case?

Nextcloud Server version

35 (master)

Start with lib/private/Preview/Storage/LocalPreviewStorage.php, especially the failing readPreview path shown in the stack trace, then follow how occ preview:generate reaches it through StorageFactory.php, PreviewFile.php, and Generator.php. Run occ preview:generate for a file whose database entry exists but local preview file is missing. Done means that case is handled automatically and preview generation can proceed without the stream error.