nc_previews : Could not create folder

I tracked down this problem to a race condition in /usr/local/bin/nc-previews. So you can not reproduce it quite simple, but I am quite sure it is that problem, as I get in /var/log/ncp.log four times the same file logged for preview if the error is thrown.

I commented the cause in /usr/local/bin/nc-previews:

echo -e "\n[ nc-previews-auto ]" >> /var/log/ncp.log
(
#    for i in $(seq 1 $(nproc)); do
      ionice -c3 nice -n20 /usr/local/bin/ncc preview:pre-generate -n -vvv &
#    done
    wait
) 2>&1 >>/var/log/ncp.log &

PID=$!
[[ "0" != 0 ]] && {
  for i in $(seq 1 1 0); do
    sleep 1
    kill -0 "$PID" &>/dev/null || break
  done
  pkill -f preview:pre-generate
  pkill -f preview:generate-all
}
wait "$PID"

This is of cause not a nice solution, as it stops using four processes. But from time to time the cron job throws this exception otherwise resulting in an e-mail to the root user...

2021-02-10T21:28:37+00:00
2021-02-10T21:28:37+00:00 In Folder.php line 167:
2021-02-10T21:28:37+00:00
2021-02-10T21:28:37+00:00 [OCP\Files\NotPermittedException]
2021-02-10T21:28:37+00:00 Could not create folder/appdata_oc0iazbq1rtc/preview/b/3/9/f/5/d/3/487265
2021-02-10T21:28:37+00:00
2021-02-10T21:28:37+00:00
2021-02-10T21:28:37+00:00 Exception trace:
2021-02-10T21:28:37+00:00 at /var/www/nextcloud/lib/private/Files/Node/Folder.php:167
2021-02-10T21:28:37+00:00 OC\Files\Node\Folder->newFolder() at /var/www/nextcloud/lib/private/Files/AppData/AppData.php:157
2021-02-10T21:28:37+00:00 OC\Files\AppData\AppData->newFolder() at /var/www/nextcloud/lib/private/Preview/Storage/Root.php:75
2021-02-10T21:28:37+00:00 OC\Preview\Storage\Root->newFolder() at /var/www/nextcloud/lib/private/Preview/Generator.php:471
2021-02-10T21:28:37+00:00 OC\Preview\Generator->getPreviewFolder() at /var/www/nextcloud/lib/private/Preview/Generator.php:132
2021-02-10T21:28:37+00:00 OC\Preview\Generator->generatePreviews() at /var/www/nextcloud/lib/private/PreviewManager.php:205
2021-02-10T21:28:37+00:00 OC\PreviewManager->generatePreviews() at /var/www/nextcloud/apps/previewgenerator/lib/Command/PreGenerate.php:217
2021-02-10T21:28:37+00:00 OCA\PreviewGenerator\Command\PreGenerate->processFile() at /var/www/nextcloud/apps/previewgenerator/lib/Command/PreGenerate.php:195
2021-02-10T21:28:37+00:00 OCA\PreviewGenerator\Command\PreGenerate->processRow() at /var/www/nextcloud/apps/previewgenerator/lib/Command/PreGenerate.php:163
2021-02-10T21:28:37+00:00 OCA\PreviewGenerator\Command\PreGenerate->startProcessing() at /var/www/nextcloud/apps/previewgenerator/lib/Command/PreGenerate.php:131
2021-02-10T21:28:37+00:00 OCA\PreviewGenerator\Command\PreGenerate->execute() at /var/www/nextcloud/3rdparty/symfony/console/Command/Command.php:255
2021-02-10T21:28:37+00:00 Symfony\Component\Console\Command\Command->run() at /var/www/nextcloud/3rdparty/symfony/console/Application.php:1000
2021-02-10T21:28:37+00:00 Symfony\Component\Console\Application->doRunCommand() at /var/www/nextcloud/3rdparty/symfony/console/Application.php:271
2021-02-10T21:28:37+00:00 Symfony\Component\Console\Application->doRun() at /var/www/nextcloud/3rdparty/symfony/console/Application.php:147
2021-02-10T21:28:37+00:00 Symfony\Component\Console\Application->run() at /var/www/nextcloud/lib/private/Console/Application.php:215
2021-02-10T21:28:37+00:00 OC\Console\Application->run() at /var/www/nextcloud/console.php:100
2021-02-10T21:28:37+00:00 require_once() at /var/www/nextcloud/occ:11
2021-02-10T21:28:37+00:00
2021-02-10T21:28:37+00:00 preview:pre-generate
2021-02-10T21:28:37+00:00
2021-02-10T21:28:37+00:00
2021-02-10T21:28:37+00:00 In Folder.php line 167:
2021-02-10T21:28:37+00:00
2021-02-10T21:28:37+00:00 [OCP\Files\NotPermittedException]
2021-02-10T21:28:37+00:00 Could not create folder/appdata_oc0iazbq1rtc/preview/b/3/9/f/5/d/3/487265
2021-02-10T21:28:37+00:00
2021-02-10T21:28:37+00:00
2021-02-10T21:28:37+00:00 Exception trace:
2021-02-10T21:28:37+00:00 at /var/www/nextcloud/lib/private/Files/Node/Folder.php:167
2021-02-10T21:28:37+00:00 OC\Files\Node\Folder->newFolder() at /var/www/nextcloud/lib/private/Files/AppData/AppData.php:157
2021-02-10T21:28:37+00:00 OC\Files\AppData\AppData->newFolder() at /var/www/nextcloud/lib/private/Preview/Storage/Root.php:75
2021-02-10T21:28:37+00:00 OC\Preview\Storage\Root->newFolder() at /var/www/nextcloud/lib/private/Preview/Generator.php:471
2021-02-10T21:28:37+00:00 OC\Preview\Generator->getPreviewFolder() at /var/www/nextcloud/lib/private/Preview/Generator.php:132
2021-02-10T21:28:37+00:00 OC\Preview\Generator->generatePreviews() at /var/www/nextcloud/lib/private/PreviewManager.php:205
2021-02-10T21:28:37+00:00 OC\PreviewManager->generatePreviews() at /var/www/nextcloud/apps/previewgenerator/lib/Command/PreGenerate.php:217
2021-02-10T21:28:37+00:00 OCA\PreviewGenerator\Command\PreGenerate->processFile() at /var/www/nextcloud/apps/previewgenerator/lib/Command/PreGenerate.php:195
2021-02-10T21:28:37+00:00 OCA\PreviewGenerator\Command\PreGenerate->processRow() at /var/www/nextcloud/apps/previewgenerator/lib/Command/PreGenerate.php:163
2021-02-10T21:28:37+00:00 OCA\PreviewGenerator\Command\PreGenerate->startProcessing() at /var/www/nextcloud/apps/previewgenerator/lib/Command/PreGenerate.php:131
2021-02-10T21:28:37+00:00 OCA\PreviewGenerator\Command\PreGenerate->execute() at /var/www/nextcloud/3rdparty/symfony/console/Command/Command.php:255
2021-02-10T21:28:37+00:00 Symfony\Component\Console\Command\Command->run() at /var/www/nextcloud/3rdparty/symfony/console/Application.php:1000
2021-02-10T21:28:37+00:00 Symfony\Component\Console\Application->doRunCommand() at /var/www/nextcloud/3rdparty/symfony/console/Application.php:271
2021-02-10T21:28:37+00:00 Symfony\Component\Console\Application->doRun() at /var/www/nextcloud/3rdparty/symfony/console/Application.php:147
2021-02-10T21:28:37+00:00 Symfony\Component\Console\Application->run() at /var/www/nextcloud/lib/private/Console/Application.php:215
2021-02-10T21:28:37+00:00 OC\Console\Application->run() at /var/www/nextcloud/console.php:100
2021-02-10T21:28:37+00:00 require_once() at /var/www/nextcloud/occ:11
2021-02-10T21:28:37+00:00
2021-02-10T21:28:37+00:00 preview:pre-generate