Add occ preview:migrate to migrate previews from the old flat structure to a subfolder structure

* `php occ preview:repair` - a preview migration tool that moves existing previews into the new location introduced with #19214
* moves `appdata_INSTANCEID/previews/FILEID` to `appdata_INSTANCEID/previews/0/5/8/4/c/e/5/FILEID`
* migration tool can be stopped during migration via `CTRL+C` - it then finishes the current folder (with the previews of one file) and stops gracefully
* if a PHP memory limit is set in the `php.ini` then it will stop automatically once it has less than 25 MiB memory left (this is to avoid hard crashes in the middle of a migration)
* the tool can be used during operation - possible drawbacks:
    * there is the chance of a race condition that a new preview is generated in the moment the folder is already migrated away - so the old folder with the newly cached preview is deleted and one cached preview needs to be re-generated
    * there is the chance of a race condition during access of a preview while it is migrated to the other folder - then no preview can be shown and results in a 404 (as of now this is an accepted risk)

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

1 files changed, 3 insertions, 3 deletions

diff --git a/lib/private/Preview/Storage/Root.php b/lib/private/Preview/Storage/Root.php
index a9a72026a51..1f332c32085 100644
--- a/lib/private/Preview/Storage/Root.php
+++ b/lib/private/Preview/Storage/Root.php
@@ -39,7 +39,7 @@ class Root extends AppData {
 
 
 	public function getFolder(string $name): ISimpleFolder {
-		$internalFolder = $this->getInternalFolder($name);
+		$internalFolder = self::getInternalFolder($name);
 
 		try {
 			return parent::getFolder($internalFolder);
@@ -54,7 +54,7 @@ class Root extends AppData {
 	}
 
 	public function newFolder(string $name): ISimpleFolder {
-		$internalFolder = $this->getInternalFolder($name);
+		$internalFolder = self::getInternalFolder($name);
 		return parent::newFolder($internalFolder);
 	}
 
@@ -66,7 +66,7 @@ class Root extends AppData {
 		return [];
 	}
 
-	private function getInternalFolder(string $name): string {
+	public static function getInternalFolder(string $name): string {
 		return implode('/', str_split(substr(md5($name), 0, 7))) . '/' . $name;
 	}
 }