Empty system addressbook instead of removing it

This makes Federated Cloud Sharing not fail when synching trusted
servers addressbooks (only synching an empty one, not bumping on an
non-existent one).

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

apps/dav/lib/CardDAV/CardDavBackend.php

@@ -470,7 +470,30 @@ function deleteAddressBook($addressBookId) {
 		$query->delete($this->dbCardsPropertiesTable)
 			->where($query->expr()->eq('addressbookid', $query->createNamedParameter($addressBookId)))
 			->execute();
+	}
+
+	/**
+	 * Delete all the content from an addressbook
+	 *
+	 * @param mixed $addressBookId
+	 * @return void
+	 */
+	public function emptyAddressBook($addressBookId): void
+	{
+		$query = $this->db->getQueryBuilder();
+		$query->delete('cards')
+			->where($query->expr()->eq('addressbookid', $query->createParameter('addressbookid')))
+			->setParameter('addressbookid', $addressBookId)
+			->execute();
+
+		$query->delete('addressbookchanges')
+			->where($query->expr()->eq('addressbookid', $query->createParameter('addressbookid')))
+			->setParameter('addressbookid', $addressBookId)
+			->execute();
 
+		$query->delete($this->dbCardsPropertiesTable)
+			->where($query->expr()->eq('addressbookid', $query->createNamedParameter($addressBookId)))
+			->execute();
 	}
 
 	/**

apps/dav/lib/CardDAV/SyncService.php

@@ -345,6 +345,6 @@ public function syncInstance(\Closure $progressCallback = null) {
 	public function purgeSystemAddressBook(): void
 	{
 		$systemAddressBook = $this->getLocalSystemAddressBook();
-		$this->backend->deleteAddressBook($systemAddressBook['id']);
+		$this->backend->emptyAddressBook($systemAddressBook['id']);
 	}
 }