fix(UserMountCache): Set transaction isolation level to REPEATABLE_READ before executing insertIfNotExist without unique index

lib/private/DB/ConnectionAdapter.php

@@ -26,6 +26,13 @@ class ConnectionAdapter implements IDBConnection {
 	/** @var Connection */
 	private $inner;
 
+	/**
+	 * The currently active transaction isolation level or NULL before it has been determined.
+	 *
+	 * @var \Doctrine\DBAL\TransactionIsolationLevel::*|null
+	 */
+	private ?int $transactionIsolationLevel = null;
+
 	public function __construct(Connection $inner) {
 		$this->inner = $inner;
 	}
@@ -262,4 +269,14 @@ public function getShardDefinition(string $name): ?ShardDefinition {
 	public function getCrossShardMoveHelper(): CrossShardMoveHelper {
 		return $this->inner->getCrossShardMoveHelper();
 	}
+
+	public function setTransactionIsolation(int $level): int {
+		$this->transactionIsolationLevel = $level;
+
+		return $this->executeStatement($this->getDatabasePlatform()->getSetTransactionIsolationSQL($level));
+	}
+
+	public function getTransactionIsolation(): int {
+		return $this->transactionIsolationLevel ??= $this->getDatabasePlatform()->getDefaultTransactionIsolationLevel();
+	}
 }

lib/private/Files/Config/UserMountCache.php

@@ -7,6 +7,7 @@
  */
 namespace OC\Files\Config;
 
+use Doctrine\DBAL\TransactionIsolationLevel;
 use OC\User\LazyUser;
 use OCP\Cache\CappedMemoryCache;
 use OCP\DB\QueryBuilder\IQueryBuilder;
@@ -99,6 +100,8 @@ public function registerMounts(IUser $user, array $mounts, ?array $mountProvider
 		$changedMounts = $this->findChangedMounts($newMounts, $cachedMounts);
 
 		if ($addedMounts || $removedMounts || $changedMounts) {
+			$this->connection->setTransactionIsolation(TransactionIsolationLevel::REPEATABLE_READ);
+
 			$this->connection->beginTransaction();
 			$userUID = $user->getUID();
 			try {
@@ -124,6 +127,8 @@ public function registerMounts(IUser $user, array $mounts, ?array $mountProvider
 			} catch (\Throwable $e) {
 				$this->connection->rollBack();
 				throw $e;
+			} finally {
+				$this->connection->setTransactionIsolation(TransactionIsolationLevel::READ_COMMITTED);
 			}
 
 			// Only fire events after all mounts have already been adjusted in the database.

lib/public/IDBConnection.php

@@ -388,4 +388,26 @@ public function getShardDefinition(string $name): ?ShardDefinition;
 	 * @since 30.0.0
 	 */
 	public function getCrossShardMoveHelper(): CrossShardMoveHelper;
+
+	/**
+	 * Sets the transaction isolation level.
+	 *
+	 * @param \Doctrine\DBAL\TransactionIsolationLevel::* $level The level to set.
+	 *
+	 * @throws \Doctrine\DBAL\Exception
+	 *
+	 * @since 33.0.0
+	 */
+	public function setTransactionIsolation(int $level): int;
+
+	/**
+	 * Gets the currently active transaction isolation level.
+	 *
+	 * @return \Doctrine\DBAL\TransactionIsolationLevel::* The current transaction isolation level.
+	 *
+	 * @throws \Doctrine\DBAL\Exception
+	 *
+	 * @since 33.0.0
+	 */
+	public function getTransactionIsolation(): int;
 }