fix: forbid sharded queries from self-joining sharded tables

Signed-off-by: Robin Appelman <robin@icewind.nl>

lib/private/DB/QueryBuilder/Sharded/ShardedQueryBuilder.php

@@ -29,6 +29,7 @@ class ShardedQueryBuilder extends ExtendedQueryBuilder {
 	private ?int $offset = null;
 	/** @var array{column: string, order: string}[]  */
 	private array $sortList = [];
+	private string $mainTable = '';
 
 	public function __construct(
 		IQueryBuilder                  $builder,
@@ -175,6 +176,7 @@ public function values(array $values) {
 	}
 
 	private function actOnTable(string $table): void {
+		$this->mainTable = $table;
 		foreach ($this->shardDefinitions as $shardDefinition) {
 			if ($shardDefinition->hasTable($table)) {
 				$this->shardDefinition = $shardDefinition;
@@ -213,6 +215,9 @@ public function delete($delete = null, $alias = null) {
 
 	private function checkJoin(string $table): void {
 		if ($this->shardDefinition) {
+			if ($table === $this->mainTable) {
+				throw new InvalidShardedQueryException("Sharded query on {$this->mainTable} isn't allowed to join on itself");
+			}
 			if (!$this->shardDefinition->hasTable($table)) {
 				throw new InvalidShardedQueryException("Sharded query on {$this->shardDefinition->table} isn't allowed to join on $table");
 			}
@@ -270,7 +275,7 @@ public function orderBy($sort, $order = null) {
 		return parent::orderBy($sort, $order);
 	}
 
-	private function registerOrder(string $column, string $order) {
+	private function registerOrder(string $column, string $order): void {
 		// handle `mime + 0` and similar by just sorting on the first part of the expression
 		[$column] = explode(' ', $column);
 		$column = trim($column, '`');