Reset nested transaction level on deadlock exceptions

src/Connection.php

@@ -15,6 +15,7 @@
 use Doctrine\DBAL\Event\TransactionCommitEventArgs;
 use Doctrine\DBAL\Event\TransactionRollBackEventArgs;
 use Doctrine\DBAL\Exception\ConnectionLost;
+use Doctrine\DBAL\Exception\DeadlockException;
 use Doctrine\DBAL\Exception\DriverException;
 use Doctrine\DBAL\Exception\InvalidArgumentException;
 use Doctrine\DBAL\Platforms\AbstractPlatform;
@@ -1276,6 +1277,8 @@ public function transactional(Closure $func)
             $this->commit();
 
             return $res;
+        } catch (DeadlockException $e) {
+            throw $e;
         } catch (Throwable $e) {
             $this->rollBack();
 
@@ -1938,6 +1941,11 @@ private function handleDriverException(
             $this->close();
         }
 
+        if ($exception instanceof DeadlockException) {
+            //Reset transaction nesting level since deadlocks always rollback.
+            $this->transactionNestingLevel = 0;
+        }
+
         return $exception;
     }
 

tests/Functional/Driver/PDO/MySQL/DeadlockTest.php

@@ -0,0 +1,77 @@
+<?php
+
+namespace Doctrine\DBAL\Tests\Functional\Driver\PDO\MySQL;
+
+use Doctrine\DBAL\Exception\DeadlockException;
+use Doctrine\DBAL\Schema\Table;
+use Doctrine\DBAL\Tests\FunctionalTestCase;
+use Doctrine\DBAL\Tests\TestUtil;
+
+use function implode;
+use function pcntl_fork;
+use function sleep;
+use function sprintf;
+
+/** @require extension pcntl */
+class DeadlockTest extends FunctionalTestCase
+{
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $supportedDrivers = ['pdo_mysql', 'mysqli', 'pdo_pgsql'];
+        if (! TestUtil::isDriverOneOf(...$supportedDrivers)) {
+            self::markTestSkipped(sprintf('This supports one of %s drivers', implode(', ', $supportedDrivers)));
+        }
+
+        $table = new Table('test1');
+        $table->addColumn('id', 'integer');
+        $this->dropAndCreateTable($table);
+
+        $table = new Table('test2');
+        $table->addColumn('id', 'integer');
+        $this->dropAndCreateTable($table);
+
+        $this->connection->executeStatement('INSERT INTO test1 VALUES(1)');
+        $this->connection->executeStatement('INSERT INTO test2 VALUES(1)');
+    }
+
+    public function testNestedTransactionsDeadlockExceptionHandling(): void
+    {
+        $this->connection->setNestTransactionsWithSavepoints(true);
+
+        $this->connection->beginTransaction();
+        $this->connection->beginTransaction();
+        $this->connection->executeStatement('DELETE FROM test1');
+
+        $pid = pcntl_fork();
+        if ($pid) {
+            $connection = TestUtil::getConnection();
+            $connection->beginTransaction();
+            $connection->executeStatement('DELETE FROM test2');
+            $connection->executeStatement('DELETE FROM test1');
+            $connection->commit();
+            $connection->close();
+
+            $this->markTestSkipped('Gracefully skip child process in deadlock test');
+        }
+
+        try {
+            $this->waitForTableLock();
+            $this->connection->executeStatement('DELETE FROM test2');
+            $this->connection->commit();
+            $this->connection->commit();
+        } catch (DeadlockException $ex) {
+            self::assertFalse($this->connection->isTransactionActive());
+
+            return;
+        }
+
+        $this->fail('Expected deadlock exception did not happen.');
+    }
+
+    private function waitForTableLock(): void
+    {
+        sleep(5);
+    }
+}