Convert driver exceptions when starting transactions

For long-running processes, it's possible that the wrapped connection is
no longer valid and starting a transaction will lead to driver
exceptions.

DBAL Connection wasn't converting these, leaking internal details of the
platform and not providing a consistent API (all other methods would
throw a `ConnectionLost` exception for the scenario above).

This ensures we do have the expected behaviour for this edge-case.

Signed-off-by: Luís Cobucci <lcobucci@gmail.com>

src/Connection.php

@@ -1026,10 +1026,16 @@ public function beginTransaction(): void
 
         ++$this->transactionNestingLevel;
 
-        if ($this->transactionNestingLevel === 1) {
-            $connection->beginTransaction();
-        } else {
+        if ($this->transactionNestingLevel !== 1) {
             $this->createSavepoint($this->_getNestedTransactionSavePointName());
+
+            return;
+        }
+
+        try {
+            $connection->beginTransaction();
+        } catch (Driver\Exception $e) {
+            throw $this->convertException($e);
         }
     }
 

tests/ConnectionTest.php

@@ -224,6 +224,35 @@ public function testCommitStartsTransactionInNoAutoCommitMode(): void
         self::assertTrue($conn->isTransactionActive());
     }
 
+    public function testDriverExceptionsAreConvertedDuringBeingTransaction(): void
+    {
+        $driverConnectionMock = self::createStub(DriverConnection::class);
+        $driverConnectionMock
+            ->method('beginTransaction')
+            ->willThrowException(
+                new Driver\PDO\Exception(
+                    'SQLSTATE[HY000]: General error: 2006 MySQL server has gone away',
+                    'HY000',
+                    2006,
+                ),
+            );
+
+        $mysqlConverter = new Driver\API\MySQL\ExceptionConverter();
+
+        $driver = self::createStub(Driver::class);
+        $driver
+            ->method('getExceptionConverter')
+            ->willReturn($mysqlConverter);
+        $driver
+            ->method('connect')
+            ->willReturn($driverConnectionMock);
+
+        $conn = new Connection([], $driver);
+
+        $this->expectException(Exception\ConnectionLost::class);
+        $conn->beginTransaction();
+    }
+
     public function testBeginTransactionFailureAfterCommitInNoAutoCommitMode(): void
     {
         $driverConnectionMock = $this->createMock(DriverConnection::class);