Doctrine setAutoCommit(false) method and "there is no active transaction" message - symfony

Question

EDIT: The question is about why using setAutoCommit(false) is a solution for the "there is no active transaction" exception. Forget this question since this is not the correct solution (at least in my case). I will leave the question here in case somebody encounters the same problem. See my answer below for more details.
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The following code worked fine in Symfony 2.7, but after an update to Symfony 2.8 (and to the latest DoctrineBundle version), a There is no active transaction exception is thrown:
private function getSynchronization() {
$lock_repo = $this->entityManager->getRepository('MyAppBundle\Entity\DBLock');
$this->entityManager->getConnection()->beginTransaction();
try {
$sync = $lock_repo->findOneByUser($this->getUser());
if (!$lock) {
$lock = new DBLock();
} else {
if ($lock->isActive()) {
// ... Exception: Process already running
}
$expected_version = $lock->getVersion();
$this->entityManager->lock($lock, LockMode::OPTIMISTIC, $expected_version);
}
$sync->setActive(false);
$this->entityManager->persist($sync);
$this->entityManager->flush();
$this->entityManager->getConnection()->commit();
// EXCEPTION on this line
$this->entityManager->lock($lock, LockMode::NONE);
}
catch(\Exception $e) {
$this->entityManager->getConnection()->rollback();
throw new ProcessException($e->getMessage());
}
...
}
After some searching I found a solution in another post. After adding the following line everything works fine:
private function getSynchronization() {
$lock_repo = $this->entityManager->getRepository('MyAppBundle\Entity\DBLock');
$this->entityManager->getConnection()->beginTransaction();
// ADDED LINE
$this->entityManager->getConnection()->setAutoCommit(false);
try {
...
So, the question is not how to solve the problem, but how the solution works...
I am quite confused by the Doctrine docs of the setAutoCommit() method:
To have a connection automatically open up a new transaction on
connect() and after commit() or rollBack(), you can disable
auto-commit mode with setAutoCommit(false)
I do not understand this.
Does this mean, that the transaction that was started/created with beginTransaction() is now automatically closed when using commit()? So in order to be able to use lock(...) after using commit() I have to begin a new transaction first. I can either do this manually by calling beginTransaction() again, or auotmatically by setting setAutoCommit(false) before. Is that correct?
Is this a change in on of the latest Doctrine versions? I did not found anything about in in the updates notes and before the update of Symfony/Doctrine the code worked just fine.
Thank you very much!

Answer 1

As described before I encountered the problem, that calling lock($lock, LockMode::NONE) suddenly threw a There is no active transaction exception after the Update from Doctrine 2.4 to 2.5.
My solution was to add setAutoCommit(false), which automatically created a new transaction after calling commit(). It worked and the exception did not occur again. However, this is not the real/correct solution, it creates other problems as side effects.
After re-reading the Doctrine Update Notes I found out, that the correct solution is to use lock($lock, null) instead of lock($lock, LockMode::NONE). This is BC Break between Doctrine 2.4 and 2.5.
Maybe my question and answer helps someone else who encounters the same problem.