core-bundle/src/Resources/contao/library/Contao/Folder.php
public function synchronize()
	{
		if (!$this->isUnsynchronized())
		{
			return;
		}

		// Check if the .nosync file exists
		if (!file_exists($this->strRootDir . '/' . $this->strFolder . '/.nosync'))
		{
			throw new \RuntimeException(sprintf('Cannot synchronize the folder "%s" because one of its parent folders is unsynchronized', $this->strFolder));
		}

		(new File($this->strFolder . '/.nosync'))->delete();
	}
public function synchronize()
	{
		if (!$this->isUnsynchronized())
		{
			return;
		}

		// Check if the .nosync file exists
		if (!file_exists($this->strRootDir . '/' . $this->strFolder . '/.nosync'))
		{
			throw new \RuntimeException(sprintf('Cannot synchronize the folder "%s" because one of its parent folders is unsynchronized', $this->strFolder));
		}

		(new File($this->strFolder . '/.nosync'))->delete();
	}
Synchronize the folder by removing the .nosync file

@throws \RuntimeException If one of the parent folders is unsynchronized