Export incorrect when QueuedJob takes more than 1 cycle to complete

[SanderHamaka]

Thanks for a very useful module!

Situation:
7000+ rows export on a high load machine

Problem:
QueuedJobService has a default memory limit of 256Mb. Their method runJob has a check isMemoryTooHigh. In our case, that limit was met. The idea behind the way QueuedJobService handles this situation seems to be to release memory and let the next iteration of ProcessJobQueueTask take up where we left it.

It looks like silverstripe-gridfieldqueuedexport can not handle this situation. In the end I have an export with only the records from the last restart. This is easy to spot because the column headers are missing in the first line.

Then the part I am not sure about:
It looks like the problem might be that silverstripe-gridfieldqueuedexport uses write mode and not append mode for file handling in GenerateCSVJob->getCSVWriter():

$csvWriter = Writer::createFromPath($this->getOutputPath(), 'w');

Maybe the module is not build to account for this situation. In that case I think it is worth mentioning it in the README.

[SanderHamaka]

I think this issue is still relevant and can be fixed this way:

$csvWriter = Writer::createFromPath($this->getOutputPath(), 'a');
$csvWriter->setNewline("");

Without clearing the newline there will be an empty line between each record since file mode a already inserts a new line.
For my situatie this fix works but it is not tested in any other setup.

[GuySartorelli]

Would you be willing to create a pull request for this change @SanderHamaka?