Solves issue silverstripe#47 If an export job stalls and restarts the…

… CSV writer overwrites the file resulting in an export only containing the records from the last (re)start and no header row.

By changing the mode to append instead of write every record is appended on a new line. Because of that, the setNewLine has to be reset otherwise there is an empty line between each record in the file.

src/Jobs/GenerateCSVJob.php

@@ -224,10 +224,10 @@ protected function getOutputPath()
     protected function getCSVWriter()
     {
         if (!$this->writer) {
-            $csvWriter = Writer::createFromPath($this->getOutputPath(), 'w');
+            $csvWriter = Writer::createFromPath($this->getOutputPath(), 'a');
 
             $csvWriter->setDelimiter($this->Seperator);
-            $csvWriter->setNewline("\r\n"); //use windows line endings for compatibility with some csv libraries
+            $csvWriter->setNewline("");
             $csvWriter->setOutputBOM(Writer::BOM_UTF8);
 
             if (!Config::inst()->get(GridFieldExportButton::class, 'xls_export_disabled')) {