added PID file to recurring Invoice cron

cron/modules/SalesOrder/RecurringInvoice.service

@@ -15,6 +15,15 @@ require_once('include/logging.php');
 
 // Get the list of Invoice for which Recurring is enabled.
 
+$svcname = "RecurringInvoice";
+$servicePIDFile = "logs/{$svcname}-service.pid";
+
+if(file_exists($servicePIDFile)) {
+	die("Service $svcname already running! Check $servicePIDFile");
+} else {
+	$servicePIDFp = fopen($servicePIDFile, 'a');
+}
+
 global $adb, $log;
 $log =& LoggerManager::getLogger('RecurringInvoice');
 $log->debug("invoked RecurringInvoice");
@@ -100,6 +109,12 @@ for($i=0; $i<$no_of_salesorder;$i++) {
 	}
 }
 
+/** Close and remove the PID file. */
+if($servicePIDFp) {
+	fclose($servicePIDFp);
+	unlink($servicePIDFile);
+}
+
 /* Function to create a new Invoice using the given Sales Order id */
 function createInvoice($salesorder_id) {
 	require_once('include/utils/utils.php');