Parent process isn't aware of batch finished state

Problem/Motivation

In the execute() function of views_data_export_plugin_display_export, the execute_initial() function actually creates and processes the batch. After returning from the batch, the parent process doesn't set the batch_state as finished. So the parent process continues and does the execute() method again.

Steps to reproduce

Watch it in the debugger. I first realized that my preprocess header function was being called again after the batch had finished.

In our case, we first realized there was a problem during a long-running export (> 10 hours). When the batch finishes, the parent database connection has timed out. So we got this error:
PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away

The bug exists even for exports that don't run long, it is just benign in that case.

Proposed resolution

Patch forthcoming.