Error when task is exported multiple times

Problem

Exporting task while there is already running export can lead to an error, where the latest export worker can not find the export folder because it is already cleaned-up by the older running export.

To reproduce

1. export a task which contains larger files (so the export takes more time)
2. after the initial export, run one more export
3. no we have 2 export running. The initial export did create the export folder, and because there is no export versioning, the second export will use the same folder
4. after export 1 is finished, the zip worker will generate the zip out of the export folder and it will remove the export folder. When the export 2 finishes, the zip worker for export 2 can not the find the export folder -> this leads to this error:

   E1146

   The task export folder does not exist, no export ZIP file can be created.

Solution

We implement blocking, where do not allow another export while we have a running one