Redmine

Why?
Exporting large amounts of issues and/or issues with a lot of custom fields as CSV or PDF can take a long time, to the point where users run into HTTP timeouts and cannot successfully do their exports.

How?
This patch which has been extracted from Planio allows for such exports to be run in a queueing backend like DelayedJob (that's what we use, but any supported ActiveJob backend should be fine). Exports with less than 1000 issues or exports that take less than 30 seconds to finish will be delivered to the user immediately as a download (just as it is without this patch), but after 30 seconds, the controller gives up waiting and returns the user to the previous issues listing. An email with a download link will be sent to the user once the export is finished.

Without a configured ActiveJob backend everything will be as before and all exports are processed in-request. The background export logic itself is generic and we use the same approach for running time entry exports in the background as well. I'll post that as a separate patch if there's interest in integrating this into Redmine at all.

Patches are against current master. Patch no 1 adds a lazy_issues method to IssueQuery which fetches issues in batches (to avoid high memory usage). Patch no 2 implements the background execution.