keep thumbnails on disk for only the last ≈N visited images

[tpapp]

Is your feature request related to a problem? Please describe.

High-resolution thumbnails are very useful, eg for culling images. As RAM is usually a limiting factor, Darktable's ability to save thumbnails to disk is a great feature that speeds up processing. One downside of the current implementation is that thumbnails are not deleted from the cache automatically, so the user either deletes the thumbnail cache from time to time, or accepts that a lot of disk space is consumed.

Describe the solution you'd like

It would be great to have an option that makes Darktable keep the thumbnails for the most recent N visited images, with a default like N = 1000 or similar. This is sufficient for most use cases, yet would save a lot of disk space for large collections.

The implementation can be approximate, eg only remove items from the cache when the number of images grows above some threshold N + T. T would not need to be exposed in the preferences GUI.

Alternatives

Manual cleaning is available currently, but it is not very convenient. This feature would automate it.

Additional context

See discussion where it was suggested that I open an issue.

[wpferguson]

What should be considered as a visited image?

• opened in darkroom?
• included in a collection opened in lighttable?
• selected image in lighttable?
• accessed during culling?
• had metadata applied to it?
• a whole bunch more I haven't thought of.....

Would a time based cleaning work, i.e. remove the cache for

• any collection with a creation date older then N days?
• any thumbnail older than N days?

darktable doesn't track when images were accessed. It tracks when they were imported, changed, exported, and printed.