maps fills up /tmp with temporary image files with External Storage

Steps to reproduce

• Enable the maps application on an nextcloud instance
• Have smb storages mounted via nextcloud external storage that has many images

Expected behaviour:

• maps does not create copies of images in /tmp file system
• maps does not fill the /tmp file system

Actual behaviour:

• in /tmp many oc_tmp_*-.JPG, oc_tmp_*-.tif files and empty oc_tmp_* files are created which eventually fill up /tmp
• the files are deleted after some time and a new set of files is created
• in our monitoring we observe a saw tooth pattern like this
• when disabling maps the saw tooth pattern stops. The last block in the graph is after disabling maps followed by manual deletion of the files after a grace period (shouldn't the app clean up when it disabled?).

If your /tmp partition is not separate from / this leads to problems (e.g. redis can not write back to disk). As a workaround I created a separate /tmp partition with 10 GB, this is what you see in the monitoring, before the root file system was used, which had ~4 GB of free space which were then filled by oc_tmp* files.

I do not know why the files are copied and deleted again shortly after. My guess is, this is connected to external smb shares since I saw a lot of traffic from the smb server to the nextcloud instance, but it might be unrelated.

Also, this instance has some memory available (12 GB of memory of which 5 to 10 GB where available all the time), which could have been used, but instead the files were written out to disk.

Server configuration

• Ubuntu 18.04
• Apache 2.4.29
• postgresql 10
• php 7.2
• nextcloud 18.0.7.1

Server is behind (apache) reverse proxy with ssl offloading.

Please let me know if you need additional information.

Start by reproducing the issue with the Maps application enabled and an SMB external storage containing many images, while monitoring /tmp for oc_tmp_* files. Trace which Maps entry point or dependency creates and removes these files. Done means identifying the trigger and preventing the temporary files from filling /tmp, including cleanup when Maps is disabled.