Description

The remote thumbnails path and filename are derived from media_id when they're generated and stored with dynamic_thumbnails set to true. The said thumbnails are looked up by filesystem_id which fails every time. Synapse uses a lot of CPU regenerating the thumbnails all the time. When dynamic_thumbnails option is then turned off Synapse can't find the thumbnails generated when the said option was turned on and just returns 404.

Steps to reproduce

• set dynamic_thumbnails to true
• try to load a thumbnail of a remote media
• now set dynamic_thumbnails to false
• try to load the same thumbnail of the remote media
• it returns 404

Local media thumbnails are generated and retrieved just fine. For example, I'm trying to retrieve a media with id zOUYnsGQKLQypwBoQkRpnkXh and corresponding filesystem id WFzEWtlvXlCaZShcaIthgzrG. The media itself is stored at /var/lib/matrix-synapse/media/bc/remote_content/matrix.org/WF/zE/WtlvXlCaZShcaIthgzrG and I can retrieve it from my server. The thumbnail should be located at /var/lib/matrix-synapse/media/bc/remote_thumbnail/matrix.org/WF/zE/WtlvXlCaZShcaIthgzrG/30-30-image-png but it's actually stored at /var/lib/matrix-synapse/media/bc/remote_thumbnail/matrix.org/zO/UY/nsGQKLQypwBoQkRpnkXh/30-30-image-png. Why do we even need two identifiers? It confuses things pretty much (it's hard to find an offending file in the storage if you know its id without looking it up in the database) and doesn't add any security.

Version information

• Homeserver: own server

• Version: 1.3.1+buster1

• Install method: apt repository

• Platform: Debian stable amd64