/thumbnail should return more obvious error when dynamic_thumbnails is disabled #13016
Description
Description:
Currently /thumbnail will return a 404 M_NOT_FOUND when dynamic_thumbnails is disabled in homeserver.yaml (it is disabled by default). The response code is pretty confusing because the media does exist, it just can't be dynamically thumbnailed. We should instead return a 400 M_UNKNOWN with a friendly error explaining why (already in spec).
Maybe this also depends on no thumbnail_sizes being defined in homeserver.yaml as well.
Actual
✅ Works: http://localhost:8008/_matrix/media/r0/download/hs1/tefQeZhmVxoiBfuFQUKRzJxc
❌ Doesn't load, 404: http://localhost:8008/_matrix/media/r0/thumbnail/hs1/tefQeZhmVxoiBfuFQUKRzJxc?width=400&height=195&method=scale
{
"errcode": "M_NOT_FOUND",
"error": "Not found [b'hs1', b'tefQeZhmVxoiBfuFQUKRzJxc']"
}This really confused me because obviously the media does exist. The Synapse logs show:
2022-06-13 19:15:45,125 - synapse.rest.media.v1.thumbnail_resource - 382 - INFO - GET-319 - Failed to find any generated thumbnails
Expected
When dynamic_thumbnails is turned off, http://localhost:8008/_matrix/media/r0/thumbnail/hs1/tefQeZhmVxoiBfuFQUKRzJxc?width=400&height=195&method=scale should return 400 M_UNKNOWN
400: The request does not make sense to the server, or the server cannot thumbnail the content. For example, the client requested non-integer dimensions or asked for negatively-sized images.
-- https://spec.matrix.org/v1.1/client-server-api/#get_matrixmediav3thumbnailservernamemediaid
{
"errcode": "M_UNKNOWN",
"error": "Cannot find existing thumbnail for the requested content and cannot generate one because `dynamic_thumbnails` is disabled (see `homeserver.yaml`)"
}