Rich Media parsing was previously handled on-demand with a 2 second HTTP request timeout and retained only in Cachex. Every time a Pleroma instance is restarted it will have to request and parse the data for each status with a URL detected. When fetching a batch of statuses they were processed in parallel to attempt to keep the maximum latency at 2 seconds, but often resulted in a timeline appearing to hang during loading due to a URL that could not be successfully reached. URLs which had images links that expire (Amazon AWS) were parsed and inserted with a TTL to ensure the image link would not break.
Rich Media data is now cached in the database and fetched asynchronously. Cachex is used as a read-through cache. When the data becomes available we stream an update to the clients. If the result is returned quickly the experience is almost seamless. Activities were already processed for their Rich Media data during ingestion to warm the cache, so users should not normally encounter the asynchronous loading of the Rich Media data.
Implementation notes:
- The async worker is a Task with a globally unique process name to prevent duplicate processing of the same URL
- The Task will attempt to fetch the data 3 times with increasing sleep time between attempts
- The HTTP request obeys the default HTTP request timeout value instead of 2 seconds
- URLs that cannot be successfully parsed due to an unexpected error receives a negative cache entry for 15 minutes
- URLs that fail with an expected error will receive a negative cache with no TTL
- Activities that have no detected URLs insert a nil value in the Cachex :scrubber_cache so we do not repeat parsing the object content with Floki every time the activity is rendered
- Expiring image URLs are handled with an Oban job
- There is no automatic cleanup of the Rich Media data in the database, but it is safe to delete at any time
- The post draft/preview feature makes the URL processing synchronous so the rendered post preview will have an accurate rendering
Overall performance of timelines and creating new posts which contain URLs is greatly improved.
Due to JSON-LD compaction the full address of public scope
may also occur in shorter forms and the spec requires us to treat them
all equivalently. To save us the pain of repeatedly checking for all
variants internally, normalise inbound data to just one form.
See note at: https://www.w3.org/TR/activitypub/#public-addressing
This needs to happen very early, even before the other addressing fixes
else an earlier validator will reject the object. This in turn required
to move the list-tpye normalisation earlier as well, but since I was
unsure about putting empty lists into the data when no such field
existed before, I excluded this case and thus the later fixing had to be
kept as well.
Fixes: https://akkoma.dev/AkkomaGang/akkoma/issues/670
literally nothing uses C2S AP, and it's another route into core
systems which requires analysis and maintenance. A second API
is just extra surface for potentially bad things so let's take
it out back and obliterate it
We were overzealous with matching on a raw error from the object fetch that should have never been relied on like this. If we can't fetch successfully we should assume that the collection is private.
Building a more expressive and universal error struct to match on may be something to consider.
"id" is used for the canonical link to the AS2 representation of an object.
"url" is typically used for the canonical link to the HTTP representation.
It is what we use, for example, when following the "external source" link
in the frontend. However, it's not the link we include in the post contents
for quote posts.
Using URL instead means we include a more user-friendly URL for Mastodon,
and a working (in the browser) URL for Threads
Since we always followed redirects (and until recently allowed fuzzy id
matches), the ap_id of the received object might differ from the iniital
fetch url. This lead to us mistakenly trying to insert a new user with
the same nickname, ap_id, etc as an existing user (which will fail due
to uniqueness constraints) instead of updating the existing one.
In order to properly process incoming notes we need
to be able to map the key id back to an actor.
Also, check collections actually belong to the same server.
Key ids of Hubzilla and Bridgy samples were updated to what
modern versions of those output. If anything still uses the
old format, we would not be able to verify their posts anyway.
To save on bandwith and avoid OOMs with large files.
Ofc, this relies on the remote server
(a) sending a content-length header and
(b) being honest about the size.
Common fedi servers seem to provide the header and (b) at least raises
the required privilege of an malicious actor to a server infrastructure
admin of an explicitly allowed host.
A more complete defense which still works when faced with
a malicious server requires changes in upstream Finch;
see https://github.com/sneako/finch/issues/224
Certain attacks rely on predictable paths for their payloads.
If we weren’t so overly lax in our (id, URL) check, the current
counterfeit activity exploit would be one of those.
It seems plausible for future attacks to hinge on
or being made easier by predictable paths too.
In general, letting remote actors place arbitrary data at
a path within our domain of their choosing (sans prefix)
just doesn’t seem like a good idea.
Using fully random filenames would have worked as well, but this
is less friendly for admins checking emoji dirs.
The generated suffix should still be more than enough;
an attacker needs on average 140 trillion attempts to
correctly guess the final path.
This will decouple filenames from shortcodes and
allow more image formats to work instead of only
those included in the auto-load glob. (Albeit we
still saved other formats to disk, wasting space)
Furthermore, this will allow us to make
final URL paths infeasible to predict.
Since 3 commits ago we restrict shortcodes to a subset of
the POSIX Portable Filename Character Set, therefore
this can never have a directory component.
E.g. *key’s emoji URLs typically don’t have file extensions, but
until now we just slapped ".png" at its end hoping for the best.
Furthermore, this gives us a chance to actually reject non-images,
which before was not feasible exatly due to those extension-less URLs
As suggested in b387f4a1c1, only steal
emoji with alphanumerc, dash, or underscore characters.
Also consolidate all validation logic into a single function.
===
Taken from akkoma#703 with cosmetic tweaks
This matches our existing validation logic from Pleroma.Emoji,
and apart from excluding the dot also POSIX’s Portable Filename
Character Set making it always safe for use in filenames.
Mastodon is even stricter also disallowing U+002D HYPEN-MINUS
and requiring at least two characters.
Given both we and Mastodon reject shortcodes excluded
by this anyway, this doesn’t seem like a loss.
The default refresh interval of 1 day is woefully inadequate here;
users expect to be able to add the alias to their new account and
press the move button on their old account and have it work.
This allows callers to specify a maximum age before a refetch is
triggered. We set that to 5s for the move code, as a nice compromise
between Making Things Work and ensuring that this can't be used
to hammer a remote server
Mastodon at the very least seems to prevent the creation of emoji with
dots in their name (and refuses to accept them in federation). It feels
like being cautious in what we accept is reasonable here.
Colons are the emoji separator and so obviously should be blocked.
Perhaps instead of filtering out things like this we should just
do a regex match on `[a-zA-Z0-9_-]`? But that's plausibly a decision
for another day
Perhaps we should also have a centralised "is this a valid emoji shortcode?"
function
Currently our own frontend doesn’t show backgrounds of other users, this
property is already publicly readable via REST API and likely was always
intended to be shown and federated.
Recently Sharkey added support for profile backgrounds and
immediately made them federate and be displayed to others.
We use the same AP field as Sharkey here which should make
it interoperable both ways out-of-the-box.
Ref.: 4e64397635
