Avoid accumulation of stale data in websockets

We’ve received reports of some specific instances slowly accumulating
more and more binary data over time up to OOMs and globally setting
ERL_FULLSWEEP_AFTER=0 has proven to be an effective countermeasure.
However, this incurs increased cpu perf costs everywhere and is
thus not suitable to apply out of the box.

There are many reports unrelated to Akkoma of long-lived Phoenix
websockets getting into a state unfavourable for the garbage
collector depending on usage pattern, resulting in exactly the
observed behaviour.
Therefore it seems likely affected instances are using timeline
streaming and do so in just the right way to trigger this. We
can tune the garbage collector just for websocket processes
and use a more lenient value of 20 to keep the added perf cost
in check.

Unfortunately none of the affected instances responded to inquieries
to test this more selective gc tuning, so this is not fully veriefied.
However, given the general reports regarding websockets and Pleroma —
as it turns out — also applying and having properly tested a very
similar tweak seems to support this theory.

Ref.:
  https://www.erlang.org/doc/man/erlang#ghlink-process_flag-2-idp226
  https://blog.guzman.codes/using-phoenix-channels-high-memory-usage-save-money-with-erlfullsweepafter
  https://git.pleroma.social/pleroma/pleroma/-/merge_requests/4060

lib/pleroma/web/mastodon_api/websocket_handler.ex

@@ -18,6 +18,8 @@ defmodule Pleroma.Web.MastodonAPI.WebsocketHandler do
   @timeout :timer.seconds(60)
   # Hibernate every X messages
   @hibernate_every 100
+  # Tune garabge collect for long-lived websocket process
+  @fullsweep_after 20
 
   def init(%{qs: qs} = req, state) do
     with params <- Enum.into(:cow_qs.parse_qs(qs), %{}),
@@ -59,6 +61,10 @@ defmodule Pleroma.Web.MastodonAPI.WebsocketHandler do
       "#{__MODULE__} accepted websocket connection for user #{(state.user || %{id: "anonymous"}).id}, topic #{state.topic}"
     )
 
+    # process is long-lived and can sometimes accumulate stale data in such a way it's
+    # not freed by young garbage cycles, thus make full collection sweeps more frequent
+    :erlang.process_flag(:fullsweep_after, @fullsweep_after)
+
     Streamer.add_socket(state.topic, state.oauth_token)
     {:ok, %{state | timer: timer()}}
   end