endpoint: Use custom Multipart module for dynamic configuration

lib/pleroma/web/endpoint.ex

@@ -101,13 +101,10 @@ defmodule Pleroma.Web.Endpoint do
   plug(Plug.Logger, log: :debug)
 
   plug(Plug.Parsers,
-    parsers: [
-      :urlencoded,
-      {:multipart, length: {Config, :get, [[:instance, :upload_limit]]}},
-      :json
-    ],
+    parsers: [:urlencoded, Pleroma.Web.Multipart, :json],
     pass: ["*/*"],
     json_decoder: Jason,
+    # Note: this is compile-time only, won't work for database-config
     length: Config.get([:instance, :upload_limit]),
     body_reader: {Pleroma.Web.Plugs.DigestPlug, :read_body, []}
   )

lib/pleroma/web/multipart.ex

@@ -0,0 +1,22 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2023 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+# <https://hexdocs.pm/plug/Plug.Parsers.MULTIPART.html#module-dynamic-configuration>
+defmodule Pleroma.Web.Multipart do
+  @multipart Plug.Parsers.MULTIPART
+
+  def init(opts) do
+    opts
+  end
+
+  def parse(conn, "multipart", subtype, headers, opts) do
+    length = Pleroma.Config.get([:instance, :upload_limit])
+    opts = @multipart.init([length: length] ++ opts)
+    @multipart.parse(conn, "multipart", subtype, headers, opts)
+  end
+
+  def parse(conn, _type, _subtype, _headers, _opts) do
+    {:next, conn}
+  end
+end