Publisher: check out a connection for inserting publish_one jobs

Related to #1474, federation of one post on my istance creates in
best-case 360 jobs, so if they for some reason take a while to insert,
it will exhaust the connection pool. This fixes it by checking out one
dedicated connection for inserting them.

lib/pleroma/web/activity_pub/publisher.ex

@@ -9,6 +9,7 @@ defmodule Pleroma.Web.ActivityPub.Publisher do
   alias Pleroma.HTTP
   alias Pleroma.Instances
   alias Pleroma.Object
+  alias Pleroma.Repo
   alias Pleroma.User
   alias Pleroma.Web.ActivityPub.Relay
   alias Pleroma.Web.ActivityPub.Transmogrifier
@@ -188,31 +189,35 @@ def publish(%User{} = actor, %{data: %{"bcc" => bcc}} = activity)
 
     recipients = recipients(actor, activity)
 
-    recipients
-    |> Enum.filter(&User.ap_enabled?/1)
-    |> Enum.map(fn %{source_data: data} -> data["inbox"] end)
-    |> Enum.filter(fn inbox -> should_federate?(inbox, public) end)
-    |> Instances.filter_reachable()
-    |> Enum.each(fn {inbox, unreachable_since} ->
-      %User{ap_id: ap_id} =
-        Enum.find(recipients, fn %{source_data: data} -> data["inbox"] == inbox end)
+    inboxes =
+      recipients
+      |> Enum.filter(&User.ap_enabled?/1)
+      |> Enum.map(fn %{source_data: data} -> data["inbox"] end)
+      |> Enum.filter(fn inbox -> should_federate?(inbox, public) end)
+      |> Instances.filter_reachable()
 
-      # Get all the recipients on the same host and add them to cc. Otherwise, a remote
-      # instance would only accept a first message for the first recipient and ignore the rest.
-      cc = get_cc_ap_ids(ap_id, recipients)
+    Repo.checkout(fn ->
+      Enum.each(inboxes, fn {inbox, unreachable_since} ->
+        %User{ap_id: ap_id} =
+          Enum.find(recipients, fn %{source_data: data} -> data["inbox"] == inbox end)
 
-      json =
-        data
-        |> Map.put("cc", cc)
-        |> Jason.encode!()
+        # Get all the recipients on the same host and add them to cc. Otherwise, a remote
+        # instance would only accept a first message for the first recipient and ignore the rest.
+        cc = get_cc_ap_ids(ap_id, recipients)
 
-      Pleroma.Web.Federator.Publisher.enqueue_one(__MODULE__, %{
-        inbox: inbox,
-        json: json,
-        actor_id: actor.id,
-        id: activity.data["id"],
-        unreachable_since: unreachable_since
-      })
+        json =
+          data
+          |> Map.put("cc", cc)
+          |> Jason.encode!()
+
+        Pleroma.Web.Federator.Publisher.enqueue_one(__MODULE__, %{
+          inbox: inbox,
+          json: json,
+          actor_id: actor.id,
+          id: activity.data["id"],
+          unreachable_since: unreachable_since
+        })
+      end)
     end)
   end