Add a startup error for modified Repo pool_size

lib/pleroma/application_requirements.ex

@@ -24,6 +24,7 @@ defmodule Pleroma.ApplicationRequirements do
     |> check_migrations_applied!()
     |> check_welcome_message_config!()
     |> check_rum!()
+    |> check_repo_pool_size!()
     |> handle_result()
   end
 
@@ -188,6 +189,24 @@ defmodule Pleroma.ApplicationRequirements do
 
   defp check_system_commands!(result), do: result
 
+  defp check_repo_pool_size!(:ok) do
+    if Pleroma.Config.get([Pleroma.Repo, :pool_size], 10) != 10 and
+         not Pleroma.Config.get([:dangerzone, :override_repo_pool_size], false) do
+      Logger.error("""
+      !!!CONFIG WARNING!!!
+      The database pool size has been altered from the recommended value of 10.\n
+      Please revert or ensure your database is tuned appropriately and then set\n
+      `config :pleroma, :dangerzone, override_repo_pool_size: true`.
+      """)
+
+      {:error, "Repo.pool_size above recommended value."}
+    else
+      :ok
+    end
+  end
+
+  defp check_repo_pool_size!(result), do: result
+
   defp check_filter(filter, command_required) do
     filters = Config.get([Pleroma.Upload, :filters])