Gopher: Fix crash on (re)boot when ConfigDB is enabled

Ranch listener wasn't being properly stopped when the Gopher GenServer
received a shutdown message due Restarter rebooting Pleroma to apply
ConfigDB configuration (originating from
Config.TransferTask.load_and_update_env/2) when ConfigDB is enabled.

Handle by trapping exits in the GenServer, which causes the terminate/2
function to be called and the Ranch listener to be stopped from there.

23:22:29.871 [error] GenServer Restarter.Pleroma terminating
** (MatchError) no match of right hand side value:

    {:error,
     {{:shutdown,
       {:failed_to_start_child, Pleroma.Gopher.Server,
        {{:badmatch, {:error, {:already_started, #PID<0.4801.0>}}},
         [
           {Pleroma.Gopher.Server, :init, 1,
            [file: ~c"lib/pleroma/gopher/server.ex", line: 25]},
           {:gen_server, :init_it, 2, [file: ~c"gen_server.erl", line: 2276]},
           {:gen_server, :init_it, 6, [file: ~c"gen_server.erl", line: 2236]},
           {:proc_lib, :init_p_do_apply, 3, [file: ~c"proc_lib.erl", line: 333]}
         ]}}}, {Pleroma.Application, :start, [:normal, []]}}}

    (restarter 0.1.0) lib/pleroma.ex:104: Restarter.Pleroma.do_restart/1
    (restarter 0.1.0) lib/pleroma.ex:96: Restarter.Pleroma.handle_cast/2
    (stdlib 7.2) gen_server.erl:2460: :gen_server.try_handle_cast/3
    (stdlib 7.2) gen_server.erl:2418: :gen_server.handle_msg/3
    (stdlib 7.2) proc_lib.erl:333: :proc_lib.init_p_do_apply/3
Last message: {:"$gen_cast", {:after_boot, :dev}}
State: %{rebooted: false, need_reboot: false, after_boot: false}

changelog.d/gopher-genserver-crash-on-boot.fix

@@ -0,0 +1 @@
+Gopher: Fix Ranch listener not being stopped properly on Pleroma restart when database configuration is enabled

lib/pleroma/gopher/server.ex

@@ -21,10 +21,13 @@ defmodule Pleroma.Gopher.Server do
 
   def init([ip, port]) do
     Logger.info("Starting gopher server on #{port}")
+    Process.flag(:trap_exit, true)
+
+    listener = :gopher
 
     {:ok, _pid} =
       :ranch.start_listener(
-        :gopher,
+        listener,
         :ranch_tcp,
         %{
           num_acceptors: 100,
@@ -35,7 +38,11 @@ defmodule Pleroma.Gopher.Server do
         []
       )
 
-    {:ok, %{ip: ip, port: port}}
+    {:ok, %{ip: ip, port: port, listener: listener}}
+  end
+
+  def terminate(_reason, state) do
+    :ranch.stop_listener(state.listener)
   end
 end