Connection Pool: add tests

diff --git a/test/gun/conneciton_pool_test.exs b/test/gun/conneciton_pool_test.exs
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+++ b/test/gun/conneciton_pool_test.exs
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+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2020 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Gun.ConnectionPoolTest do
+  use Pleroma.DataCase
+
+  import Mox
+  import ExUnit.CaptureLog
+  alias Pleroma.Config
+  alias Pleroma.Gun.ConnectionPool
+
+  defp gun_mock(_) do
+    Pleroma.GunMock
+    |> stub(:open, fn _, _, _ -> Task.start_link(fn -> Process.sleep(100) end) end)
+    |> stub(:await_up, fn _, _ -> {:ok, :http} end)
+    |> stub(:set_owner, fn _, _ -> :ok end)
+
+    :ok
+  end
+
+  setup :set_mox_from_context
+  setup :gun_mock
+
+  test "gives the same connection to 2 concurrent requests" do
+    Enum.map(
+      [
+        "http://www.korean-books.com.kp/KBMbooks/en/periodic/pictorial/20200530163914.pdf",
+        "http://www.korean-books.com.kp/KBMbooks/en/periodic/pictorial/20200528183427.pdf"
+      ],
+      fn uri ->
+        uri = URI.parse(uri)
+        task_parent = self()
+
+        Task.start_link(fn ->
+          {:ok, conn} = ConnectionPool.get_conn(uri, [])
+          ConnectionPool.release_conn(conn)
+          send(task_parent, conn)
+        end)
+      end
+    )
+
+    [pid, pid] =
+      for _ <- 1..2 do
+        receive do
+          pid -> pid
+        end
+      end
+  end
+
+  test "connection limit is respected with concurrent requests" do
+    clear_config([:connections_pool, :max_connections]) do
+      Config.put([:connections_pool, :max_connections], 1)
+      # The supervisor needs a reboot to apply the new config setting
+      Process.exit(Process.whereis(Pleroma.Gun.ConnectionPool.WorkerSupervisor), :kill)
+
+      on_exit(fn ->
+        Process.exit(Process.whereis(Pleroma.Gun.ConnectionPool.WorkerSupervisor), :kill)
+      end)
+    end
+
+    capture_log(fn ->
+      Enum.map(
+        [
+          "https://ninenines.eu/",
+          "https://youtu.be/PFGwMiDJKNY"
+        ],
+        fn uri ->
+          uri = URI.parse(uri)
+          task_parent = self()
+
+          Task.start_link(fn ->
+            result = ConnectionPool.get_conn(uri, [])
+            # Sleep so that we don't end up with a situation,
+            # where request from the second process gets processed
+            # only after the first process already released the connection
+            Process.sleep(50)
+
+            case result do
+              {:ok, pid} ->
+                ConnectionPool.release_conn(pid)
+
+              _ ->
+                nil
+            end
+
+            send(task_parent, result)
+          end)
+        end
+      )
+
+      [{:error, :pool_full}, {:ok, _pid}] =
+        for _ <- 1..2 do
+          receive do
+            result -> result
+          end
+        end
+        |> Enum.sort()
+    end)
+  end
+end