adding a test for clearing stale connections

activerecord/test/cases/connection_pool_test.rb

+require "cases/helper"
+
+module ActiveRecord
+  module ConnectionAdapters
+    class ConnectionPoolTest < ActiveRecord::TestCase
+      def test_clear_stale_cached_connections!
+        pool = ConnectionPool.new ActiveRecord::Base.connection_pool.spec
+
+        threads = [
+          Thread.new { pool.connection },
+          Thread.new { pool.connection }]
+
+        threads.map { |t| t.join }
+
+        pool.extend Module.new {
+          attr_accessor :checkins
+          def checkin conn
+            @checkins << conn
+            conn.object_id
+          end
+        }
+        pool.checkins = []
+
+        cleared_threads = pool.clear_stale_cached_connections!
+        assert((cleared_threads - threads.map { |x| x.object_id }).empty?,
+               "threads should have been removed")
+        assert_equal pool.checkins.length, threads.length
+      end
+    end
+  end
+end