timeout when doing possibly unnecessary complicated counts for pagination

fixes USERS-274

test plan:
 * have a lot of users in an account (like 500)
 * go to /accounts/self/users
 * it should load, and show you how many pages there are
 * set the timeout to something absurdly low (like Setting.set('pagination_count_timeout', '1ms'))
 * go to the page again
 * it should still load the first page quickly, but the pages thingy at the bottom should only list
   1 and 2, and then an ellipsis

Change-Id: Ibc07036b39a9f4ed19d8824bb9254b23679298d1
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/234557
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

config/initializers/folio.rb

@@ -15,4 +15,44 @@
 # You should have received a copy of the GNU Affero General Public License along
 # with this program. If not, see <http://www.gnu.org/licenses/>.
 
-require 'folio/core_ext/enumerable'
+require 'folio/core_ext/enumerable'
+
+module Folio::WillPaginate::ActiveRecord::Pagination
+  def paginate(options={})
+    if !options.has_key?(:total_entries)
+      scope = if ::Rails.version < '4'
+        self.scoped
+      elsif self.is_a?(::ActiveRecord::Relation)
+        self
+      elsif self < ::ActiveRecord::Base
+        self.all
+      else
+        self.scope
+      end
+      begin
+        scope.connection.transaction(requires_new: true) do
+          scope.connection.execute("SET LOCAL statement_timeout='#{Setting.get('pagination_count_timeout', '5s')}'")
+          group_values = scope.group_values
+          unless group_values.empty?
+            # total_entries left to an auto-count, but the relation being
+            # paginated has a grouping. we need to do a special count, lest
+            # self.count give us a hash instead of the integer we expect.
+            having_clause_empty = Rails.version < '5' ? scope.having_values.empty? : scope.having_clause.empty?
+            if having_clause_empty && group_values.length == 1 # multi-column distinct counts are broken right now (as of rails 4.2.5) :(
+              if Rails.version < '5'
+                options[:total_entries] = except(:group, :select).select(group_values).uniq.count
+              else
+                options[:total_entries] = except(:group, :select).select(group_values).distinct.count
+              end
+            else
+              options[:total_entries] = unscoped.from("(#{to_sql}) a").count
+            end
+          end
+        end
+      rescue ActiveRecord::QueryCanceled
+        options[:total_entries] = nil
+      end
+    end
+    super(options).to_a
+  end
+end

spec/initializers/folio_spec.rb

@@ -0,0 +1,29 @@
+#
+# Copyright (C) 2020 - present Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+
+require_relative '../spec_helper'
+
+describe Folio do
+  it "skips the count for a grouped query that takes a long time" do
+    User.create!
+    User.create!
+    Setting.set('pagination_count_timeout', '5ms')
+    result = User.group(:id).where("pg_sleep(0.1) IS NOT NULL").paginate(per_page: 1)
+    expect(result.length).to eq 1
+    expect(result.total_entries).to be_nil
+  end
+end