fix issues using timestamp as a pagination bookmark

when a timestamp is round-tripped through a page parameter,
it comes back as a string which was failing to line up with
the bookmarked row for one of two reasons:
 1. the date string was in local time (with a UTC offset
    that the database was ignoring)
 2. the date string truncated the fractional seconds part

solve both issues by explicitly converting timestamps to
UTC and preserving fractional seconds

test plan:
 - have two courses in different shards selected in your
   calendar, having assignments with due dates in both
   (at least 50 such assignments in one of the courses)
 - all the assignments should show up on your calendar
   (just once each)

fixes LS-2156

flag = none

Change-Id: Ia2010e08653520923e1c3f05d17697e1a3cd8826
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/263546
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Eric Saupe <eric.saupe@instructure.com>
QA-Review: Eric Saupe <eric.saupe@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

gems/bookmarked_collection/lib/bookmarked_collection/collection.rb

@@ -76,7 +76,7 @@ class BookmarkedCollection::Collection < Array
   attr_accessor :include_bookmark
 
   def bookmark_to_page(bookmark)
-    bookmark && "bookmark:#{::JSONToken.encode(bookmark)}"
+    bookmark && "bookmark:#{::JSONToken.encode(format_dates(bookmark))}"
   end
 
   def page_to_bookmark(page)
@@ -118,4 +118,15 @@ class BookmarkedCollection::Collection < Array
   def has_more!
     self.next_bookmark = bookmark_for(last)
   end
+
+  def format_dates(bookmark)
+    # to_json will record local time + UTC offset with seconds truncated, both of which will screw up pagination
+    if bookmark.respond_to?(:iso8601)
+      bookmark.utc.iso8601(6)
+    elsif bookmark.is_a?(Array)
+      bookmark.map { |e| format_dates(e) }
+    else
+      bookmark
+    end
+  end
 end

gems/bookmarked_collection/spec/bookmarked_collection/collection_spec.rb

@@ -102,6 +102,24 @@ describe "BookmarkedCollection::Collection" do
     end
   end
 
+  describe "round-tripping dates" do
+    it "converts to UTC" do
+      timestamp = DateTime.parse('2020-12-31T22:00:00-09:00').in_time_zone('Alaska')
+      page = @collection.bookmark_to_page([1, timestamp])
+      expect(page).to match(/^bookmark:/)
+      bookmark = @collection.page_to_bookmark(page)
+      expect(bookmark).to eq([1, "2021-01-01T07:00:00.000000Z"])
+    end
+
+    it "preserves fractional times" do
+      timestamp = DateTime.parse('2020-02-22T22:22:22.22Z')
+      page = @collection.bookmark_to_page([1, [2, timestamp]])
+      expect(page).to match(/^bookmark:/)
+      bookmark = @collection.page_to_bookmark(page)
+      expect(bookmark).to eq([1, [2, "2020-02-22T22:22:22.220000Z"]])
+    end
+  end
+
   describe "#current_page=" do
     it "should set current_bookmark to nil if nil" do
       @collection.current_bookmark = "some value"

spec/apis/v1/calendar_events_api_spec.rb

@@ -2363,6 +2363,41 @@ describe CalendarEventsApiController, type: :request do
         end
       end
     end
+
+    context "sharding" do
+      specs_require_sharding
+
+      before :once do
+        @c0 = @course
+        @a0 = @c0.assignments.create(workflow_state: 'published', due_at: 1.day.from_now)
+        @shard1.activate do
+          @shard1_account = Account.create!
+          @c1 = course_with_teacher(user: @me, account: @shard1_account, enrollment_state: 'active').course
+          @a1 = @c1.assignments.create(workflow_state: 'published', due_at: 2.days.from_now)
+        end
+      end
+
+      it "paginates assignments from multiple shards correctly" do
+        json = api_call(:get, "/api/v1/calendar_events?type=assignment&context_codes[]=course_#{@c0.id}&context_codes[]=course_#{@c1.id}&all_events=1&per_page=1",
+                        controller: 'calendar_events_api', action: 'index', format: 'json', type: 'assignment',
+                        context_codes: [@c0.asset_string, @c1.global_asset_string], all_events: 1, per_page: 1)
+        expect(json.size).to eq 1
+        expect(json[0]['id']).to eq @a0.asset_string
+
+        links = Api.parse_pagination_links(response.headers['Link'])
+        next_link = links.detect { |link| link[:rel] == 'next' }
+        expect(next_link).not_to be_nil
+
+        json = api_call(:get, next_link[:uri].to_s, controller: 'calendar_events_api', action: 'index',
+                        format: 'json', type: 'assignment', context_codes: [@c0.asset_string, @c1.global_asset_string],
+                        all_events: 1, per_page: 1, page: next_link['page'])
+        expect(json.size).to eq 1
+        expect(json[0]['id']).to eq @a1.asset_string
+
+        links = Api.parse_pagination_links(response.headers['Link'])
+        expect(links.detect { |link| link[:rel] == 'next' }).to be_nil
+      end
+    end
   end
 
   context "user index" do