Change cursor from bigint to string

Working on the upcoming support for tasks backed by custom enumerators
surfaced the need for non-numeric cursors.

For instance, one might have a task that iterates over resources backed
by some API providing its own opaque string cursor, which would be
impossible to serialize to a bigint column.

As such, this changes the maintenance_tasks_runs.cursor column to be a
string. For this to work, we must deserialize the cursor back to an
integer in the Array and CSV collection cases, while the ActiveRecord
case is handled transparently.

Co-authored-by: Adrianna Chang <[email protected]>
Co-authored-by: Sam Bostock <[email protected]>

app/jobs/concerns/maintenance_tasks/task_job_concern.rb

@@ -56,14 +56,14 @@ def build_enumerator(_run, cursor:)
           batch_size: collection.batch_size,
         )
       when Array
-        enumerator_builder.build_array_enumerator(collection, cursor: cursor)
+        enumerator_builder.build_array_enumerator(collection, cursor: cursor&.to_i)
       when BatchCsvCollectionBuilder::BatchCsv
         JobIteration::CsvEnumerator.new(collection.csv).batches(
           batch_size: collection.batch_size,
-          cursor: cursor,
+          cursor: cursor&.to_i,
         )
       when CSV
-        JobIteration::CsvEnumerator.new(collection).rows(cursor: cursor)
+        JobIteration::CsvEnumerator.new(collection).rows(cursor: cursor&.to_i)
       else
         raise ArgumentError, <<~MSG.squish
           #{@task.class.name}#collection must be either an

db/migrate/20210219212931_change_cursor_to_string.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class ChangeCursorToString < ActiveRecord::Migration[6.0]
+  # This migration will clear all existing data in the cursor column for
+  # both MySQL and PostgresQL. Ensure no Tasks are paused when this migration
+  # is deployed, or they will be resumed from the start.
+  # Running tasks are able to gracefully handle this change, even if
+  # interrupted.
+  def up
+    change_table(:maintenance_tasks_runs) do |t|
+      t.change(:cursor, :string)
+    end
+  end
+
+  def down
+    change_table(:maintenance_tasks_runs) do |t|
+      t.change(:cursor, :bigint)
+    end
+  end
+end

test/dummy/db/schema.rb

@@ -47,7 +47,7 @@
     t.bigint "tick_count", default: 0, null: false
     t.bigint "tick_total"
     t.string "job_id"
-    t.bigint "cursor"
+    t.string "cursor"
     t.string "status", default: "enqueued", null: false
     t.string "error_class"
     t.string "error_message"

test/jobs/maintenance_tasks/task_job_test.rb

@@ -233,11 +233,11 @@ class << self
 
       TaskJob.perform_now(@run)
 
-      assert_equal 0, @run.reload.cursor
+      assert_equal "0", @run.reload.cursor
     end
 
     test ".perform_now starts job from cursor position when job resumes" do
-      @run.update!(cursor: 0)
+      @run.update!(cursor: "0")
 
       Maintenance::TestTask.any_instance.expects(:process).once.with(2)