DEV: Introduce debugging rake task for PostRevision issues (#17998)

See https://meta.discourse.org/t/232885 for context. This rake task enumerates problematic records and prints out a sample of each failure.

lib/tasks/revisions.rake

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+desc 'Print debug information about post_revisions which cannot be deserialized'
+task 'revisions:debug_deserialization' => :environment do
+  puts "Checking #{PostRevision.count} PostRevision records in batches of 1000... this may take some time..."
+  sleep 1
+
+  counts = Hash.new(0)
+  examples = Hash.new()
+
+  PostRevision.find_each do |revision|
+    revision.modifications
+  rescue Psych::DisallowedClass => e
+    class_name = e.message.sub("Tried to load unspecified class: ", "")
+    counts[class_name] += 1
+    examples[class_name] ||= revision
+    puts "#{Discourse.base_url}/p/#{revision.post_id} (revision number:#{revision.number} id:#{revision.id}) #{e.message}"
+  end
+
+  puts
+  puts "Done"
+  puts
+
+  puts "---- Summary ----"
+  puts "Checked records: #{PostRevision.count}"
+  counts.each_pair { |k, v| puts "  #{k}: #{v}" }
+
+  puts
+  puts "---- Examples ----"
+  puts
+  examples.each_pair do |class_name, revision|
+    puts "-- BEGIN Example #{class_name} --"
+    puts
+    puts revision.modifications_before_type_cast
+    puts
+    puts "-- END Example #{class_name} --"
+  end
+end