optimize CrocodocDocument.update_process_states

there is an index on process_states, but postgres can't use it cause
find_in_batches is ordering by id. so use the slave instead, which
allows (nay, forces) using a cursor, and no ordering, so postgres
can use it's index. (still do updates on the master)

Change-Id: I6f7a18f7ffa7a1a1971e4eb5195af890e2038fbd
Reviewed-on: https://gerrit.instructure.com/23593
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

app/models/crocodoc_document.rb

@@ -125,33 +125,37 @@ class CrocodocDocument < ActiveRecord::Base
 
   def self.update_process_states
     bs = Setting.get('crocodoc_status_check_batch_size', '45').to_i
-    CrocodocDocument.where(:process_state => %w(QUEUED PROCESSING)).find_in_batches do |docs|
-      statuses = []
-      docs.each_slice(bs) do |sub_docs|
-        statuses.concat CrocodocDocument.crocodoc_api.status(sub_docs.map(&:uuid))
-      end
+    Shackles.activate(:slave) do
+      CrocodocDocument.where(:process_state => %w(QUEUED PROCESSING)).find_in_batches do |docs|
+        Shackles.activate(:master) do
+          statuses = []
+          docs.each_slice(bs) do |sub_docs|
+            statuses.concat CrocodocDocument.crocodoc_api.status(sub_docs.map(&:uuid))
+          end
 
-      statuses.each do |status|
-        uuid, state = status['uuid'], status['status']
-        CrocodocDocument.
-            where(:uuid => status['uuid']).
-            update_all(:process_state => status['status'])
-        if status['status'] == 'ERROR'
-          error = status['error'] || 'No explanation given'
-          ErrorReport.log_error 'crocodoc', :message => error
+          statuses.each do |status|
+            uuid, state = status['uuid'], status['status']
+            CrocodocDocument.
+                where(:uuid => status['uuid']).
+                update_all(:process_state => status['status'])
+            if status['status'] == 'ERROR'
+              error = status['error'] || 'No explanation given'
+              ErrorReport.log_error 'crocodoc', :message => error
+            end
+          end
+
+          error_uuids = statuses.select { |s|
+            s['status'] == 'ERROR'
+          }.map { |s| s['uuid'] }
+          if error_uuids.present?
+            error_docs = CrocodocDocument.where(:uuid => error_uuids)
+            attachment_ids = error_docs.map(&:attachment_id)
+            Attachment.send_later_enqueue_args :submit_to_scribd,
+              {:n_strand => 'scribd', :max_attempts => 1},
+              attachment_ids
+          end
         end
       end
-
-      error_uuids = statuses.select { |s|
-        s['status'] == 'ERROR'
-      }.map { |s| s['uuid'] }
-      if error_uuids.present?
-        error_docs = CrocodocDocument.where(:uuid => error_uuids)
-        attachment_ids = error_docs.map(&:attachment_id)
-        Attachment.send_later_enqueue_args :submit_to_scribd,
-          {:n_strand => 'scribd', :max_attempts => 1},
-          attachment_ids
-      end
     end
   end
 end