fix double-_localstorage_ in Attachment#infer_namespace

test plan:
 - in a Rails development environment using local file storage,
   1. use the content migration API to import the contents
      of a zip file into a folder in a course
      (use zip_file_importer as the migration_type, and set
       settings[folder_id] to the id of a folder in the course)
   2. repeat step (1), but specify a different folder id
   3. confirm the files can be downloaded and/or deleted in the
      second folder without a "Cannot find Account with id=0"
      error

fixes CNVS-15726

Change-Id: Icb73efcf5b933f4a6b9dea9c0e2c884bfb82afd5
Reviewed-on: https://gerrit.instructure.com/41583
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

Author: jstanley0

app/models/attachment.rb

@@ -426,7 +426,7 @@ def infer_namespace
     ns ||= self.context.account.file_namespace rescue nil
     if Rails.env.development? && Attachment.local_storage?
       ns ||= ""
-      ns = "_localstorage_/#{ns}"
+      ns = "_localstorage_/#{ns}" unless ns.start_with?('_localstorage_/')
     end
     ns = nil if ns && ns.empty?
     ns

spec/models/attachment_spec.rb

@@ -1325,4 +1325,16 @@ def configure_crocodoc
     end
   end
 
+  describe "infer_namespace" do
+    it "should infer the correct namespace from the root attachment" do
+      local_storage!
+      Rails.env.stubs(:development?).returns(true)
+      course
+      a1 = attachment_model(context: @course, uploaded_data: default_uploaded_data)
+      a2 = attachment_model(context: @course, uploaded_data: default_uploaded_data)
+      a2.root_attachment.should eql(a1)
+      a2.namespace.should eql(a1.namespace)
+    end
+  end
+
 end