EquationImagesController#show can support alternate latex service

refs CNVS-29513

- Replace `+` with `%2B` in params[:id]
- Update setting identifier and default value for service url so that
  service interface can be different between existing code cogs
  integration and future integrations.
- Add a predeploy migration to move existing
  `codecogs.equation_image_link` settings to `equation_image.url`, and
  update the values to include a trailing `?`.
- Add a postdeploy migration to remove `codecogs.equation_image_link`
  settings.

test plan:
- Navigate to form to create new assignment.
- Via TinyMCE, open the equation editor and add an equation / formula to
  the assignment description.
- Observe that upon submitting the form to insert the equation, it is
  displayed in the TinyMCE editor text area.
- Observe that upon creating the assignnment and viewing it, the
  equation is visible in the assignment description.

Change-Id: Ie4ffff640e51f9269a68fb3df81aae7d1f5ca33b
Reviewed-on: https://gerrit.instructure.com/80404
Reviewed-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins
QA-Review: Benjamin Christian Nelson <bcnelson@instructure.com>
Product-Review: John Corrigan <jcorrigan@instructure.com>

app/controllers/equation_images_controller.rb

@@ -1,12 +1,16 @@
 class EquationImagesController < ApplicationController
 
-  # Facade to codecogs API so we retain control
+  # Facade to codecogs API for gif generation or microservice MathMan for svg
   def show
-    # TODO: escape id here, and stop double escaping it in
-    # public/javascripts/tinymce/jscripts/tiny_mce/plugins/instructure_equation/editor_plugin.js
-    # this will require a corresponding data migration to fix
-    base_url = Setting.get('codecogs.equation_image_link', 'http://latex.codecogs.com/gif.latex')
-    redirect_to base_url + '?' + params[:id]
+    base_url = Setting.get('equation_image_url', 'http://latex.codecogs.com/gif.latex?')
+
+    # At the moment, the latex string is stored in the db double escaped. By
+    # the time the value gets here as `params[:id]` it has been unescaped once.
+    # This is nearly how we want it to pass it on to the next service, except
+    # `+` signs are in tact. Since normally the `+` signifies a space and we
+    # want the `+` signs for real, we need to encode them.
+    latex = params[:id].gsub('+', '%2B')
+    redirect_to base_url + latex
   end
 
 end

db/migrate/20160601192206_update_setting_equation_svg_url_default.rb

@@ -0,0 +1,21 @@
+class UpdateSettingEquationSvgUrlDefault < ActiveRecord::Migration
+  tag :predeploy
+
+  def up
+    return unless Shard.current.default?
+
+    setting = Setting.where(name: 'codecogs.equation_image_link').take
+    if setting.present?
+      Setting.set('equation_image_url', "#{setting.value}?")
+    end
+  end
+
+  def down
+    return unless Shard.current.default?
+
+    setting = Setting.where(name: 'equation_image_url').take
+    if setting.present?
+      Setting.set('codecogs.equation_image_link', setting.value.sub(/\?$/, ''))
+    end
+  end
+end

db/migrate/20160601195833_remove_codecogs_equation_image_setting_key.rb

@@ -0,0 +1,15 @@
+class RemoveCodecogsEquationImageSettingKey < ActiveRecord::Migration
+  tag :postdeploy
+
+  def up
+    return unless Shard.current.default?
+
+    Setting.remove('codecogs.equation_image_link')
+  end
+
+  def down
+    return unless Shard.current.default?
+
+    Setting.remove('equation_image_url')
+  end
+end

spec/controllers/equation_images_controller_spec.rb

@@ -7,4 +7,9 @@ describe EquationImagesController do
     expect(response).to redirect_to('http://latex.codecogs.com/gif.latex?foo')
   end
 
+  it 'encodes `+` signs properly' do
+    latex = '5%5E5%5C%3A+%5C%3A%5Csqrt%7B9%7D'
+    get :show, id: latex
+    expect(response).to redirect_to(/\%2B/)
+  end
 end