Handle NaN in GradeCalculator

Fixes CNVS-32980

Test plan: smoke test grade calculator
* As a teacher, create an assignment
* As a student, submit something for that assignment
* As a teacher, grade the submission
* Make sure the grade saved properly and is viewable by teacher
  and student.

Change-Id: Ie789492c3966be660bb726079dc2191cc1cbf8fe
Reviewed-on: https://gerrit.instructure.com/99065
Reviewed-by: Jeremy Neander <jneander@instructure.com>
Reviewed-by: Keith T. Garner <kgarner@instructure.com>
Tested-by: Jenkins
QA-Review: Anju Reddy <areddy@instructure.com>
Product-Review: Neil Gupta <ngupta@instructure.com>

lib/grade_calculator.rb

@@ -147,6 +147,13 @@ class GradeCalculator
     end
   end
 
+  def number_or_null(score)
+    # GradeCalculator sometimes divides by 0 somewhere,
+    # resulting in NaN. Treat that as null here
+    score = nil if score.try(:nan?)
+    score || 'NULL'
+  end
+
   def save_scores
     raise "Can't save scores when ignore_muted is false" unless @ignore_muted
 
@@ -164,7 +171,7 @@ class GradeCalculator
               current_score = CASE enrollment_id
                 #{@current_updates.map do |user_id, score|
                   enrollments_by_user[user_id].map do |enrollment|
-                    "WHEN #{enrollment.id} THEN #{score || 'NULL'}"
+                    "WHEN #{enrollment.id} THEN #{number_or_null(score)}"
                   end.join(' ')
                 end.join(' ')}
                 ELSE current_score
@@ -172,7 +179,7 @@ class GradeCalculator
               final_score = CASE enrollment_id
                 #{@final_updates.map do |user_id, score|
                   enrollments_by_user[user_id].map do |enrollment|
-                    "WHEN #{enrollment.id} THEN #{score || 'NULL'}"
+                    "WHEN #{enrollment.id} THEN #{number_or_null(score)}"
                   end.join(' ')
                 end.join(' ')}
                 ELSE final_score
@@ -190,7 +197,7 @@ class GradeCalculator
               CASE enrollments.id
                 #{@current_updates.map do |user_id, score|
                   enrollments_by_user[user_id].map do |enrollment|
-                    "WHEN #{enrollment.id} THEN #{score || 'NULL'}"
+                    "WHEN #{enrollment.id} THEN #{number_or_null(score)}"
                   end.join(' ')
                 end.join(' ')}
                 ELSE NULL
@@ -198,7 +205,7 @@ class GradeCalculator
               CASE enrollments.id
                 #{@final_updates.map do |user_id, score|
                   enrollments_by_user[user_id].map do |enrollment|
-                    "WHEN #{enrollment.id} THEN #{score || 'NULL'}"
+                    "WHEN #{enrollment.id} THEN #{number_or_null(score)}"
                   end.join(' ')
                 end.join(' ')}
                 ELSE NULL

spec/lib/grade_calculator_spec.rb

@@ -456,6 +456,26 @@ describe GradeCalculator do
     end
   end
 
+  describe '#number_or_null' do
+    it "should return a valid score" do
+      calc = GradeCalculator.new [@user.id], @course.id
+      score = 23.4
+      expect(calc.send(:number_or_null, score)).to eql(score)
+    end
+
+    it "should convert NaN to NULL" do
+      calc = GradeCalculator.new [@user.id], @course.id
+      score = 0/0.0
+      expect(calc.send(:number_or_null, score)).to eql('NULL')
+    end
+
+    it "should convert nil to NULL" do
+      calc = GradeCalculator.new [@user.id], @course.id
+      score = nil
+      expect(calc.send(:number_or_null, score)).to eql('NULL')
+    end
+  end
+
   describe '#compute_and_save_scores' do
     before(:once) do
       @first_period, @second_period = grading_periods(count: 2)