lint: discourage stubbing in before(:once) blocks, fixes SD-861

test plan:
* see test commit

Change-Id: I34433152aae370618c6bae1a23078e7c99c8b084
Reviewed-on: https://gerrit.instructure.com/74741
Tested-by: Jenkins
Reviewed-by: Jon Jensen <jon@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>

Author: Landon Wilkins

gems/rubocop-canvas/lib/rubocop_canvas.rb

@@ -17,6 +17,7 @@
 require 'rubocop_canvas/cops/lint/freeze_constants'
 require 'rubocop_canvas/cops/lint/sleep'
 require 'rubocop_canvas/cops/lint/specs_before_all'
+require 'rubocop_canvas/cops/lint/specs_before_once_stubs'
 require 'rubocop_canvas/cops/lint/specs_ensure_spec_extension'
 require 'rubocop_canvas/cops/lint/specs_execute_script'
 require 'rubocop_canvas/cops/lint/specs_f_over_fj'

gems/rubocop-canvas/lib/rubocop_canvas/cops/lint/specs_before_once_stubs.rb

@@ -0,0 +1,36 @@
+module RuboCop
+  module Cop
+    module Lint
+      class SpecsBeforeOnceStubs < Cop
+        MSG = "Stubs in a `before(:once)` block won't carry over to the examples; you should move this to a `before(:each)`"
+        # http://gofreerange.com/mocha/docs/Mocha/Mock.html
+        # - stubs
+        # - returns
+        # homegrown:
+        # - stub_file_data
+        # - stub_kaltura
+        # - stub_png_data
+        STUB_METHODS = %i[
+          stubs
+          returns
+          stub_file_data
+          stub_kaltura
+          stub_png_data
+        ].freeze
+
+        BLOCK_METHOD = :before
+        BLOCK_ARG = :once
+
+        def on_send(node)
+          _receiver, method_name, *_args = *node
+          return unless STUB_METHODS.include? method_name
+          return unless node.ancestors.find do |ancestor|
+            ancestor.children[0].to_a[1] == BLOCK_METHOD &&
+              ancestor.children[0].to_a[2].children[0] == BLOCK_ARG
+          end
+          add_offense node, :expression, MSG
+        end
+      end
+    end
+  end
+end