Allow comments when reading the JSON cache files (fixes load failure on json 3.0)

CHANGELOG.md

@@ -1,5 +1,6 @@
 Unreleased
 -----
+* Fix loading the JSON conversion table cache under json 3.0, which no longer accepts comments by default. (@jules-w2)
 
 3.2.1
 -----

lib/measured/cache/json.rb

@@ -15,7 +15,10 @@ def exist?
 
     def read
       return unless exist?
-      decode(JSON.load(File.read(@path), nil, freeze: true))
+      # The cache files written by Measured::Cache::JsonWriter start with a `//` comment line, which is not
+      # valid JSON. json < 3.0 accepted comments by default, json >= 3.0 requires opting in. The option is
+      # ignored by json < 2.20, where comments are accepted anyway.
+      decode(JSON.load(File.read(@path), nil, freeze: true, allow_comments: true))
     end
 
     def write(table)

test/cache/json_test.rb

@@ -53,6 +53,13 @@ class Measured::Cache::JsonTest < ActiveSupport::TestCase
     assert_equal @table_hash, @cache.read
   end
 
+  test "#read loads a file that starts with the comment header written by the writer" do
+    commented_json = "// Do not modify this file directly. Regenerate it with 'rake cache:write'.\n#{@table_json}"
+    File.expects(:exist?).with(@cache.path).returns(true)
+    File.expects(:read).with(@cache.path).returns(commented_json)
+    assert_equal @table_hash, @cache.read
+  end
+
   test "#write raises not implemented" do
     assert_raises(ArgumentError) do
       @cache.write({})