Fix load failure under SimpleCov 1.x json_formatter shim

simplecov 1.x ships its own simplecov_json_formatter.rb that can win the
require over the standalone gem depending on $LOAD_PATH order, leaving
SimpleCovJSONFormatter::ResultHashFormatter undefined (green locally, broke
CI). Require the standalone gem's classes by their unambiguous subpaths, and
make UndercoverSimplecovFormatter own its #format instead of inheriting the
version-divergent JSONFormatter#format.

Author: grodowski

lib/undercover/simplecov_formatter.rb

@@ -1,7 +1,14 @@
 # frozen_string_literal: true
 
 require 'simplecov'
-require 'simplecov_json_formatter'
+require 'json'
+
+# simplecov >= 1.0 ships its own simplecov_json_formatter.rb shim that can win
+# the require over the standalone gem depending on $LOAD_PATH order. Require the
+# standalone gem's classes by their subpaths (which simplecov does not ship) so
+# the SimpleCovJSONFormatter::* constants are always present.
+require 'simplecov_json_formatter/result_hash_formatter'
+require 'simplecov_json_formatter/result_exporter'
 
 # Patch ResultExporter to allow setting a custom export_path
 module SimpleCovJSONFormatter
@@ -125,11 +132,20 @@ def format_files
     end
   end
 
-  class UndercoverSimplecovFormatter < SimpleCov::Formatter::JSONFormatter
+  class UndercoverSimplecovFormatter
     class << self
       attr_accessor :output_filename
     end
 
+    # Own #format instead of inheriting SimpleCov::Formatter::JSONFormatter's:
+    # simplecov >= 1.0 reimplemented it to bypass #format_result and always
+    # write coverage.json, dropping undercover's meta and custom filename.
+    def format(result)
+      result_hash = format_result(result)
+      SimpleCovJSONFormatter::ResultExporter.new(result_hash).export
+      result_hash
+    end
+
     def format_result(result)
       result_hash_formater = ResultHashFormatterWithRoot.new(result)
       result_hash_formater.format

spec/undercover_simplecov_formatter_spec.rb

@@ -70,6 +70,19 @@
       output = formatter.format_result(result)
       expect(output).to eq({meta: {}, coverage: {}})
     end
+
+    it 'writes the formatted result through ResultExporter and returns the hash' do
+      result = double('result')
+      formatter = Undercover::UndercoverSimplecovFormatter.new
+      result_hash = {meta: {timestamp: 1}, coverage: {}}
+      allow(formatter).to receive(:format_result).with(result).and_return(result_hash)
+
+      exporter = instance_double(SimpleCovJSONFormatter::ResultExporter)
+      expect(SimpleCovJSONFormatter::ResultExporter).to receive(:new).with(result_hash).and_return(exporter)
+      expect(exporter).to receive(:export)
+
+      expect(formatter.format(result)).to eq(result_hash)
+    end
   end
 
   describe 'Undercover::SimplecovResultAdapter.parse' do