[Fix rubocop#3396] Concise error when config. file not found

Instead of a stack trace, prints a friendly (concise) message
to stderr when a configuration file, e.g. inherit_from, cannot
be found.

We use the constant Errno::ENOENT because it is present on
all major operating systems. According to the ruby documenation,
there may be some odd platforms that do not support this standard.
If we find that someone is using rubocop on such a platform, we
can rescue SystemCallError instead, but that will make it hard
to exit with the correct exit code.

.rubocop_todo.yml

@@ -13,7 +13,7 @@ Metrics/AbcSize:
 # Offense count: 37
 # Configuration parameters: CountComments.
 Metrics/ClassLength:
-  Max: 180
+  Max: 186
 
 # Offense count: 157
 # Configuration parameters: CountComments.

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Bug fixes
 
+* [#3396](https://github.com/bbatsov/rubocop/issues/3396): Concise error when config. file not found. ([@jaredbeck][])
 * [#4881](https://github.com/bbatsov/rubocop/issues/4881): Fix a false positive for `Performance/HashEachMethods` when unused argument(s) exists in other blocks. ([@pocke][])
 * [#4883](https://github.com/bbatsov/rubocop/pull/4883): Fix auto-correction for `Performance/HashEachMethods`. ([@pocke][])
 * [#4896](https://github.com/bbatsov/rubocop/pull/4896): Fix Style/DateTime wrongly triggered on classes `...::DateTime`. ([@dpostorivo][])

lib/rubocop/config_loader.rb

@@ -211,7 +211,7 @@ def transform(config)
       end
 
       def load_yaml_configuration(absolute_path)
-        yaml_code = IO.read(absolute_path, encoding: Encoding::UTF_8)
+        yaml_code = read_file(absolute_path)
         hash = yaml_safe_load(yaml_code, absolute_path) || {}
 
         puts "configuration from #{absolute_path}" if debug?
@@ -223,6 +223,16 @@ def load_yaml_configuration(absolute_path)
         hash
       end
 
+      # Read the specified file, or exit with a friendly, concise message on
+      # stderr. Care is taken to use the standard OS exit code for a "file not
+      # found" error.
+      def read_file(absolute_path)
+        IO.read(absolute_path, encoding: Encoding::UTF_8)
+      rescue Errno::ENOENT
+        warn(format('Configuration file not found: %s', absolute_path))
+        exit(Errno::ENOENT::Errno)
+      end
+
       def yaml_safe_load(yaml_code, filename)
         if YAML.respond_to?(:safe_load) # Ruby 2.1+
           if defined?(SafeYAML) && SafeYAML.respond_to?(:load)

spec/rubocop/config_loader_spec.rb

@@ -548,18 +548,6 @@ class FilePath < Cop
       end
     end
 
-    context 'when a file inherits from a non http/https url' do
-      let(:file_path) { '.rubocop.yml' }
-
-      before do
-        create_file(file_path, ['inherit_from: c:\\\\foo\\bar.yml'])
-      end
-
-      it 'fails to load the resulting path' do
-        expect { configuration_from_file }.to raise_error(Errno::ENOENT)
-      end
-    end
-
     context 'EnabledByDefault / DisabledByDefault' do
       def cop_enabled?(cop_class)
         configuration_from_file.for_cop(cop_class).fetch('Enabled')
@@ -783,6 +771,20 @@ def cop_enabled?(cop_class)
         end
       end
     end
+
+    context 'when the file does not exist' do
+      let(:configuration_path) { 'file_that_does_not_exist.yml' }
+
+      it 'prints a friendly (concise) message to stderr and exits' do
+        expect { load_file }.to(
+          output(/Configuration file not found/).to_stderr.and(
+            raise_error(SystemExit) do |e|
+              expect(e.status).to(eq(Errno::ENOENT::Errno))
+            end
+          )
+        )
+      end
+    end
   end
 
   describe '.merge' do