Fix resolving inherit_gem in remote configs

When a file inherits from a url inheriting from a gem, like so:

```yaml
# .rubocop.yml
inherit_from: http://example.com/default.yml
```

```yaml
# http://example.com/default.yml
inherit_gem:
    somegem: default.yml
```

```yaml
# somegem's default.yml
Layout/LineLength:
    Max: 48
```

Rubocop would then incorrectly try to resolve a URL like this:

`http://example.com//private/var/folders/69/bjrqv8v1225f7yqnxknm3fb80000gn/T/d20200518-94245-6dqeew/work/gems/somegem/default.yml`

With this change, local gem paths are no longer taken by mistake for relative remote paths.

CHANGELOG.md

@@ -26,6 +26,7 @@
 * [#7886](https://github.com/rubocop-hq/rubocop/issues/7886): Fix a bug in `AllowComments` logic in `Lint/SuppressedException`. ([@jonas054][])
 * [#7991](https://github.com/rubocop-hq/rubocop/issues/7991): Fix an error for `Layout/EmptyLinesAroundAttributeAccessor` when attribute method is method chained. ([@koic][])
 * [#7993](https://github.com/rubocop-hq/rubocop/issues/7993): Fix a false positive for `Migration/DepartmentName` when a disable comment contains an unexpected character for department name. ([@koic][])
+* [#7990](https://github.com/rubocop-hq/rubocop/issues/7990): Fix resolving `inherit_gem` in remote configs. ([@CvX][])
 
 ### Changes
 
@@ -4538,3 +4539,4 @@
 [@jeffcarbs]: https://github.com/jeffcarbs
 [@laurmurclar]: https://github.com/laurmurclar
 [@jethrodaniel]: https://github.com/jethrodaniel
+[@CvX]: https://github.com/CvX

lib/rubocop/config_loader_resolver.rb

@@ -171,10 +171,20 @@ def base_configs(path, inherit_from, file)
 
     def inherited_file(path, inherit_from, file)
       if remote_file?(inherit_from)
+        # A remote configuration, e.g. `inherit_from: http://example.com/rubocop.yml`.
         RemoteConfig.new(inherit_from, File.dirname(path))
+      elsif Pathname.new(inherit_from).absolute?
+        # An absolute path to a config, e.g. `inherit_from: /Users/me/rubocop.yml`.
+        # The path may come from `inherit_gem` option, where a gem name is expanded
+        # to an absolute path to that gem.
+        print 'Inheriting ' if ConfigLoader.debug?
+        inherit_from
       elsif file.is_a?(RemoteConfig)
+        # A path relative to a URL, e.g. `inherit_from: configs/default.yml`
+        # in a config included with `inherit_from: http://example.com/rubocop.yml`
         file.inherit_from_remote(inherit_from, path)
       else
+        # A local relative path, e.g. `inherit_from: default.yml`
         print 'Inheriting ' if ConfigLoader.debug?
         File.expand_path(inherit_from, File.dirname(path))
       end

spec/rubocop/config_loader_spec.rb

@@ -781,7 +781,7 @@ class Loop < Cop
 
     context 'when a file inherits from a known gem' do
       let(:file_path) { '.rubocop.yml' }
-      let(:gem_root) { 'gems' }
+      let(:gem_root) { File.expand_path('gems') }
 
       before do
         create_file("#{gem_root}/gemone/config/rubocop.yml",
@@ -851,6 +851,35 @@ class Loop < Cop
       end
     end
 
+    context 'when a file inherits from a url inheriting from a gem' do
+      let(:file_path) { '.rubocop.yml' }
+      let(:cache_file) { '.rubocop-http---example-com-default-yml' }
+      let(:gem_root) { File.expand_path('gems') }
+      let(:gem_name) { 'somegem' }
+
+      before do
+        create_file(file_path, ['inherit_from: http://example.com/default.yml'])
+
+        stub_request(:get, %r{example.com/default})
+          .to_return(status: 200, body: "inherit_gem:\n    #{gem_name}: default.yml")
+
+        create_file("#{gem_root}/#{gem_name}/default.yml", ["Layout/LineLength:\n    Max: 48"])
+
+        mock_spec = OpenStruct.new(gem_dir: File.join(gem_root, gem_name))
+        allow(Gem::Specification).to receive(:find_by_name)
+          .with(gem_name).and_return(mock_spec)
+        allow(Gem).to receive(:path).and_return([gem_root])
+      end
+
+      after do
+        File.unlink cache_file if File.exist? cache_file
+      end
+
+      it 'resolves the inherited config' do
+        expect(configuration_from_file['Layout/LineLength']['Max']).to eq(48)
+      end
+    end
+
     context 'when a file inherits from a url' do
       let(:file_path) { '.rubocop.yml' }
       let(:cache_file) { '.rubocop-http---example-com-rubocop-yml' }