Allow matching only the repo-root for CODEOWNERS

When wanting to match files in the repo root, but only on the repo's
root. Users would supply `/*` as the pattern.

In our old implementation we would match this pattern as `*`, but
allowing `*` to match directory separators.

In this we treat the `/*` pattern as an entirely separate case:
Matching all files in the root, but only one level deep.

ee/changelogs/unreleased/bvl-codeowners-root-dir.yml

+---
+title: Allow matching only the repo-root for CODEOWNERS
+merge_request: 8708
+author:
+type: fixed

ee/lib/gitlab/code_owners/file.rb

@@ -3,6 +3,8 @@
 module Gitlab
   module CodeOwners
     class File
+      ROOT_DIR_PATTERN = '/*'
+
       def initialize(blob)
         @blob = blob
       end
@@ -52,6 +54,8 @@ module Gitlab
       end
 
       def normalize_pattern(pattern)
+        return ROOT_DIR_PATTERN if pattern == ROOT_DIR_PATTERN
+
         # Remove `\` when escaping `\#`
         pattern = pattern.sub(/\A\\#/, '#')
         # Replace all whitespace preceded by a \ with a regular whitespace
@@ -74,7 +78,11 @@ module Gitlab
       def path_matches?(pattern, path)
         flags = ::File::FNM_DOTMATCH
 
-        if pattern.ends_with?('/*')
+        if pattern == ROOT_DIR_PATTERN
+          # Matching everyting on the root, but only one level deep
+          flags |= ::File::FNM_PATHNAME
+          ::File.fnmatch?('*', path, flags)
+        elsif pattern.ends_with?('/*')
           # Then the pattern ends in a wildcard, we only want to go one level deep
           # setting `::File::FNM_PATHNAME` makes the `*` not match directory
           # separators

ee/spec/lib/gitlab/code_owners/file_spec.rb

@@ -160,5 +160,23 @@ describe Gitlab::CodeOwners::File do
         expect(owners).not_to include('username', 'and-email@lookalikes.com')
       end
     end
+
+    context 'a glob on the root directory' do
+      let(:file_content) do
+        '/* @user-1 @user-2'
+      end
+
+      it 'matches files in the root directory' do
+        owners = file.owners_for_path('README.md')
+
+        expect(owners).to include('user-1', 'user-2')
+      end
+
+      it 'does not match nested files' do
+        owners = file.owners_for_path('nested/path/README.md')
+
+        expect(owners).to be_nil
+      end
+    end
   end
 end