Add branch names field to repository type

app/graphql/resolvers/repository_branch_names_resolver.rb

+# frozen_string_literal: true
+
+module Resolvers
+  class RepositoryBranchNamesResolver < BaseResolver
+    type ::GraphQL::STRING_TYPE, null: false
+
+    calls_gitaly!
+
+    argument :search_pattern, GraphQL::STRING_TYPE,
+      required: true,
+      description: 'The pattern to search for branch names by.'
+
+    def resolve(search_pattern:)
+      Repositories::BranchNamesFinder.new(object, search: search_pattern).execute
+    end
+  end
+end

app/graphql/types/repository_type.rb

@@ -16,5 +16,8 @@ module Types
           description: 'Tree of the repository.'
     field :blobs, Types::Tree::BlobType.connection_type, null: true, resolver: Resolvers::BlobsResolver, calls_gitaly: true,
           description: 'Blobs contained within the repository'
+    field :branch_names, [GraphQL::STRING_TYPE], null: true, calls_gitaly: true,
+          complexity: 170, description: 'Names of branches available in this repository that match the search pattern.',
+          resolver: Resolvers::RepositoryBranchNamesResolver
   end
 end

changelogs/unreleased/mc-backstage-graphql-list-available-branches.yml

+---
+title: Add branch names field to repository GraphQL type.
+merge_request: 55074
+author:
+type: changed

doc/api/graphql/reference/index.md

@@ -5444,6 +5444,7 @@ Autogenerated return type of RepositionImageDiffNote.
 | Field | Type | Description |
 | ----- | ---- | ----------- |
 | `blobs` | [`BlobConnection`](#blobconnection) | Blobs contained within the repository. |
+| `branchNames` | [`[String!]`](#string) | Names of branches available in this repository that match the search pattern. |
 | `empty` | [`Boolean!`](#boolean) | Indicates repository has no visible content. |
 | `exists` | [`Boolean!`](#boolean) | Indicates a corresponding Git repository exists on disk. |
 | `rootRef` | [`String`](#string) | Default branch of the repository. |

spec/graphql/resolvers/repository_branch_names_resolver_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Resolvers::RepositoryBranchNamesResolver do
+  include GraphqlHelpers
+
+  let(:project) { create(:project, :repository) }
+
+  describe '#resolve' do
+    subject(:resolve_branch_names) do
+      resolve(
+        described_class,
+        obj: project.repository,
+        args: { search_pattern: pattern },
+        ctx: { current_user: project.creator }
+      )
+    end
+
+    context 'with empty search pattern' do
+      let(:pattern) { '' }
+
+      it 'returns nil' do
+        expect(resolve_branch_names).to eq(nil)
+      end
+    end
+
+    context 'with a valid search pattern' do
+      let(:pattern) { 'mas*' }
+
+      it 'returns matching branches' do
+        expect(resolve_branch_names).to match_array(['master'])
+      end
+    end
+  end
+end

spec/graphql/types/repository_type_spec.rb

@@ -14,4 +14,6 @@ RSpec.describe GitlabSchema.types['Repository'] do
   specify { expect(described_class).to have_graphql_field(:exists, calls_gitaly?: true, complexity: 2) }
 
   specify { expect(described_class).to have_graphql_field(:blobs) }
+
+  specify { expect(described_class).to have_graphql_field(:branch_names, calls_gitaly?: true, complexity: 170) }
 end