fix #35290 Make read-only API for public merge requests available without authentication

app/finders/issuable_finder.rb

@@ -244,6 +244,8 @@ class IssuableFinder
   end
 
   def by_scope(items)
+    return items.none if current_user_related? && !current_user
+
     case params[:scope]
     when 'created-by-me', 'authored'
       items.where(author_id: current_user.id)

changelogs/unreleased/35290_allow_public_project_apis.yml

+---
+title: made read-only APIs for public merge requests available without authentication
+merge_request: 13291
+author: haseebeqx

lib/api/merge_requests.rb

@@ -2,7 +2,7 @@ module API
   class MergeRequests < Grape::API
     include PaginationParams
 
-    before { authenticate! }
+    before { authenticate_non_get! }
 
     helpers ::Gitlab::IssuableMetadata
 
@@ -55,6 +55,7 @@ module API
                          desc: 'Return merge requests for the given scope: `created-by-me`, `assigned-to-me` or `all`'
       end
       get do
+        authenticate! unless params[:scope] == 'all'
         merge_requests = find_merge_requests
 
         options = { with: Entities::MergeRequestBasic,

spec/requests/api/merge_requests_spec.rb

@@ -28,10 +28,29 @@ describe API::MergeRequests do
 
   describe 'GET /merge_requests' do
     context 'when unauthenticated' do
-      it 'returns authentication error' do
-        get api('/merge_requests')
+      it 'returns an array of all merge requests' do
+        get api('/merge_requests', user), scope: 'all'
 
-        expect(response).to have_gitlab_http_status(401)
+        expect(response).to have_http_status(200)
+        expect(json_response).to be_an Array
+      end
+
+      it "returns authentication error without any scope" do
+        get api("/merge_requests")
+
+        expect(response).to have_http_status(401)
+      end
+
+      it "returns authentication error  when scope is assigned-to-me" do
+        get api("/merge_requests"), scope: 'assigned-to-me'
+
+        expect(response).to have_http_status(401)
+      end
+
+      it "returns authentication error  when scope is created-by-me" do
+        get api("/merge_requests"), scope: 'created-by-me'
+
+        expect(response).to have_http_status(401)
       end
     end
 
@@ -134,10 +153,18 @@ describe API::MergeRequests do
 
   describe "GET /projects/:id/merge_requests" do
     context "when unauthenticated" do
-      it "returns authentication error" do
+      it 'returns merge requests for public projects' do
         get api("/projects/#{project.id}/merge_requests")
 
-        expect(response).to have_gitlab_http_status(401)
+        expect(response).to have_http_status(200)
+        expect(json_response).to be_an Array
+      end
+
+      it "returns 404 for non public projects" do
+        project = create(:project, :private)
+        get api("/projects/#{project.id}/merge_requests")
+
+        expect(response).to have_http_status(404)
       end
     end