Allow CI to clone public projects when HTTP protocol is disabled

GitLab has a mechanism that allows CI to clone repositories via HTTP
even when the HTTP protocol is disabled. This works as expected
when a project is private or internal. However, when a project is
public CI gets an error message that HTTP is not allowed. This
happens because Git only sends auth in a subsequent request after a
401 is returned first. For public projects, GitLab grabs onto that
unauthenticated request and sends it through since it recognizes
that Guests are ordinarily allowed to access the repository.
Later on this leads to a 403 since HTTP protocol is disabled.
Fix this by only continuing with unauthenticated requests when
HTTP is allowed.

app/controllers/projects/git_http_client_controller.rb

@@ -49,7 +49,8 @@ class Projects::GitHttpClientController < Projects::ApplicationController
         send_final_spnego_response
         return # Allow access
       end
-    elsif project && download_request? && Guest.can?(:download_code, project)
+    elsif project && download_request? && http_allowed? && Guest.can?(:download_code, project)
+
       @authentication_result = Gitlab::Auth::Result.new(nil, project, :none, [:download_code])
 
       return # Allow access
@@ -113,4 +114,8 @@ class Projects::GitHttpClientController < Projects::ApplicationController
   def ci?
     authentication_result.ci?(project)
   end
+
+  def http_allowed?
+    Gitlab::ProtocolAccess.allowed?('http')
+  end
 end

changelogs/unreleased/dblessing-fix-public-project-ssh-only-ci-failure.yml

+---
+title: Allow CI to clone public projects when HTTP protocol is disabled
+merge_request: 31632
+author:
+type: fixed

spec/controllers/projects/git_http_controller_spec.rb

@@ -12,4 +12,15 @@ describe Projects::GitHttpController do
       expect(response.status).to eq(403)
     end
   end
+
+  describe 'GET #info_refs' do
+    it 'returns 401 for unauthenticated requests to public repositories when http protocol is disabled' do
+      stub_application_setting(enabled_git_access_protocol: 'ssh')
+      project = create(:project, :public, :repository)
+
+      get :info_refs, params: { service: 'git-upload-pack', namespace_id: project.namespace.to_param, project_id: project.path + '.git' }
+
+      expect(response.status).to eq(401)
+    end
+  end
 end