Do not allow a linktobranch for metapackages like _pattern

Fixes #17305

src/api/app/controllers/source_controller.rb

@@ -1068,6 +1068,10 @@ def package_command_deleteuploadrev
 
   # POST /source/<project>/<package>?cmd=linktobranch
   def package_command_linktobranch
+    if @package.nil?
+      render_error status: 404
+      return
+    end
     pkg_rev = params[:rev]
     pkg_linkrev = params[:linkrev]
 

src/api/spec/controllers/source_controller_spec.rb

@@ -207,4 +207,34 @@
       it { expect(response).to have_http_status(:ok) }
     end
   end
+
+  describe 'POST #linktobranch', vcr: false do
+    context 'when accessing a regular package' do
+      let(:package) { create(:package, name: 'some_package', project: project) }
+
+      before do
+        login user
+
+        post :package_command, params: {
+          cmd: 'linktobranch', project: package.project, package: package
+        }
+      end
+
+      it { expect(response).to have_http_status(:found) }
+    end
+
+    context 'when accessing a _patterns package' do
+      let(:package) { create(:package, name: 'some_package', project: project) }
+
+      before do
+        login user
+
+        post :package_command, params: {
+          cmd: 'linktobranch', project: package.project, package: '_pattern'
+        }
+      end
+
+      it { expect(response).to have_http_status(:not_found) }
+    end
+  end
 end