[webui] Fix build log for multibuild and local link packages

The @Package variable was set in the check_build_log_access before filter.
However, for multibuild and local link packages this was set to the parent package (follow_mulitbuild / follow_project_links)
which does not contain a log file. This resulted that set_initial_offset returned an empty log and calculated no offset
which eventually resulted in cutting the end of the log files.
Fix #2651.

src/api/app/controllers/webui/package_controller.rb

@@ -838,6 +838,7 @@ def update_build_log
 
     check_ajax
 
+    @package = params[:package]
     # Make sure objects don't contain invalid chars (eg. '../')
     @repo = @project.repositories.find_by(name: params[:repository]).try(:name)
     unless @repo
@@ -869,7 +870,6 @@ def update_build_log
       return
     end
 
-    @package = params[:package]
     begin
       chunk = get_log_chunk(@project, @package, @repo, @arch, @offset, @offset + @maxsize)
 

src/api/spec/controllers/webui/package_controller_spec.rb

@@ -819,4 +819,33 @@ def do_request(params)
       end
     end
   end
+
+  describe 'GET #update_build_log' do
+    let(:architecture) { repo_for_source_project.architectures.first }
+    let(:params) {
+      { project:    source_project,
+        package:    "#{source_package}:multibuild-package",
+        repository: repo_for_source_project.name,
+        arch:       architecture.name}
+    }
+
+    context 'for multibuild package' do
+      context 'instance variables' do
+        before do
+          get :update_build_log, params: params, xhr: true
+        end
+
+        # This is fine as backend does not need to run to test this. Otherwise it would be necessary to create first a multibuild package in the backend
+        it { expect(assigns(:log_chunk)).to match(/No live log available:/) }
+        it { expect(assigns(:package)).to eq("#{source_package}:multibuild-package") }
+        it { expect(assigns(:project)).to eq(source_project) }
+      end
+
+      it "should call 'get_size_of_log' with appropriate arguments" do
+        expect(controller).to receive(:get_size_of_log).
+          with(source_project, "#{source_package}:multibuild-package", repo_for_source_project.name, architecture.name)
+        get :update_build_log, params: params, xhr: true
+      end
+    end
+  end
 end