bug fix (& spec): After parent has completed (via #wait, #value, etc)…

…, creating a child promise (via #then, #on_success, etc) and calling child_promise.execute.state return :unscheduled. It should return :pending

lib/concurrent-ruby/concurrent/promise.rb

@@ -250,6 +250,7 @@ def execute
           realize(@promise_body)
         end
       else
+        compare_and_set_state(:pending, :unscheduled)
         @parent.execute
       end
       self

spec/concurrent/promise_spec.rb

@@ -195,6 +195,14 @@ def get_ivar_from_args(opts)
             end_latch.count_down
           end
         end
+
+        context "when called on child after parent completes" do
+          let(:parent_promise) { Concurrent::Promise.execute { 1 + 1 }.tap { |p| p.wait } }
+          it 'sets state to :pending immediately' do
+            child_promise = parent_promise.then { |two| two + 2 }.execute
+            expect(child_promise.state).to eq(:pending)
+          end
+        end
       end
     end