test: 투표 동시성 테스트 추가한다

src/test/java/life/offonoff/ab/application/service/TopicServiceConcurrencyTest.java

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+package life.offonoff.ab.application.service;
+
+import jakarta.persistence.EntityManager;
+import jakarta.transaction.Transactional;
+import life.offonoff.ab.application.service.member.MemberService;
+import life.offonoff.ab.application.service.request.VoteRequest;
+import life.offonoff.ab.application.testutil.AbCleaner;
+import life.offonoff.ab.domain.member.Member;
+import life.offonoff.ab.domain.topic.Topic;
+import life.offonoff.ab.domain.topic.TopicSide;
+import life.offonoff.ab.domain.topic.choice.Choice;
+import life.offonoff.ab.domain.topic.choice.ChoiceOption;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.test.context.transaction.TestTransaction;
+
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.stream.IntStream;
+
+import static life.offonoff.ab.domain.TestEntityUtil.createRandomMember;
+import static life.offonoff.ab.domain.TestEntityUtil.createRandomTopicByMemberWithChoices;
+import static org.assertj.core.api.Assertions.assertThat;
+
+@Transactional
+@SpringBootTest
+public class TopicServiceConcurrencyTest {
+
+    @Autowired
+    private MemberService memberService;
+
+    @Autowired
+    private TopicService topicService;
+
+    @Autowired
+    private EntityManager em;
+
+    @Autowired
+    private AbCleaner cleaner;
+
+    @AfterEach
+    void tearDown() {
+        cleaner.cleanTables();
+    }
+
+    @Test
+    void voteForTopicByMember() throws InterruptedException {
+        // given
+        final int COUNT = 10;
+        final ExecutorService executorService = Executors.newFixedThreadPool(COUNT);
+
+        // 토픽 생성
+        Member topicAuthor = createRandomMember();
+        em.persist(topicAuthor);
+        Topic topic = createRandomTopicByMemberWithChoices(
+                topicAuthor, TopicSide.TOPIC_A, ChoiceOption.CHOICE_A, ChoiceOption.CHOICE_B);
+        em.persist(topic);
+
+        // 투표자 COUNT만큼 생성
+        List<Member> voters = IntStream.range(0, COUNT)
+                .mapToObj(__ -> {
+                    Member voter = createRandomMember();
+                    em.persist(voter);
+                    return voter;
+                })
+                .toList();
+
+        TestTransaction.flagForCommit();
+        TestTransaction.end();
+        TestTransaction.start();
+
+        // when
+        long votedAt = System.currentTimeMillis() / 1000;
+        final CountDownLatch latch = new CountDownLatch(COUNT);
+        voters.forEach(voter -> {
+            executorService.execute(() -> {
+                topicService.voteForTopicByMember(
+                        topic.getId(), voter.getId(), new VoteRequest(ChoiceOption.CHOICE_A, votedAt));
+                latch.countDown();
+            });
+        });
+        latch.await();
+
+        // then
+        Topic updatedTopic = em.find(Topic.class, topic.getId());
+        Choice votedChoice = updatedTopic.getChoices().stream().filter(c -> c.getChoiceOption().equals(ChoiceOption.CHOICE_A)).findAny().get();
+        // 투표수는 COUNT와 동일해야함
+        assertThat(updatedTopic.getVoteCount()).isEqualTo(COUNT);
+        assertThat(votedChoice.getVoteCount()).isEqualTo(COUNT);
+    }
+
+}