[BE] [REFACTOR] 좋아요 동시성 문제 해결

- post_id + user_id 유니크 제약 선언
- advisory lock, insert on conflict do nothing, row count 반환 delete 추가
- likeCount 원자 증가/감소 쿼리 추가
- 실제 insert/delete가 일어난 경우에만 count 변경
- 기존 중복 row 정리, like_count 재계산, 유니크 제약/인덱스 추가
- 추가/취소/insert 경합/delete 경합 테스트 추가

Author: discipline24

backend/src/main/java/org/sejongisc/backend/board/entity/PostLike.java

@@ -7,6 +7,8 @@
 import jakarta.persistence.Id;
 import jakarta.persistence.JoinColumn;
 import jakarta.persistence.ManyToOne;
+import jakarta.persistence.Table;
+import jakarta.persistence.UniqueConstraint;
 import java.util.UUID;
 import lombok.AllArgsConstructor;
 import lombok.Builder;
@@ -17,6 +19,15 @@
 import org.sejongisc.backend.user.entity.User;
 
 @Entity
+@Table(
+    name = "post_like",
+    uniqueConstraints = {
+        @UniqueConstraint(
+            name = "uk_post_like_post_user",
+            columnNames = {"post_id", "user_id"}
+        )
+    }
+)
 @Getter
 @Setter
 @NoArgsConstructor

backend/src/main/java/org/sejongisc/backend/board/repository/PostLikeRepository.java

@@ -1,11 +1,13 @@
 package org.sejongisc.backend.board.repository;
 
 import java.util.List;
-import org.sejongisc.backend.board.entity.PostLike;
-import org.springframework.data.jpa.repository.JpaRepository;
-
 import java.util.Optional;
 import java.util.UUID;
+import org.sejongisc.backend.board.entity.PostLike;
+import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Modifying;
+import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.repository.query.Param;
 import org.springframework.transaction.annotation.Transactional;
 
 public interface PostLikeRepository extends JpaRepository<PostLike, UUID> {
@@ -16,6 +18,46 @@ public interface PostLikeRepository extends JpaRepository<PostLike, UUID> {
 
   List<PostLike> findAllByPostPostId(UUID postId);
 
+  @Query(
+      value = """
+      select pg_advisory_xact_lock(
+        hashtext(cast(:postId as text)),
+        hashtext(cast(:userId as text))
+      )
+      """,
+      nativeQuery = true
+  )
+  Object acquirePostUserLikeToggleLock(
+      @Param("postId") UUID postId,
+      @Param("userId") UUID userId
+  );
+
+  @Modifying(flushAutomatically = true, clearAutomatically = true)
+  @Query(
+      value = """
+      insert into post_like (post_like_id, post_id, user_id, created_date, updated_date)
+      values (:postLikeId, :postId, :userId, now(), now())
+      on conflict (post_id, user_id) do nothing
+      """,
+      nativeQuery = true
+  )
+  int insertIgnore(
+      @Param("postLikeId") UUID postLikeId,
+      @Param("postId") UUID postId,
+      @Param("userId") UUID userId
+  );
+
+  @Modifying(flushAutomatically = true, clearAutomatically = true)
+  @Query("""
+      delete from PostLike pl
+      where pl.post.postId = :postId
+        and pl.user.userId = :userId
+      """)
+  int deleteByPostIdAndUserId(
+      @Param("postId") UUID postId,
+      @Param("userId") UUID userId
+  );
+
   @Transactional
   void deleteAllByPostPostId(UUID postId);
 }

backend/src/main/java/org/sejongisc/backend/board/service/PostInteractionService.java

@@ -177,29 +177,34 @@ public void toggleLike(UUID postId, UUID userId) {
     User user = userRepository.findById(userId)
         .orElseThrow(() -> new CustomException(ErrorCode.USER_NOT_FOUND));
 
-    // 이미 좋아요를 눌렀는지 확인
+    // 토글은 같은 사용자-게시글 조합 안에서 순서가 의미를 가지므로 해당 조합만 직렬화합니다.
+    postLikeRepository.acquirePostUserLikeToggleLock(postId, userId);
+
     Optional<PostLike> existingLike = postLikeRepository.findByPostPostIdAndUserUserId(postId, userId);
 
     if (existingLike.isPresent()) {
-      // 좋아요가 이미 있으면 -> 삭제 (좋아요 취소)
-      postLikeRepository.delete(existingLike.get());
-      post.setLikeCount(post.getLikeCount() - 1); // Post 엔티티 카운트 감소
-    } else {
-      // 좋아요가 없으면 -> 생성 (좋아요)
-      PostLike newLike = PostLike.builder()
-          .post(post)
-          .user(user)
-          .build();
-      postLikeRepository.save(newLike);
-      post.setLikeCount(post.getLikeCount() + 1); // Post 엔티티 카운트 증가
-      eventPublisher.publishEvent(new ActivityEvent(
-              user.getUserId(),
-              user.getName(),
-              ActivityType.BOARD_LIKE,
-              "[" + post.getTitle() + "]에 좋아요를 눌렀습니다.",
-              post.getPostId(),
-              post.getBoard().getBoardName()));
+      int deletedRows = postLikeRepository.deleteByPostIdAndUserId(postId, userId);
+      if (deletedRows > 0) {
+        // 실제 좋아요 row가 삭제된 경우에만 캐시 카운트를 원자적으로 감소시킵니다.
+        postRepository.decrementLikeCount(postId);
+      }
+      return;
     }
+
+    int insertedRows = postLikeRepository.insertIgnore(UUID.randomUUID(), postId, userId);
+    if (insertedRows == 0) {
+      return;
+    }
+
+    // 실제 좋아요 row가 추가된 경우에만 캐시 카운트를 원자적으로 증가시킵니다.
+    postRepository.incrementLikeCount(postId);
+    eventPublisher.publishEvent(new ActivityEvent(
+            user.getUserId(),
+            user.getName(),
+            ActivityType.BOARD_LIKE,
+            "[" + post.getTitle() + "]에 좋아요를 눌렀습니다.",
+            post.getPostId(),
+            post.getBoard().getBoardName()));
   }
 
   // 북마크 등록/삭제

backend/src/main/resources/db/migration/V4__post_like_concurrency.sql

@@ -0,0 +1,51 @@
+-- Make board post likes concurrency-safe.
+-- 1) Collapse legacy duplicate likes per (post_id, user_id).
+-- 2) Reconcile cached post.like_count from the canonical post_like table.
+-- 3) Add a DB-level uniqueness guard so duplicates cannot be reintroduced.
+
+delete from post_like
+where ctid in (
+  select like_ctid
+  from (
+    select
+      ctid as like_ctid,
+      row_number() over (
+        partition by post_id, user_id
+        order by created_date asc nulls last, post_like_id asc
+      ) as row_number
+    from post_like
+    where post_id is not null
+      and user_id is not null
+  ) ranked_likes
+  where ranked_likes.row_number > 1
+);
+
+update post p
+set like_count = coalesce(like_totals.like_count, 0)
+from (
+  select p_inner.post_id, count(pl.post_like_id)::integer as like_count
+  from post p_inner
+  left join post_like pl on pl.post_id = p_inner.post_id
+  group by p_inner.post_id
+) like_totals
+where p.post_id = like_totals.post_id;
+
+alter table post
+  alter column like_count set default 0,
+  alter column like_count set not null;
+
+do $$
+begin
+  if not exists (
+    select 1
+    from pg_constraint
+    where conname = 'uk_post_like_post_user'
+      and conrelid = 'post_like'::regclass
+  ) then
+    alter table post_like
+      add constraint uk_post_like_post_user unique (post_id, user_id);
+  end if;
+end $$;
+
+create index if not exists idx_post_like_post_id on post_like(post_id);
+create index if not exists idx_post_like_user_id on post_like(user_id);