keep alive attachment variant

src/main/java/com/imyeyu/api/modules/common/service/implement/AttachmentVariantServiceImplement.java

@@ -112,7 +112,10 @@ public class AttachmentVariantServiceImplement implements AttachmentVariantServi
 			now
 		);
 		if (cached != null) {
-			return cached;
+			Attachment refreshed = refreshCachedThumb(cached);
+			if (refreshed != null) {
+				return refreshed;
+			}
 		}
 
 		String lockKey = buildThumbLockKey(source, width, height, fitMode);
@@ -131,7 +134,10 @@ public class AttachmentVariantServiceImplement implements AttachmentVariantServi
 				Time.now()
 			);
 			if (cached != null) {
-				return cached;
+				Attachment refreshed = refreshCachedThumb(cached);
+				if (refreshed != null) {
+					return refreshed;
+				}
 			}
 			AttachmentVariant oldVariant = variantMapper.selectByKey(
 				source.getId(),
@@ -197,7 +203,10 @@ public class AttachmentVariantServiceImplement implements AttachmentVariantServi
 						Time.now()
 					);
 					if (concurrent != null) {
-						return concurrent;
+						Attachment refreshed = refreshCachedThumb(concurrent);
+						if (refreshed != null) {
+							return refreshed;
+						}
 					}
 					throw e;
 				}
@@ -223,6 +232,20 @@ public class AttachmentVariantServiceImplement implements AttachmentVariantServi
 		}
 	}
 
+	/// 延长已命中缩略图缓存的有效期
+	///
+	/// @param cached 已命中的缩略图
+	/// @return 更新成功时返回刷新后的缩略图，记录已不存在或已过期时返回 `null`
+	private Attachment refreshCachedThumb(Attachment cached) {
+		long now = Time.now();
+		long expireAt = now + THUMB_CACHE_TTL;
+		if (!(0 < variantMapper.updateExpireAt(cached.getId(), now, expireAt))) {
+			return null;
+		}
+		cached.setDestroyAt(expireAt);
+		return cached;
+	}
+
 	private String buildThumbLockKey(Attachment source, int width, int height, ThumbnailFitMode fitMode) {
 		return "%s%s:%s:%s:%s:%s".formatted(
 			THUMB_LOCK_KEY_PREFIX,