fix(tasks): 任务队列 - 限制缓存回源并修复并发租约与过期写入

backend/internal/service/text_replay_cache.go

@@ -0,0 +1,51 @@
+package service
+
+import (
+	"context"
+	"errors"
+	"time"
+
+	"infinite-canvas/backend/internal/model"
+)
+
+type textReplayCacheKey struct {
+	userID, taskID string
+	after          int64
+}
+
+func (s *Service) initReadCaches() {
+	s.readCachesOnce.Do(func() {
+		s.concurrencyReadCache = newBoundedReadCache[string, RuntimeTaskPolicy](1, 1024, 1, 2*time.Second)
+		s.textReplayReadCache = newBoundedReadCache[textReplayCacheKey, *TextReplayResult](256, 8<<20, 8, 750*time.Millisecond)
+		s.routeVersionReadCache = newBoundedReadCache[string, int64](1, 1024, 1, 250*time.Millisecond)
+	})
+}
+
+// SSE 展示专用，不用于写路径和权限决策；userID 进入 key，回源仍检查任务归属。
+// 游标保留在 key 中，避免重连读取到其他客户端已经越过的增量窗口。
+func (s *Service) CachedTaskTextReplay(ctx context.Context, userID, taskID string, after int64) (*TextReplayResult, error) {
+	s.initReadCaches()
+	ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
+	defer cancel()
+	value, err := s.textReplayReadCache.get(ctx, textReplayCacheKey{userID, taskID, after}, func(ctx context.Context) (*TextReplayResult, int, error) {
+		reader := &Service{repo: s.repo.WithContext(ctx)}
+		value, err := reader.TaskTextReplay(userID, taskID, after)
+		if err != nil {
+			return nil, 0, err
+		}
+		bytes := 512 + len(value.TextDraft) + len(value.FinalText) + len(value.Error) + len(value.Stage)
+		for _, delta := range value.Deltas {
+			bytes += 256 + len(delta.Content) + len(delta.ID) + len(delta.UserID) + len(delta.TaskID)
+		}
+		return value, bytes, nil
+	})
+	if errors.Is(err, errReadCacheBusy) {
+		return nil, &AppError{Status: 503, Code: 503, Message: "任务状态查询繁忙，请稍后重试", Retryable: true}
+	}
+	if err != nil {
+		return nil, err
+	}
+	copy := *value
+	copy.Deltas = append([]model.TaskTextDelta(nil), value.Deltas...)
+	return &copy, nil
+}