autoresearch iter 14: stop a cache read error from clobbering the buffered task log

AppendTaskExecutionLog discarded the error from its read of the buffer, so a
transient cache failure looked like an empty buffer and the very next write
replaced the whole accumulated log with just the newest line. Flush already
distinguished miss from failure; append now does the same.

backend/plugins/domain/admin/repository/flush_log_test.go

@@ -6,7 +6,9 @@ package repository_test
 import (
 	"context"
 	"errors"
+	"fmt"
 	"testing"
+	"time"
 
 	"github.com/alicebob/miniredis/v2"
 	"github.com/redis/go-redis/v9"
@@ -112,3 +114,37 @@ func TestFlushTaskExecutionLogPersistsAndClears(t *testing.T) {
 	err = svc.Get(ctx, repository.TaskExecutionLogRedisKey(taskID), &buf)
 	assert.True(t, errors.Is(err, contracts.ErrCacheMiss), "flush 后缓存应清空, got %v", err)
 }
+
+// readFailCache 读取永远报错而写入成功，用于区分「未命中」与「缓存故障」两种语义。
+type readFailCache struct {
+	writes []string
+}
+
+func (c *readFailCache) Get(context.Context, string, any) error {
+	return errors.New("cache unavailable")
+}
+
+func (c *readFailCache) Set(_ context.Context, key string, value any, _ time.Duration) error {
+	c.writes = append(c.writes, fmt.Sprintf("%s=%v", key, value))
+	return nil
+}
+
+func (c *readFailCache) Delete(context.Context, string) error { return nil }
+
+func (c *readFailCache) GetOrSet(context.Context, string, any, time.Duration, func() (any, error)) error {
+	return errors.New("cache unavailable")
+}
+
+func (c *readFailCache) Invalidate(context.Context, string) error { return nil }
+
+// TestAppendTaskExecutionLogKeepsBufferOnCacheReadError 回归：缓存读取失败（而非未命中）时
+// 不得把「读不到」当成「没有缓冲」继续写入，否则整段任务日志会被最新一行覆盖丢失。
+func TestAppendTaskExecutionLogKeepsBufferOnCacheReadError(t *testing.T) {
+	fake := &readFailCache{}
+	repository.SetCacheService(fake)
+	defer repository.SetCacheService(nil)
+
+	err := repository.AppendTaskExecutionLog(context.Background(), "append-err-task", "step-2")
+	assert.Error(t, err, "缓存故障必须上抛，而不是覆盖缓冲")
+	assert.Empty(t, fake.writes, "读取失败时不得写入，避免覆盖已缓冲日志")
+}

backend/plugins/domain/admin/repository/task.go

@@ -125,7 +125,13 @@ func AppendTaskExecutionLog(ctx context.Context, taskID, logLine string) error {
 	key := TaskExecutionLogRedisKey(taskID)
 
 	var existing string
-	_ = cacheSvc.Get(ctx, key, &existing)
+	if err := cacheSvc.Get(ctx, key, &existing); err != nil {
+		// 只有未命中才代表「尚无缓冲」；其余读取失败若被当作空缓冲继续写入，
+		// 会用这一行覆盖掉整段已缓冲的任务日志。
+		if !errors.Is(err, contracts.ErrCacheMiss) {
+			return fmt.Errorf("load buffered task execution log: %w", err)
+		}
+	}
 	return cacheSvc.Set(ctx, key, existing+line, taskExecutionLogExpiration)
 }