fix[syncer]: cannot run prune task (#18344)

### Summary

As title

internal/syncer/syncer_test.go

@@ -617,6 +617,64 @@ func TestNATSTaskWorkerAcksUnclaimableMessage(t *testing.T) {
 	}
 }
 
+func TestNATSTaskWorkerRetriesStillScheduledUnclaimedTask(t *testing.T) {
+	db := setupSyncerDB(t)
+	insertTaskContext(t, db, "conn-1", "kb-1", "task-1", dao.TaskTypeSync)
+	insertSyncLog(t, db, "conn-1", "kb-1", "task-2", dao.TaskTypePrune)
+	if err := db.Model(&entity.SyncLogs{}).Where("id = ?", "task-1").Update("status", dao.SyncStatusRunning).Error; err != nil {
+		t.Fatalf("mark sync running: %v", err)
+	}
+	if err := db.Model(&entity.Connector{}).Where("id = ?", "conn-1").Update("status", dao.SyncStatusRunning).Error; err != nil {
+		t.Fatalf("mark connector running: %v", err)
+	}
+
+	taskService := service.NewSyncTaskService(dao.NewSyncTaskDAO(db))
+	scheduler := NewNATSScheduler(make(chan TaskEnvelope, 1), taskService, &fakeSyncTaskBroker{})
+	worker := NewTaskWorker(make(chan TaskEnvelope, 1), taskService, newCoordinator(taskService, newTestRegistry(nil), &fakeSink{}, nil, fakeStore{}), NewConnectorLock()).WithScheduler(scheduler)
+	handle := &fakeTaskHandle{msg: common.TaskMessage{TaskID: "task-2", TaskType: common.TaskTypeSyncer}}
+
+	worker.handle(t.Context(), TaskEnvelope{TaskID: "task-2", Handle: handle})
+
+	if handle.acks != 1 || handle.nacks != 0 {
+		t.Fatalf("settlement acks=%d nacks=%d", handle.acks, handle.nacks)
+	}
+	scheduler.timerMu.Lock()
+	timer := scheduler.timers["task-2"]
+	if timer != nil {
+		timer.Stop()
+		delete(scheduler.timers, "task-2")
+	}
+	scheduler.timerMu.Unlock()
+	if timer == nil {
+		t.Fatalf("scheduled prune retry timer was not registered")
+	}
+}
+
+func TestNATSTaskWorkerDoesNotRetryCompletedUnclaimedTask(t *testing.T) {
+	db := setupSyncerDB(t)
+	insertTaskContext(t, db, "conn-1", "kb-1", "task-1", dao.TaskTypeSync)
+	if err := db.Model(&entity.SyncLogs{}).Where("id = ?", "task-1").Update("status", dao.SyncStatusDone).Error; err != nil {
+		t.Fatalf("mark done: %v", err)
+	}
+
+	taskService := service.NewSyncTaskService(dao.NewSyncTaskDAO(db))
+	scheduler := NewNATSScheduler(make(chan TaskEnvelope, 1), taskService, &fakeSyncTaskBroker{})
+	worker := NewTaskWorker(make(chan TaskEnvelope, 1), taskService, newCoordinator(taskService, newTestRegistry(nil), &fakeSink{}, nil, fakeStore{}), NewConnectorLock()).WithScheduler(scheduler)
+	handle := &fakeTaskHandle{msg: common.TaskMessage{TaskID: "task-1", TaskType: common.TaskTypeSyncer}}
+
+	worker.handle(t.Context(), TaskEnvelope{TaskID: "task-1", Handle: handle})
+
+	if handle.acks != 1 || handle.nacks != 0 {
+		t.Fatalf("settlement acks=%d nacks=%d", handle.acks, handle.nacks)
+	}
+	scheduler.timerMu.Lock()
+	_, scheduled := scheduler.timers["task-1"]
+	scheduler.timerMu.Unlock()
+	if scheduled {
+		t.Fatalf("completed task should not be scheduled for retry")
+	}
+}
+
 // TestSameConnectorDifferentKBsRunInParallel verifies one datasource can sync into different KBs concurrently.
 func TestSameConnectorDifferentKBsRunInParallel(t *testing.T) {
 	db := setupSyncerDB(t)

internal/syncer/task_worker.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"fmt"
 	"ragflow/internal/common"
+	"ragflow/internal/dao"
 	"ragflow/internal/service"
 	"sync"
 	"time"
@@ -91,6 +92,7 @@ func (w *TaskWorker) handle(ctx context.Context, envelope TaskEnvelope) {
 		}
 		if !claimed {
 			_ = envelope.Handle.Ack() // this task has been claimed by other worker
+			w.scheduleRetryIfTaskScheduled(ctx, envelope.TaskID, 3*time.Second)
 			return
 		}
 	}
@@ -187,6 +189,21 @@ func (w *TaskWorker) scheduleRetry(ctx context.Context, taskID string, delay tim
 	}
 }
 
+func (w *TaskWorker) scheduleRetryIfTaskScheduled(ctx context.Context, taskID string, delay time.Duration) {
+	if w.scheduler == nil || taskID == "" {
+		return
+	}
+	taskContext, err := w.taskService.GetContext(ctx, taskID)
+	if err != nil {
+		common.Warn("syncer retry task lookup failed", zap.String("task_id", taskID), zap.Error(err))
+		return
+	}
+	if taskContext.Task.Status != dao.SyncStatusSchedule {
+		return
+	}
+	w.scheduleRetry(ctx, taskID, delay)
+}
+
 // transientRetryDelay return retry delay
 func transientRetryDelay(attempts int64) time.Duration {
 	if attempts < 1 {