feat: Add flow build cancellation endpoint and functionality (#6813)

* feat: Add flow build cancellation endpoint and support

Implement new functionality to cancel ongoing flow build jobs:
- Add `cancel_flow_build` function in build.py to handle job cancellation
- Create new `/build/{job_id}/cancel` endpoint in chat.py
- Introduce `CancelFlowResponse` schema for cancellation response
- Implement robust error handling and logging for job cancellation process

* [autofix.ci] apply automated fixes

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

src/backend/base/langflow/api/build.py

@@ -426,3 +426,47 @@ async def generate_flow_events(
         raise
     event_manager.on_end(data={})
     await event_manager.queue.put((None, None, time.time()))
+
+
+async def cancel_flow_build(
+    *,
+    job_id: str,
+    queue_service: JobQueueService,
+) -> bool:
+    """Cancel an ongoing flow build job.
+
+    Args:
+        job_id: The unique identifier of the job to cancel
+        queue_service: The service managing job queues
+
+    Returns:
+        True if the job was successfully canceled or doesn't need cancellation
+        False if the cancellation failed
+
+    Raises:
+        ValueError: If the job doesn't exist
+    """
+    # Get the event task and event manager for the job
+    _, _, event_task = queue_service.get_queue_data(job_id)
+
+    if event_task is None:
+        logger.warning(f"No event task found for job_id {job_id}")
+        return True  # Nothing to cancel is still a success
+
+    if event_task.done():
+        logger.info(f"Task for job_id {job_id} is already completed")
+        return True  # Nothing to cancel is still a success
+
+    # Store the task reference to check status after cleanup
+    task_before_cleanup = event_task
+
+    # Perform cleanup using the queue service
+    await queue_service.cleanup_job(job_id)
+
+    # Verify that the task was actually cancelled
+    # The task should be done (cancelled) after cleanup
+    if task_before_cleanup.cancelled():
+        logger.info(f"Successfully cancelled flow build for job_id {job_id}")
+        return True
+    logger.error(f"Failed to cancel flow build for job_id {job_id}, task is still running")
+    return False

src/backend/base/langflow/api/v1/chat.py

@@ -5,11 +5,12 @@ import traceback
 import uuid
 from typing import TYPE_CHECKING, Annotated
 
-from fastapi import APIRouter, BackgroundTasks, Body, Depends, HTTPException
+from fastapi import APIRouter, BackgroundTasks, Body, Depends, HTTPException, status
 from fastapi.responses import StreamingResponse
 from loguru import logger
 
 from langflow.api.build import (
+    cancel_flow_build,
     get_flow_events_response,
     start_flow_build,
 )
@@ -25,6 +26,7 @@ from langflow.api.utils import (
     parse_exception,
 )
 from langflow.api.v1.schemas import (
+    CancelFlowResponse,
     FlowDataRequest,
     InputValueRequest,
     ResultDataResponse,
@@ -177,6 +179,32 @@ async def get_build_events(
     )
 
 
+@router.post("/build/{job_id}/cancel", response_model=CancelFlowResponse)
+async def cancel_build(
+    job_id: str,
+    queue_service: Annotated[JobQueueService, Depends(get_queue_service)],
+):
+    """Cancel a specific build job."""
+    try:
+        # Cancel the flow build and check if it was successful
+        cancellation_success = await cancel_flow_build(job_id=job_id, queue_service=queue_service)
+
+        if cancellation_success:
+            # Cancellation succeeded or wasn't needed
+            return CancelFlowResponse(success=True, message="Flow build cancelled successfully")
+        # Cancellation was attempted but failed
+        return CancelFlowResponse(success=False, message="Failed to cancel flow build")
+
+    except ValueError as exc:
+        # Job not found
+
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=str(exc)) from exc
+    except Exception as exc:
+        # Any other unexpected error
+        logger.exception(f"Error cancelling flow build for job_id {job_id}: {exc}")
+        raise HTTPException(status_code=status.HTTP_500_INTERNAL_SERVER_ERROR, detail=str(exc)) from exc
+
+
 @router.post("/build/{flow_id}/vertices/{vertex_id}", deprecated=True)
 async def build_vertex(
     *,

src/backend/base/langflow/api/v1/schemas.py

@@ -377,3 +377,10 @@ class ConfigResponse(BaseModel):
     health_check_max_retries: int
     max_file_size_upload: int
     event_delivery: Literal["polling", "streaming"]
+
+
+class CancelFlowResponse(BaseModel):
+    """Response model for flow build cancellation."""
+
+    success: bool
+    message: str