fix(dispatcher): simplify and fix termination handling

src/datachain/query/dispatch.py

@@ -2,12 +2,16 @@
 from collections.abc import Iterable, Sequence
 from itertools import chain
 from multiprocessing import cpu_count
+from queue import Empty
 from sys import stdin
+from time import monotonic, sleep
 from typing import TYPE_CHECKING, Literal
 
+import multiprocess
 from cloudpickle import load, loads
 from fsspec.callbacks import DEFAULT_CALLBACK, Callback
-from multiprocess import get_context
+from multiprocess.context import Process
+from multiprocess.queues import Queue as MultiprocessQueue
 
 from datachain.catalog import Catalog
 from datachain.catalog.catalog import clone_catalog_with_cache
@@ -25,7 +29,6 @@
 from datachain.utils import batched, flatten, safe_closing
 
 if TYPE_CHECKING:
-    import multiprocess
     from sqlalchemy import Select, Table
 
     from datachain.data_storage import AbstractMetastore, AbstractWarehouse
@@ -101,8 +104,8 @@ def udf_worker_entrypoint(fd: int | None = None) -> int:
 
 class UDFDispatcher:
     _catalog: Catalog | None = None
-    task_queue: "multiprocess.Queue | None" = None
-    done_queue: "multiprocess.Queue | None" = None
+    task_queue: MultiprocessQueue | None = None
+    done_queue: MultiprocessQueue | None = None
 
     def __init__(self, udf_info: UdfInfo, buffer_size: int = DEFAULT_BATCH_SIZE):
         self.udf_data = udf_info["udf_data"]
@@ -121,7 +124,7 @@ def __init__(self, udf_info: UdfInfo, buffer_size: int = DEFAULT_BATCH_SIZE):
         self.buffer_size = buffer_size
         self.task_queue = None
         self.done_queue = None
-        self.ctx = get_context("spawn")
+        self.ctx = multiprocess.get_context("spawn")
 
     @property
     def catalog(self) -> "Catalog":
@@ -259,8 +262,6 @@ def run_udf_parallel(  # noqa: C901, PLR0912
         for p in pool:
             p.start()
 
-        # Will be set to True if all tasks complete normally
-        normal_completion = False
         try:
             # Will be set to True when the input is exhausted
             input_finished = False
@@ -283,10 +284,20 @@ def run_udf_parallel(  # noqa: C901, PLR0912
 
             # Process all tasks
             while n_workers > 0:
-                try:
-                    result = get_from_queue(self.done_queue)
-                except KeyboardInterrupt:
-                    break
+                while True:
+                    try:
+                        result = self.done_queue.get_nowait()
+                        break
+                    except Empty:
+                        for p in pool:
+                            exitcode = p.exitcode
+                            if exitcode not in (None, 0):
+                                message = (
+                                    f"Worker {p.name} exited unexpectedly with "
+                                    f"code {exitcode}"
+                                )
+                                raise RuntimeError(message) from None
+                        sleep(0.01)
 
                 if bytes_downloaded := result.get("bytes_downloaded"):
                     download_cb.relative_update(bytes_downloaded)
@@ -313,39 +324,50 @@ def run_udf_parallel(  # noqa: C901, PLR0912
                         put_into_queue(self.task_queue, next(input_data))
                     except StopIteration:
                         input_finished = True
-
-            # Finished with all tasks normally
-            normal_completion = True
         finally:
-            if not normal_completion:
-                # Stop all workers if there is an unexpected exception
-                for _ in pool:
-                    put_into_queue(self.task_queue, STOP_SIGNAL)
-
-                # This allows workers (and this process) to exit without
-                # consuming any remaining data in the queues.
-                # (If they exit due to an exception.)
-                self.task_queue.close()
-                self.task_queue.join_thread()
-
-                # Flush all items from the done queue.
-                # This is needed if any workers are still running.
-                while n_workers > 0:
-                    result = get_from_queue(self.done_queue)
-                    status = result["status"]
-                    if status != OK_STATUS:
-                        n_workers -= 1
-
-                self.done_queue.close()
-                self.done_queue.join_thread()
+            self._shutdown_workers(pool)
+
+    def _shutdown_workers(self, pool: list[Process]) -> None:
+        self._terminate_pool(pool)
+        self._drain_queue(self.done_queue)
+        self._drain_queue(self.task_queue)
+        self._close_queue(self.done_queue)
+        self._close_queue(self.task_queue)
+
+    def _terminate_pool(self, pool: list[Process]) -> None:
+        for proc in pool:
+            if proc.is_alive():
+                proc.terminate()
+
+        deadline = monotonic() + 1.0
+        for proc in pool:
+            if not proc.is_alive():
+                continue
+            remaining = deadline - monotonic()
+            if remaining > 0:
+                proc.join(remaining)
+            if proc.is_alive():
+                proc.kill()
+                proc.join(timeout=0.2)
+
+    def _drain_queue(self, queue: MultiprocessQueue) -> None:
+        while True:
+            try:
+                queue.get_nowait()
+            except Empty:
+                return
+            except (OSError, ValueError):
+                return
 
-            # Wait for workers to stop
-            for p in pool:
-                p.join()
+    def _close_queue(self, queue: MultiprocessQueue) -> None:
+        with contextlib.suppress(OSError, ValueError):
+            queue.close()
+        with contextlib.suppress(RuntimeError, AssertionError, ValueError):
+            queue.join_thread()
 
 
 class DownloadCallback(Callback):
-    def __init__(self, queue: "multiprocess.Queue") -> None:
+    def __init__(self, queue: MultiprocessQueue) -> None:
         self.queue = queue
         super().__init__()
 
@@ -360,7 +382,7 @@ class ProcessedCallback(Callback):
     def __init__(
         self,
         name: Literal["processed", "generated"],
-        queue: "multiprocess.Queue",
+        queue: MultiprocessQueue,
     ) -> None:
         self.name = name
         self.queue = queue
@@ -375,8 +397,8 @@ def __init__(
         self,
         catalog: "Catalog",
         udf: "UDFAdapter",
-        task_queue: "multiprocess.Queue",
-        done_queue: "multiprocess.Queue",
+        task_queue: MultiprocessQueue,
+        done_queue: MultiprocessQueue,
         query: "Select",
         table: "Table",
         cache: bool,

src/datachain/query/queue.py

@@ -1,11 +1,12 @@
 import datetime
 from collections.abc import Iterable, Iterator
-from queue import Empty, Full, Queue
+from queue import Empty, Full
 from struct import pack, unpack
 from time import sleep
 from typing import Any
 
 import msgpack
+from multiprocess.queues import Queue
 
 from datachain.query.batch import RowsOutput
 

tests/func/test_udf.py

@@ -2,7 +2,10 @@
 import os
 import pickle
 import posixpath
+import sys
+import time
 
+import multiprocess as mp
 import pytest
 
 import datachain as dc
@@ -560,6 +563,119 @@ def name_len_error(_name):
             chain.show()
 
 
+@pytest.mark.parametrize(
+    "failure_mode,expected_exit_code,error_marker",
+    [
+        ("exception", 1, "Worker 1 failure!"),
+        ("keyboard_interrupt", -2, "KeyboardInterrupt"),
+        ("sys_exit", 1, None),
+        ("os_exit", 1, None),  # os._exit - immediate termination
+    ],
+)
+def test_udf_parallel_worker_failure_exits_peers(
+    test_session_tmpfile,
+    tmp_path,
+    capfd,
+    failure_mode,
+    expected_exit_code,
+    error_marker,
+):
+    """
+    Test that when one worker fails, all other workers exit immediately.
+
+    Tests different failure modes:
+    - exception: Worker raises RuntimeError (normal exception)
+    - keyboard_interrupt: Worker raises KeyboardInterrupt (simulates Ctrl+C)
+    - sys_exit: Worker calls sys.exit() (clean Python exit)
+    - os_exit: Worker calls os._exit() (immediate process termination)
+    """
+    import platform
+
+    # Windows uses different exit codes for KeyboardInterrupt
+    # 3221225786 (0xC000013A) is STATUS_CONTROL_C_EXIT on Windows
+    # while POSIX systems use -2 (SIGINT)
+    if platform.system() == "Windows" and failure_mode == "keyboard_interrupt":
+        expected_exit_code = 3221225786
+
+    vals = list(range(100))
+
+    barrier_dir = tmp_path / "udf_workers_barrier"
+    barrier_dir_str = str(barrier_dir)
+    os.makedirs(barrier_dir_str, exist_ok=True)
+    expected_workers = 3
+
+    def slow_process(val: int) -> int:
+        proc_name = mp.current_process().name
+        with open(os.path.join(barrier_dir_str, f"{proc_name}.started"), "w") as f:
+            f.write(str(time.time()))
+
+        # Wait until all expected workers have written their markers
+        deadline = time.time() + 1.0
+        while time.time() < deadline:
+            try:
+                count = len(
+                    [n for n in os.listdir(barrier_dir_str) if n.endswith(".started")]
+                )
+            except FileNotFoundError:
+                count = 0
+            if count >= expected_workers:
+                break
+            time.sleep(0.01)
+
+        if proc_name == "Worker-UDF-1":
+            if failure_mode == "exception":
+                raise RuntimeError("Worker 1 failure!")
+            if failure_mode == "keyboard_interrupt":
+                raise KeyboardInterrupt("Worker interrupted")
+            if failure_mode == "sys_exit":
+                sys.exit(1)
+            if failure_mode == "os_exit":
+                os._exit(1)
+        time.sleep(5)
+        return val * 2
+
+    chain = (
+        dc.read_values(val=vals, session=test_session_tmpfile)
+        .settings(parallel=3)
+        .map(slow_process, output={"result": int})
+    )
+
+    start = time.time()
+    with pytest.raises(RuntimeError, match="UDF Execution Failed!") as exc_info:
+        list(chain.to_iter("result"))
+    elapsed = time.time() - start
+
+    # Verify timing: should exit immediately when worker fails
+    assert elapsed < 10, f"took {elapsed:.1f}s, should exit immediately"
+
+    # Verify multiple workers were started via barrier markers
+    try:
+        started_files = [
+            n for n in os.listdir(barrier_dir_str) if n.endswith(".started")
+        ]
+    except FileNotFoundError:
+        started_files = []
+    assert len(started_files) == 3, (
+        f"Expected all 3 workers to start, but saw markers for: {started_files}"
+    )
+
+    captured = capfd.readouterr()
+
+    # Verify the RuntimeError has a meaningful message with exit code
+    error_message = str(exc_info.value)
+    assert f"UDF Execution Failed! Exit code: {expected_exit_code}" in error_message, (
+        f"Expected exit code {expected_exit_code}, got: {error_message}"
+    )
+
+    if error_marker:
+        assert error_marker in captured.err, (
+            f"Expected '{error_marker}' in stderr for {failure_mode} mode. "
+            f"stderr output: {captured.err[:500]}"
+        )
+
+    assert "semaphore" not in captured.err
+
+
 @pytest.mark.parametrize(
     "cloud_type,version_aware",
     [("s3", True)],