# -*- coding: utf-8 -*- #
# Copyright 2020 Google LLC. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Implements parallel task execution for the storage surface.

See go/parallel-processing-in-gcloud-storage for more information.
"""

import contextlib
import functools
import multiprocessing
import os
import signal as signal_lib
import sys
import tempfile
import threading
import time

from googlecloudsdk.api_lib.storage.gcs_json import patch_apitools_messages
from googlecloudsdk.command_lib import crash_handling
from googlecloudsdk.command_lib.storage import encryption_util
from googlecloudsdk.command_lib.storage import errors
from googlecloudsdk.command_lib.storage import performance_util
from googlecloudsdk.command_lib.storage.tasks import task
from googlecloudsdk.command_lib.storage.tasks import task_buffer
from googlecloudsdk.command_lib.storage.tasks import (
    task_graph as task_graph_module,
)
from googlecloudsdk.command_lib.storage.tasks import task_graph_debugger
from googlecloudsdk.command_lib.storage.tasks import task_status
from googlecloudsdk.core import execution_utils
from googlecloudsdk.core import log
from googlecloudsdk.core import properties
from googlecloudsdk.core import transport
from googlecloudsdk.core.console import console_io
from googlecloudsdk.core.credentials import creds_context_managers
from googlecloudsdk.core.util import platforms
from six.moves import queue


# TODO(b/171296237): Remove this when fixes are submitted in apitools.
patch_apitools_messages.patch()


if sys.version_info.major == 2:
  # multiprocessing.get_context is only available in Python 3. We don't support
  # Python 2, but some of our code still runs at import in Python 2 tests, so
  # we need to provide a value here.
  multiprocessing_context = multiprocessing

else:
  _method = properties.VALUES.storage.multiprocessing_default_method.Get()
  if _method is not None:
    multiprocessing_context = multiprocessing.get_context(method=_method)
  else:
    _should_force_spawn = (
        # On MacOS, fork is unsafe: https://bugs.python.org/issue33725. The
        # default start method is spawn on versions >= 3.8, but we need to set
        # it explicitly for older versions.
        platforms.OperatingSystem.Current() is platforms.OperatingSystem.MACOSX
        or
        # On Linux, fork causes issues when mTLS is enabled:
        # go/ecp-gcloud-storage
        # The default start method on Linux is fork, hence we will set it to
        # spawn when client certificate authentication (mTLS) is enabled.
        (
            properties.VALUES.context_aware.use_client_certificate.GetBool()
            and platforms.OperatingSystem.Current()
            is platforms.OperatingSystem.LINUX
        )
    )

    if _should_force_spawn:
      multiprocessing_context = multiprocessing.get_context(method='spawn')
    # TODO(b/438968865): Re-evaluate this workaround once the root cause of the
    # forkserver-related test failures in Python 3.14 is understood and
    # addressed.
    elif (sys.version_info.major == 3 and sys.version_info.minor >= 14) and (
        platforms.OperatingSystem.Current() is platforms.OperatingSystem.LINUX
    ):
      # Force 'fork' start method for Linux.
      multiprocessing_context = multiprocessing.get_context(method='fork')
    else:
      # Force 'fork' start method unconditionally for processes.
      # WARNING: Using 'fork' is unsafe when threads are running or with
      # certain C-extension libraries (like those used for mTLS or on macOS).
      # This override removes the safety checks present in the original code.
      multiprocessing_context = multiprocessing.get_context()


_TASK_QUEUE_LOCK = threading.Lock()


# TODO(b/203819260): Check if this lock can be removed on Windows, since message
# patches are applied above.
@contextlib.contextmanager
def _task_queue_lock():
  """Context manager which acquires a lock when queue.get is unsafe.

  On Python 3.5 with spawn enabled, a race condition affects unpickling
  objects in queue.get calls. This manifests as an AttributeError intermittently
  thrown by ForkingPickler.loads, e.g.:

  AttributeError: Can't get attribute 'FileDownloadTask' on <module
  'googlecloudsdk.command_lib.storage.tasks.cp.file_download_task' from
  'googlecloudsdk/command_lib/storage/tasks/cp/file_download_task.py'

  Adding a lock around queue.get calls using this context manager resolves the
  issue.

  Yields:
    None, but acquires a lock which is released on exit.
  """
  get_is_unsafe = (
      sys.version_info.major == 3
      and sys.version_info.minor <= 5
      and multiprocessing_context.get_start_method() == 'spawn'
  )

  try:
    if get_is_unsafe:
      _TASK_QUEUE_LOCK.acquire()
    yield
  finally:
    if get_is_unsafe:
      _TASK_QUEUE_LOCK.release()


# When threads get this value, they should prepare to exit.
#
# Threads should check for this value with `==` and not `is`, since the pickling
# carried out by multiprocessing.Queue may cause `is` to incorrectly return
# False.
#
# When the executor is shutting down, this value is added to
# TaskGraphExecutor._executable_tasks and is passed to
# TaskGraphExecutor._task_queue.
_SHUTDOWN = 'SHUTDOWN'

_CREATE_WORKER_PROCESS = 'CREATE_WORKER_PROCESS'

# The generous upper bound for IPC pipes to flush and processes to respond
# to SIGTERM without forcing the user to wait indefinitely during a teardown.
_TEARDOWN_TIMEOUT_SECONDS = 5.0

# These dummy TaskWrapper and Output objects function as a "reverse poison
# pill". While a standard poison pill (like _SHUTDOWN) travels from the
# orchestrator down to the workers to signal termination, this reverse poison
# pill originates from a crashing worker and travels upwards to the
# orchestrator. Its purpose is to force the orchestrator to abort cleanly upon
# encountering unrecoverable structural failures (e.g., multiprocessing crashes
# or BrokenPipeErrors).
_STRUCTURAL_ERROR_TASK_WRAPPER = task_graph_module.TaskWrapper(
    task_id=None, task=None, dependent_task_ids=None
)

_STRUCTURAL_ERROR_OUTPUT = task.Output(
    additional_task_iterators=None,
    messages=[task.Message(topic=task.Topic.FATAL_ERROR, payload={})],
)


class _DebugSignalHandler:
  """Signal handler for collecting debug information."""

  def __init__(self):
    """Initializes the debug signal handler."""
    if (
        platforms.OperatingSystem.Current()
        is not platforms.OperatingSystem.WINDOWS
    ):
      self._debug_signal = signal_lib.SIGUSR1

  def _debug_handler(
      self, signal_number: int = None, frame: object = None
  ) -> None:
    """Logs stack traces of running threads.

    Args:
      signal_number: Signal number.
      frame: Frame object.
    """
    del signal_number, frame  # currently unused
    log.debug('Initiating crash debug information data collection.')
    stack_traces = []
    stack_traces.extend(task_graph_debugger.yield_stack_traces())
    for line in stack_traces:
      log.debug(line)

  def install(self):
    """Installs the debug signal handler."""
    if platforms.OperatingSystem.Current() is platforms.OperatingSystem.WINDOWS:
      return  # Not supported for windows systems.
    try:
      self._original_signal_handler = signal_lib.getsignal(self._debug_signal)
      signal_lib.signal(self._debug_signal, self._debug_handler)
    except ValueError:
      pass  # Can be run from the main thread only.

  def terminate(self):
    """Restores the original signal handler.

    This method should be called when the debug signal handler is no longer
    needed.
    """
    if platforms.OperatingSystem.Current() is platforms.OperatingSystem.WINDOWS:
      return  # Not supported for windows systems.
    try:
      if hasattr(self, '_original_signal_handler'):
        signal_lib.signal(self._debug_signal, self._original_signal_handler)
    except ValueError:
      pass  # Can be run from the main thread only.


class SharedProcessContext:
  """Context manager used to collect and set global state."""

  def __init__(self):
    """Collects global state in the main process."""
    if multiprocessing_context.get_start_method() == 'fork':
      return

    self._environment_variables = execution_utils.GetToolEnv()
    self._creds_context_manager = (
        creds_context_managers.CredentialProvidersManager()
    )
    self._key_store = encryption_util._key_store
    self._invocation_id = transport.INVOCATION_ID

  def __enter__(self):
    """Sets global state in child processes."""
    if multiprocessing_context.get_start_method() == 'fork':
      return

    self._environment_context_manager = execution_utils.ReplaceEnv(
        **self._environment_variables
    )

    self._environment_context_manager.__enter__()
    self._creds_context_manager.__enter__()
    encryption_util._key_store = self._key_store
    transport.INVOCATION_ID = self._invocation_id

    # Passing None causes log settings to be refreshed based on property values.
    log.SetUserOutputEnabled(None)
    log.SetVerbosity(None)

  def __exit__(self, exc_type, exc_value, exc_traceback):
    """Cleans up global state in child processes."""
    if multiprocessing_context.get_start_method() == 'fork':
      return

    self._environment_context_manager.__exit__(
        exc_type, exc_value, exc_traceback
    )
    self._creds_context_manager.__exit__(exc_type, exc_value, exc_traceback)


@crash_handling.CrashManager
def _thread_worker(
    task_queue,
    task_output_queue,
    task_status_queue,
    idle_thread_count,
    abort_event,
    thread_exit_callback=None,
):
  """A consumer thread run in a child process.

  Args:
    task_queue (multiprocessing.Queue): Holds task_graph.TaskWrapper instances.
    task_output_queue (multiprocessing.Queue): Sends information about completed
      tasks back to the main process.
    task_status_queue (multiprocessing.Queue|None): Used by task to report it
      progress to a central location.
    idle_thread_count (multiprocessing.Semaphore): Keeps track of how many
      threads are busy. Useful for spawning new workers if all threads are busy.
    abort_event (multiprocessing.Event): Global signal to abort.
    thread_exit_callback (Callable[[], None]|None): An optional function to
      execute in this thread's finally block.
  """
  # Verifiable Accounting: The thread is alive and ready for work.
  idle_thread_count.release()

  try:
    while not abort_event.is_set():
      with _task_queue_lock():
        try:
          task_wrapper = task_queue.get(timeout=1.0)
        except queue.Empty:
          continue

      if task_wrapper == _SHUTDOWN:
        break

      # We received a task. Mark ourselves as busy.
      idle_thread_count.acquire()

      try:
        task_output = task_wrapper.task.execute(
            task_status_queue=task_status_queue
        )
      # pylint: disable=broad-except
      # If any exception is raised, it will prevent the executor from exiting.
      except Exception as exception:
        log.error('%s', exception)
        log.debug(
            'Task %r failed. Traceback:',
            getattr(task_wrapper, 'id', None),
            exc_info=True,
        )

        if isinstance(exception, errors.FatalError):
          task_output = task.Output(
              additional_task_iterators=None,
              messages=[task.Message(topic=task.Topic.FATAL_ERROR, payload={})],
          )
        elif task_wrapper.task.change_exit_code:
          task_output = task.Output(
              additional_task_iterators=None,
              messages=[
                  task.Message(topic=task.Topic.CHANGE_EXIT_CODE, payload={})
              ],
          )
        else:
          task_output = None
        task_wrapper.task.exit_handler(exception, task_status_queue)
      else:
        task_wrapper.task.exit_handler(None, task_status_queue)
      finally:
        # By placing the release here, inside the finally block of the
        # execution loop, we guarantee the capacity token is returned to the
        # Producer even if the thread crashed structurally during execution.
        # This prevents capacity starvation deadlocks (the "Ghost Token" bug).
        idle_thread_count.release()

      task_output_queue.put((task_wrapper, task_output))

  except Exception as structural_exception:  # pylint: disable=broad-except
    # The Structural Guardian: Catches Semaphore crashes, BrokenPipe, etc.
    log.error('Worker thread crashed structurally: %r', structural_exception)
    try:
      # The structural error task wrapper is used here to inform the main
      # process that a worker thread caught a severe error and it cannot
      # recover or continue processing tasks. The wrapper will be processed by
      # the listener(_handle_task_output) and it will change the command exit
      # code as well as will provide a signal to stop accepting new tasks.
      task_output_queue.put(
          (_STRUCTURAL_ERROR_TASK_WRAPPER, _STRUCTURAL_ERROR_OUTPUT)
      )
    except Exception as e:  # pylint: disable=broad-except
      log.debug('Failed to send poison pill: %r', e)
    # The thread will now die. We have successfully sent the Poison Pill or
    # failed to do so but reported the failure in logs.
  finally:
    if thread_exit_callback is not None:
      try:
        thread_exit_callback()
      except Exception as e:  # pylint: disable=broad-except
        log.debug('Thread exit callback failed: %r', e)


@crash_handling.CrashManager
def _process_worker(
    task_queue,
    task_output_queue,
    task_status_queue,
    thread_count,
    idle_thread_count,
    shared_process_context,
    stack_trace_file_path,
    abort_event,
    thread_exit_callback=None,
):
  """Starts a consumer thread pool.

  Args:
    task_queue (multiprocessing.Queue): Holds task_graph.TaskWrapper instances.
    task_output_queue (multiprocessing.Queue): Sends information about completed
      tasks back to the main process.
    task_status_queue (multiprocessing.Queue|None): Used by task to report it
      progress to a central location.
    thread_count (int): Number of threads the process should spawn.
    idle_thread_count (multiprocessing.Semaphore): Passed on to worker threads.
    shared_process_context (SharedProcessContext): Holds values from global
      state that need to be replicated in child processes.
    stack_trace_file_path (str): File path to write stack traces to.
    abort_event (multiprocessing.Event): Global signal to abort.
    thread_exit_callback (Callable[[], None]|None): An optional function to
      execute in each worker thread's finally block.
  """
  if properties.VALUES.storage.use_nic_isolation.GetBool():
    performance_util.apply_nic_isolation()

  threads = []
  with shared_process_context:
    for _ in range(thread_count):
      thread = threading.Thread(
          target=_thread_worker,
          args=(
              task_queue,
              task_output_queue,
              task_status_queue,
              idle_thread_count,
              abort_event,
              thread_exit_callback,
          ),
      )
      thread.start()
      threads.append(thread)

    # TODO: b/354829547 - Update the function to catch the updated stack traces
    # of the already running worker threads while a new worker process
    # is not created.

    if task_graph_debugger.is_task_graph_debugging_enabled():
      stack_trace = task_graph_debugger.yield_stack_traces()
      task_graph_debugger.write_stack_traces_to_file(
          stack_trace, stack_trace_file_path
      )

    for thread in threads:
      timeout = 1.0 if abort_event.is_set() else None
      thread.join(timeout=timeout)

    if abort_event.is_set():
      for thread in threads:
        if thread.is_alive():
          os._exit(1)  # pylint: disable=protected-access


def _health_check_worker_processes(processes, task_output_queue, abort_event):
  """Checks if any worker process died unexpectedly and signals abort if so.

  Args:
    processes (list[multiprocessing.Process]): List of worker processes.
    task_output_queue (multiprocessing.Queue): Queue for reporting results.
    abort_event (multiprocessing.Event): Global abort signal.
  """
  for process in processes:
    if process.is_alive():
      continue
    if (
        not hasattr(process, 'exitcode')
        or process.exitcode is None
        or process.exitcode == 0
    ):
      continue

    log.error(
        'Worker process %s died silently with exit code %s',
        process.pid,
        process.exitcode,
    )
    try:
      # The structural error task wrapper is used here to inform the main
      # process that a worker process died unexpectedly.
      task_output_queue.put(
          (_STRUCTURAL_ERROR_TASK_WRAPPER, _STRUCTURAL_ERROR_OUTPUT)
      )
    except Exception:  # pylint: disable=broad-except
      pass
    abort_event.set()
    break


@crash_handling.CrashManager
def _process_factory(
    task_queue,
    task_output_queue,
    task_status_queue,
    thread_count,
    idle_thread_count,
    signal_queue,
    shared_process_context,
    stack_trace_file_path,
    abort_event,
    thread_exit_callback=None,
):
  """Create worker processes.

  This factory must run in a separate process to avoid deadlock issue,
  see go/gcloud-storage-deadlock-issue/. Although we are adding one
  extra process by doing this, it will remain idle once all the child worker
  processes are created. Thus, it does not add noticable burden on the system.

  Args:
    task_queue (multiprocessing.Queue): Holds task_graph.TaskWrapper instances.
    task_output_queue (multiprocessing.Queue): Sends information about completed
      tasks back to the main process.
    task_status_queue (multiprocessing.Queue|None): Used by task to report it
      progress to a central location.
    thread_count (int): Number of threads the process should spawn.
    idle_thread_count (multiprocessing.Semaphore): Passed on to worker threads.
    signal_queue (multiprocessing.Queue): Queue used by parent process to signal
      when a new child worker process must be created.
    shared_process_context (SharedProcessContext): Holds values from global
      state that need to be replicated in child processes.
    stack_trace_file_path (str): File path to write stack traces to.
    abort_event (multiprocessing.Event): Global signal to abort.
    thread_exit_callback (Callable[[], None]|None): An optional function to
      execute in each worker thread's finally block.
  """
  processes = []
  while not abort_event.is_set():
    # We receive one signal message for each process to be created.
    try:
      signal = signal_queue.get(timeout=1.0)
    except queue.Empty:
      _health_check_worker_processes(
          processes, task_output_queue, abort_event
      )
      if abort_event.is_set():
        break
      continue

    if signal == _SHUTDOWN:
      abort_detected = False
      for _ in processes:
        if abort_detected:
          break
        for _ in range(thread_count):
          # Ensure every worker thread receives a _SHUTDOWN token to exit.
          # Loop indefinitely because a healthy worker might be busy.
          while True:
            try:
              # Use timeout to prevent block if worker dies or queue is full.
              task_queue.put(_SHUTDOWN, timeout=1.0)
              break
            except queue.Full:
              # If aborting, abandon graceful shutdown to prevent hanging.
              if abort_event.is_set():
                abort_detected = True
                break
            except Exception:  # pylint: disable=broad-except
              # Abandon push if pipe is broken to avoid 100% CPU spin-loop.
              log.error(
                  'Aborting worker process shutdown due to unexpected error'
              )
              abort_detected = True
              break
          if abort_detected:
            break
      break
    elif signal == _CREATE_WORKER_PROCESS:
      process = multiprocessing_context.Process(
          target=_process_worker,
          args=(
              task_queue,
              task_output_queue,
              task_status_queue,
              thread_count,
              idle_thread_count,
              shared_process_context,
              stack_trace_file_path,
              abort_event,
              thread_exit_callback,
          ),
      )

      processes.append(process)
      log.debug(
          'Adding 1 process with %d threads. Total processes: %d. Total'
          ' threads: %d.',
          thread_count,
          len(processes),
          len(processes) * thread_count,
      )
      process.start()
    else:
      raise errors.Error(
          f'Received invalid signal for worker process creation: {signal}'
      )

  for process in processes:
    process.join()


def _drain_queue(queue_to_drain) -> None:
  """Drains a queue to unblock any processes/threads blocked on putting to it.

  This is necessary during early exits or aborts to ensure that worker
  processes or management threads attempting to write their final exit states
  do not block indefinitely on full IPC pipes, which would cause the entire
  shutdown sequence to deadlock.

  Args:
    queue_to_drain (multiprocessing.Queue): The queue to drain.
  """
  while True:
    try:
      queue_to_drain.get_nowait()
    except queue.Empty:
      # Happy Path: The queue is completely empty. We can safely move on.
      break
    except (BrokenPipeError, EOFError, OSError):
      # Broken Pipe / Corruption Fail-Safe: A child process died mid-write
      # or the underlying OS pipe is physically broken. Since we are in an
      # abort context and simply trying to unblock writers, a broken pipe is
      # functionally "unblocked", so we gracefully exit the drain loop.
      break


def _store_exception(target_function):
  """Decorator for storing exceptions raised from the thread targets.

  Args:
    target_function (function): Thread target to decorate.

  Returns:
    Decorator function.
  """

  @functools.wraps(target_function)
  def wrapper(self, *args, **kwargs):
    try:
      target_function(self, *args, **kwargs)
      # pylint:disable=broad-except
    except Exception as e:
      # pylint:enable=broad-except
      if not isinstance(self, TaskGraphExecutor):
        # Storing of exception is only allowed for TaskGraphExecutor.
        raise
      with self.thread_exception_lock:
        if self.thread_exception is None:
          log.debug('Storing error to raise later: %s', e)
          self.thread_exception = e
        else:
          # This indicates that the exception has been already stored for
          # another thread. We will simply log the traceback in this
          # case, since raising the error is not going to be handled by the
          # main thread anyway.
          log.error(e)
          log.debug(e, exc_info=sys.exc_info())

  return wrapper


class TaskGraphExecutor:
  """Executes an iterable of command_lib.storage.tasks.task.Task instances."""

  def __init__(
      self,
      task_iterator,
      max_process_count=multiprocessing.cpu_count(),
      thread_count=4,
      task_status_queue=None,
      progress_manager_args=None,
      thread_exit_callback=None,
  ):
    """Initializes a TaskGraphExecutor instance.

    No threads or processes are started by the constructor.

    Args:
      task_iterator (Iterable[command_lib.storage.tasks.task.Task]): Task
        instances to execute.
      max_process_count (int): The number of processes to start.
      thread_count (int): The number of threads to start per process.
      task_status_queue (multiprocessing.Queue|None): Used by task to report its
        progress to a central location.
      progress_manager_args (task_status.ProgressManagerArgs|None): Determines
        what type of progress indicator to display.
      thread_exit_callback (Callable[[], None]|None): An optional function to
        execute in each worker thread's finally block.
    """

    self._task_iterator = iter(task_iterator)
    self._max_process_count = max_process_count
    self._thread_count = thread_count
    self._task_status_queue = task_status_queue
    self._progress_manager_args = progress_manager_args
    self._thread_exit_callback = thread_exit_callback

    self._process_count = 0
    self._idle_thread_count = multiprocessing_context.Semaphore(value=0)

    self._worker_count = self._max_process_count * self._thread_count

    # Sends task_graph.TaskWrapper instances to child processes.
    # Size must be 1. go/lazy-process-spawning-addendum.
    self._task_queue = multiprocessing_context.Queue(maxsize=1)

    # Sends information about completed tasks to the main process.
    self._task_output_queue = multiprocessing_context.Queue(
        maxsize=self._worker_count
    )

    # Queue for informing worker_process_creator to create a new process.
    self._signal_queue = multiprocessing_context.Queue(
        maxsize=self._worker_count + 1
    )

    # Tracks dependencies between tasks in the executor to help ensure that
    # tasks returned by executed tasks are completed in the correct order.
    self._task_graph = task_graph_module.TaskGraph(
        top_level_task_limit=2 * self._worker_count
    )

    # Holds tasks without any dependencies.
    self._executable_tasks = task_buffer.TaskBuffer()

    # System-wide signal to abort operations on fatal failure.
    self._abort_event = multiprocessing_context.Event()

    # For storing exceptions.
    self.thread_exception = None
    self.thread_exception_lock = threading.Lock()

    self._accepting_new_tasks = True
    self._exit_code = 0
    self._debug_handler = _DebugSignalHandler()

    self.stack_trace_file_path = None
    if task_graph_debugger.is_task_graph_debugging_enabled():
      try:
        with tempfile.NamedTemporaryFile(
            prefix='stack_trace', suffix='.txt', delete=False
        ) as f:
          self.stack_trace_file_path = f.name
      except IOError as e:
        log.error('Error creating stack trace file: %s', e)

    self._management_threads_name_to_function = {}

  def _add_worker_process(self):
    """Signal the worker process spawner to create a new process."""
    self._signal_queue.put(_CREATE_WORKER_PROCESS)
    self._process_count += 1

  @_store_exception
  def _get_tasks_from_iterator(self):
    """Adds tasks from self._task_iterator to the executor.

    This involves adding tasks to self._task_graph, marking them as submitted,
    and adding them to self._executable_tasks.
    """

    while self._accepting_new_tasks and not self._abort_event.is_set():
      try:
        task_object = next(self._task_iterator)
      except StopIteration:
        break
      task_wrapper = self._task_graph.add(task_object)
      if task_wrapper is None:
        # self._task_graph rejected the task.
        continue
      task_wrapper.is_submitted = True
      # Tasks from task_iterator should have a lower priority than tasks that
      # are spawned by other tasks. This helps keep memory usage under control
      # when a workload's task graph has a large branching factor.
      self._executable_tasks.put(task_wrapper, prioritize=False)

  @_store_exception
  def _add_executable_tasks_to_queue(self):
    """Sends executable tasks to consumer threads in child processes."""
    task_wrapper = None
    while not self._abort_event.is_set():
      if task_wrapper is None:
        try:
          task_wrapper = self._executable_tasks.get(timeout=1.0)
        except queue.Empty:
          continue
        if task_wrapper == _SHUTDOWN:
          break

      reached_process_limit = self._process_count >= self._max_process_count

      try:
        # We combine `block` and `timeout` to defeat the Blocked Thread Paradox
        # (where a thread blocks indefinitely on a full/empty queue and cannot
        # receive the abort signal) while preventing CPU spinlocks.
        # 1. When `block=False` (scaling up), timeout is ignored. It fails
        #    instantly if full, rapidly triggering _add_worker_process()
        #    without latency.
        # 2. When `block=True` (fully scaled), it sleeps to save CPU cycles but
        #    wakes up every 1.0s to check the global _abort_event.
        self._task_queue.put(
            task_wrapper, block=reached_process_limit, timeout=1.0
        )
        task_wrapper = None
      except queue.Full:
        if self._idle_thread_count.acquire(block=False):
          # Idle worker will take a task. Restore semaphore count.
          self._idle_thread_count.release()
        elif not reached_process_limit:
          self._add_worker_process()

  @_store_exception
  def _handle_task_output(self):
    """Updates a dependency graph based on information from executed tasks."""
    while not self._abort_event.is_set():
      try:
        output = self._task_output_queue.get(timeout=1.0)
      except queue.Empty:
        continue
      if output == _SHUTDOWN:
        break

      executed_task_wrapper, task_output = output
      if task_output and task_output.messages:
        for message in task_output.messages:
          if message.topic in (
              task.Topic.CHANGE_EXIT_CODE,
              task.Topic.FATAL_ERROR,
          ):
            self._exit_code = 1
            if message.topic == task.Topic.FATAL_ERROR:
              self._accepting_new_tasks = False

      if executed_task_wrapper.task is None:
        continue

      submittable_tasks = self._task_graph.update_from_executed_task(
          executed_task_wrapper, task_output
      )

      for task_wrapper in submittable_tasks:
        task_wrapper.is_submitted = True
        self._executable_tasks.put(task_wrapper, prioritize=True)

  def _clean_worker_process_spawner(self, worker_process_spawner):
    """Carries out the required steps to clean up worker processes.

    Args:
      worker_process_spawner (Process): The worker parent process that we need
        to clean up.
    """
    # Shutdown all the workers.
    if worker_process_spawner.is_alive():
      try:
        self._signal_queue.put(_SHUTDOWN, timeout=1.0)
      except queue.Full:
        pass
      except Exception:  # pylint: disable=broad-except
        pass

    if self._abort_event.is_set():
      if worker_process_spawner.is_alive():
        self._drain_and_join_with_timeout(
            worker_process_spawner,
            (self._signal_queue, self._task_output_queue),
        )

        # If the process is still alive after the draining phase, it means
        # it is not responding to the shutdown signals cleanly. We forcefully
        # terminate it to avoid hanging the entire executor during teardown.
        if worker_process_spawner.is_alive():
          worker_process_spawner.terminate()
          worker_process_spawner.join(timeout=1.0)

      self._cancel_join_threads()

      if worker_process_spawner.is_alive():
        log.error(
            'Factory process refused to die after SIGTERM. Forcing OS exit.'
        )
        os._exit(1)  # pylint: disable=protected-access
    else:
      worker_process_spawner.join()

    # Restore the debug signal handler.
    self._debug_handler.terminate()

  def _drain_and_join_with_timeout(self, process_or_thread, queues_to_drain):
    """Drains given queues while joining a process or thread with a timeout."""
    abort_time_deadline = time.time() + _TEARDOWN_TIMEOUT_SECONDS
    while process_or_thread.is_alive() and time.time() < abort_time_deadline:
      for q in queues_to_drain:
        _drain_queue(q)
      process_or_thread.join(timeout=0.1)

  def _cancel_join_threads(self):
    """Cancels the background feeder threads for all IPC pipes."""
    if hasattr(self._task_queue, 'cancel_join_thread'):
      self._task_queue.cancel_join_thread()
    if hasattr(self._task_output_queue, 'cancel_join_thread'):
      self._task_output_queue.cancel_join_thread()
    if hasattr(self._signal_queue, 'cancel_join_thread'):
      self._signal_queue.cancel_join_thread()
    if self._task_status_queue is not None and hasattr(
        self._task_status_queue, 'cancel_join_thread'
    ):
      self._task_status_queue.cancel_join_thread()

  def run(self):
    """Executes tasks from a task iterator in parallel.

    Returns:
      An integer indicating the exit code. Zero indicates no fatal errors were
        raised.
    """
    shared_process_context = SharedProcessContext()
    self._debug_handler.install()
    worker_process_spawner = multiprocessing_context.Process(
        target=_process_factory,
        args=(
            self._task_queue,
            self._task_output_queue,
            self._task_status_queue,
            self._thread_count,
            self._idle_thread_count,
            self._signal_queue,
            shared_process_context,
            self.stack_trace_file_path,
            self._abort_event,
            self._thread_exit_callback,
        ),
    )

    worker_process_cleaned_up = False
    try:
      worker_process_spawner.start()
      # It is now safe to start the progress_manager thread, since new processes
      # are started by a child process.
      with task_status.progress_manager(
          self._task_status_queue, self._progress_manager_args
      ):
        try:
          self._add_worker_process()

          get_tasks_from_iterator_thread = threading.Thread(
              target=self._get_tasks_from_iterator
          )
          add_executable_tasks_to_queue_thread = threading.Thread(
              target=self._add_executable_tasks_to_queue
          )
          handle_task_output_thread = threading.Thread(
              target=self._handle_task_output
          )

          # Robustly start the management threads. If any of them
          # fails to start, ensure that the ones that did start are
          # joined properly to avoid hanging.
          threads_to_start = [
              get_tasks_from_iterator_thread,
              add_executable_tasks_to_queue_thread,
              handle_task_output_thread,
          ]
          started_threads = []
          try:
            for thread in threads_to_start:
              thread.start()
              started_threads.append(thread)
          except Exception:
            # If any thread fails to start, stop the ones that did start.
            self._abort_event.set()
            self._accepting_new_tasks = False
            try:
              self._executable_tasks.put(_SHUTDOWN, timeout=1.0)
            except queue.Full:
              pass
            try:
              self._task_output_queue.put(_SHUTDOWN, timeout=1.0)
            except queue.Full:
              pass
            # The get_tasks_from_iterator thread might be blocked on the
            # task graph semaphore. Release it to allow the thread to exit.
            # pylint: disable=protected-access
            self._task_graph._top_level_task_semaphore.release()
            # pylint: enable=protected-access
            for thread in started_threads:
              if thread.is_alive():
                # Join with a timeout to avoid hanging if the thread is
                # truly stuck.
                thread.join(timeout=1.0)
            raise

          if task_graph_debugger.is_task_graph_debugging_enabled():
            self._management_threads_name_to_function[
                'get_tasks_from_iterator'
            ] = get_tasks_from_iterator_thread

            self._management_threads_name_to_function[
                'add_executable_tasks_to_queue'
            ] = add_executable_tasks_to_queue_thread

            self._management_threads_name_to_function['handle_task_output'] = (
                handle_task_output_thread
            )

            task_graph_debugger.start_thread_for_task_graph_debugging(
                self._management_threads_name_to_function,
                self.stack_trace_file_path,
                self._task_graph,
                self._executable_tasks,
            )

          get_tasks_from_iterator_thread.join()
          try:
            while not self._task_graph.is_empty.wait(timeout=1.0):
              if self._abort_event.is_set():
                break
              if self.thread_exception or not worker_process_spawner.is_alive():
                self._exit_code = 1
                self._abort_event.set()
                break
          except console_io.OperationCancelledError:
            self._abort_event.set()

          if self._abort_event.is_set():

            self._drain_and_join_with_timeout(
                add_executable_tasks_to_queue_thread, (self._task_queue,)
            )
            self._drain_and_join_with_timeout(
                handle_task_output_thread, (self._task_output_queue,)
            )
          else:
            try:
              self._executable_tasks.put(_SHUTDOWN, timeout=1.0)
            except Exception:  # pylint: disable=broad-except
              pass
            try:
              self._task_output_queue.put(_SHUTDOWN, timeout=1.0)
            except Exception:  # pylint: disable=broad-except
              pass

          if self._abort_event.is_set():
            if (
                handle_task_output_thread.is_alive()
                or add_executable_tasks_to_queue_thread.is_alive()
            ):
              log.error('Management threads refused to die. Forcing OS exit.')
              os._exit(1)  # pylint: disable=protected-access
          else:
            handle_task_output_thread.join()
            add_executable_tasks_to_queue_thread.join()
        finally:
          if task_graph_debugger.is_task_graph_debugging_enabled():
            task_graph_debugger.stop_thread_for_task_graph_debugging()
          # By calling the clean in the finally block, we ensure that the
          # progress manager exit is called first.
          # We also handle the scenario where an exception may be thrown by the
          # progress manager it self.
          self._clean_worker_process_spawner(worker_process_spawner)
          worker_process_cleaned_up = True
    except Exception:  # pylint: disable=broad-exception-caught
      # In case we get an exception occurs while spinning up the worker process
      # spawner or during start of progress manager context, we need to
      # do a clean up, hence we use the following method which carries out
      # the neccesary steps.
      # Note that the clean up only occurs if an exception occurs. There is
      # another finally block within the progress manager context which will
      # execute if there is any exception or in case of compleition of internal
      # logic. If that is invoked, there is a small chance of this block being
      # invoked as well, but for that, we have the worker process clean-up flag.
      if not worker_process_cleaned_up:
        self._clean_worker_process_spawner(worker_process_spawner)

      # Force close all pipes to unblock any stuck listeners (e.g. progress
      # indicator thread or caller).
      self._cancel_join_threads()

      self._task_queue.close()
      self._task_output_queue.close()
      self._signal_queue.close()
      if self._task_status_queue is not None:
        self._task_status_queue.close()

      # Raise it back as we still want main process to exit
      raise

    # Queue close calls need to be outside the worker process spawner context
    # manager since the task queue need to be open for the shutdown logic.
    self._task_queue.close()
    self._task_output_queue.close()

    with self.thread_exception_lock:
      if self.thread_exception:
        raise self.thread_exception  # pylint: disable=raising-bad-type

    return self._exit_code
