Refactor pipeline thread executor creation

Move the creation of executor to builder so that later
it can reuse the executor

src/spdl/pipeline/_builder.py

@@ -19,7 +19,7 @@
     Iterable,
     Sequence,
 )
-from concurrent.futures import Executor
+from concurrent.futures import Executor, ThreadPoolExecutor
 from contextlib import asynccontextmanager, contextmanager
 from functools import partial
 from typing import TypeVar
@@ -893,4 +893,8 @@ def build(self, *, num_threads: int | None = None) -> Pipeline:
             ]
             num_threads = max(concurrencies) if concurrencies else 4
         assert num_threads is not None
-        return Pipeline(coro, queues, num_threads, desc=self._get_desc())
+        executor = ThreadPoolExecutor(
+            max_workers=num_threads,
+            thread_name_prefix="spdl_",
+        )
+        return Pipeline(coro, queues, executor, desc=self._get_desc())

src/spdl/pipeline/_pipeline.py

@@ -13,6 +13,7 @@
 import warnings
 from asyncio import AbstractEventLoop, Queue as AsyncQueue
 from collections.abc import Coroutine, Iterator
+from concurrent.futures import ThreadPoolExecutor
 from contextlib import contextmanager
 from enum import IntEnum
 from threading import Event as SyncEvent, Thread
@@ -36,9 +37,13 @@
 # This class has a bit exessive debug logs, because it is tricky to debug
 # it from the outside.
 class _EventLoop:
-    def __init__(self, coro: Coroutine[None, None, None], num_threads: int) -> None:
+    def __init__(
+        self,
+        coro: Coroutine[None, None, None],
+        executor: ThreadPoolExecutor,
+    ) -> None:
         self._coro = coro
-        self._num_threads = num_threads
+        self._executor = executor
 
         self._loop: AbstractEventLoop | None = None
 
@@ -62,14 +67,8 @@ def __str__(self) -> str:
 
     async def _execute_task(self) -> None:
         _LG.debug("The event loop thread coroutine is started.")
-        _LG.debug("Initializing the thread pool of size=%d.", self._num_threads)
         self._loop = asyncio.get_running_loop()
-        self._loop.set_default_executor(
-            concurrent.futures.ThreadPoolExecutor(
-                max_workers=self._num_threads,
-                thread_name_prefix="spdl_",
-            )
-        )
+        self._loop.set_default_executor(self._executor)
 
         _LG.debug("Starting the task.")
 
@@ -263,7 +262,7 @@ def __init__(
         self,
         coro: Coroutine[None, None, None],
         queues: list[AsyncQueue],
-        num_threads: int,
+        executor: ThreadPoolExecutor,
         *,
         desc: list[str],
     ) -> None:
@@ -272,7 +271,7 @@ def __init__(
         self._str: str = "\n".join([repr(self), *desc])
 
         self._output_queue: AsyncQueue = queues[-1]
-        self._event_loop = _EventLoop(coro, num_threads)
+        self._event_loop = _EventLoop(coro, executor)
         self._event_loop_state: _EventLoopState = _EventLoopState.NOT_STARTED
 
         try: