Add executor to zimscraperlib

Author: benoit74

src/zimscraperlib/executor.py

@@ -0,0 +1,173 @@
+import datetime
+import queue
+import threading
+from collections.abc import Callable
+
+from zimscraperlib import logger
+
+_shutdown = False
+# Lock that ensures that new workers are not created while the interpreter is
+# shutting down. Must be held while mutating _threads_queues and _shutdown.
+_global_shutdown_lock = threading.Lock()
+
+
+def excepthook(args):  # pragma: no cover
+    logger.error(f"UNHANDLED Exception in {args.thread.name}: {args.exc_type}")
+    logger.exception(args.exc_value)
+
+
+threading.excepthook = excepthook
+
+
+class ScraperExecutor(queue.Queue):
+    """Custom FIFO queue based Executor that's less generic than ThreadPoolExec one
+
+    Providing more flexibility for the use cases we're interested about:
+    - halt immediately (sort of) upon exception (if requested)
+    - able to join() then restart later to accomodate successive steps
+
+    See: https://github.com/python/cpython/blob/3.8/Lib/concurrent/futures/thread.py
+    """
+
+    def __init__(
+        self,
+        queue_size: int = 10,
+        nb_workers: int = 1,
+        executor_name: str = "executor",
+        thread_deadline_sec: int = 60,
+    ):
+        super().__init__(queue_size)
+        self.executor_name = executor_name
+        self._shutdown_lock = threading.Lock()
+        self.nb_workers = nb_workers
+        self.exceptions = []
+        self.thread_deadline_sec = thread_deadline_sec
+
+    @property
+    def exception(self):
+        """Exception raises in any thread, if any"""
+        try:
+            return self.exceptions[0:1].pop()
+        except IndexError:
+            return None
+
+    @property
+    def alive(self):
+        """whether it should continue running"""
+        return not self._shutdown
+
+    def submit(self, task: Callable, **kwargs):
+        """Submit a callable and its kwargs for execution in one of the workers"""
+        with self._shutdown_lock, _global_shutdown_lock:
+            if not self.alive:
+                raise RuntimeError("cannot submit task to dead executor")
+            if self.no_more:
+                raise RuntimeError(
+                    "cannot submit task to a joined executor, restart it first"
+                )
+            if _shutdown:
+                raise RuntimeError(  # pragma: no cover
+                    "cannot submit task after interpreter shutdown"
+                )
+
+        while True:
+            try:
+                self.put((task, kwargs), block=True, timeout=3.0)
+            except queue.Full:
+                if self.no_more:
+                    # rarely happens except if submit and join are done in different
+                    # threads, but we need this to escape the while loop
+                    break  # pragma: no cover
+            else:
+                break
+
+    def start(self):
+        """Enable executor, starting requested amount of workers
+
+        Workers are started always, not provisioned dynamically"""
+        self.drain()
+        self._workers: set[threading.Thread] = set()
+        self.no_more = False
+        self._shutdown = False
+        self.exceptions[:] = []
+
+        for n in range(self.nb_workers):
+            t = threading.Thread(target=self.worker, name=f"{self.executor_name}-{n}")
+            t.daemon = True
+            t.start()
+            self._workers.add(t)
+
+    def worker(self):
+        while self.alive or self.no_more:
+            try:
+                func, kwargs = self.get(block=True, timeout=2.0)
+            except queue.Empty:
+                if self.no_more:
+                    break
+                continue
+            except TypeError:  # pragma: no cover
+                # received None from the queue. most likely shuting down
+                return
+
+            raises = kwargs.pop("raises") if "raises" in kwargs.keys() else False
+            callback = kwargs.pop("callback") if "callback" in kwargs.keys() else None
+            dont_release = kwargs.pop("dont_release", False)
+
+            try:
+                func(**kwargs)
+            except Exception as exc:
+                logger.error(f"Error processing {func} with {kwargs=}")
+                logger.exception(exc)
+                if raises:  # to cover when raises = False
+                    self.exceptions.append(exc)
+                    self.shutdown()
+            finally:
+                # user will manually release the queue for this task.
+                # most likely in a libzim-written callback
+                if not dont_release:
+                    self.task_done()
+                if callback:
+                    callback.__call__()
+
+    def drain(self):
+        """Empty the queue without processing the tasks (tasks will be lost)"""
+        while True:
+            try:
+                self.get_nowait()
+            except queue.Empty:
+                break
+
+    def join(self):
+        """Await completion of workers, requesting them to stop taking new task"""
+        logger.debug(f"joining all threads for {self.executor_name}")
+        self.no_more = True
+        for num, t in enumerate(self._workers):
+            deadline = datetime.datetime.now(tz=datetime.UTC) + datetime.timedelta(
+                seconds=self.thread_deadline_sec
+            )
+            logger.debug(
+                f"Giving {self.executor_name}-{num} {self.thread_deadline_sec}s to join"
+            )
+            e = threading.Event()
+            while t.is_alive() and datetime.datetime.now(tz=datetime.UTC) < deadline:
+                t.join(1)
+                e.wait(timeout=2)
+            if t.is_alive():
+                logger.debug(
+                    f"Thread {self.executor_name}-{num} is not joining. Skipping…"
+                )
+            else:
+                logger.debug(f"Thread {self.executor_name}-{num} joined")
+        logger.debug(f"all threads joined for {self.executor_name}")
+
+    def shutdown(self, *, wait=True):
+        """stop the executor, either somewhat immediately or awaiting completion"""
+        logger.debug(f"shutting down {self.executor_name} with {wait=}")
+        with self._shutdown_lock:
+            self._shutdown = True
+
+            # Drain all work items from the queue
+            if not wait:
+                self.drain()
+        if wait:
+            self.join()