# coding: utf-8 import contextvars import threading import queue as stdlib_queue import functools from itertools import count import attr import inspect import outcome from sniffio import current_async_library_cvar import trio from ._sync import CapacityLimiter from ._core import ( enable_ki_protection, disable_ki_protection, RunVar, TrioToken, start_thread_soon, ) from ._util import coroutine_or_error # Global due to Threading API, thread local storage for trio token TOKEN_LOCAL = threading.local() _limiter_local = RunVar("limiter") # I pulled this number out of the air; it isn't based on anything. Probably we # should make some kind of measurements to pick a good value. DEFAULT_LIMIT = 40 _thread_counter = count() def current_default_thread_limiter(): """Get the default `~trio.CapacityLimiter` used by `trio.to_thread.run_sync`. The most common reason to call this would be if you want to modify its :attr:`~trio.CapacityLimiter.total_tokens` attribute. """ try: limiter = _limiter_local.get() except LookupError: limiter = CapacityLimiter(DEFAULT_LIMIT) _limiter_local.set(limiter) return limiter # Eventually we might build this into a full-fledged deadlock-detection # system; see https://github.com/python-trio/trio/issues/182 # But for now we just need an object to stand in for the thread, so we can # keep track of who's holding the CapacityLimiter's token. @attr.s(frozen=True, eq=False, hash=False) class ThreadPlaceholder: name = attr.ib() @enable_ki_protection async def to_thread_run_sync(sync_fn, *args, cancellable=False, limiter=None): """Convert a blocking operation into an async operation using a thread. These two lines are equivalent:: sync_fn(*args) await trio.to_thread.run_sync(sync_fn, *args) except that if ``sync_fn`` takes a long time, then the first line will block the Trio loop while it runs, while the second line allows other Trio tasks to continue working while ``sync_fn`` runs. This is accomplished by pushing the call to ``sync_fn(*args)`` off into a worker thread. From inside the worker thread, you can get back into Trio using the functions in `trio.from_thread`. Args: sync_fn: An arbitrary synchronous callable. *args: Positional arguments to pass to sync_fn. If you need keyword arguments, use :func:`functools.partial`. cancellable (bool): Whether to allow cancellation of this operation. See discussion below. limiter (None, or CapacityLimiter-like object): An object used to limit the number of simultaneous threads. Most commonly this will be a `~trio.CapacityLimiter`, but it could be anything providing compatible :meth:`~trio.CapacityLimiter.acquire_on_behalf_of` and :meth:`~trio.CapacityLimiter.release_on_behalf_of` methods. This function will call ``acquire_on_behalf_of`` before starting the thread, and ``release_on_behalf_of`` after the thread has finished. If None (the default), uses the default `~trio.CapacityLimiter`, as returned by :func:`current_default_thread_limiter`. **Cancellation handling**: Cancellation is a tricky issue here, because neither Python nor the operating systems it runs on provide any general mechanism for cancelling an arbitrary synchronous function running in a thread. This function will always check for cancellation on entry, before starting the thread. But once the thread is running, there are two ways it can handle being cancelled: * If ``cancellable=False``, the function ignores the cancellation and keeps going, just like if we had called ``sync_fn`` synchronously. This is the default behavior. * If ``cancellable=True``, then this function immediately raises `~trio.Cancelled`. In this case **the thread keeps running in background** – we just abandon it to do whatever it's going to do, and silently discard any return value or errors that it raises. Only use this if you know that the operation is safe and side-effect free. (For example: :func:`trio.socket.getaddrinfo` uses a thread with ``cancellable=True``, because it doesn't really affect anything if a stray hostname lookup keeps running in the background.) The ``limiter`` is only released after the thread has *actually* finished – which in the case of cancellation may be some time after this function has returned. If :func:`trio.run` finishes before the thread does, then the limiter release method will never be called at all. .. warning:: You should not use this function to call long-running CPU-bound functions! In addition to the usual GIL-related reasons why using threads for CPU-bound work is not very effective in Python, there is an additional problem: on CPython, `CPU-bound threads tend to "starve out" IO-bound threads `__, so using threads for CPU-bound work is likely to adversely affect the main thread running Trio. If you need to do this, you're better off using a worker process, or perhaps PyPy (which still has a GIL, but may do a better job of fairly allocating CPU time between threads). Returns: Whatever ``sync_fn(*args)`` returns. Raises: Exception: Whatever ``sync_fn(*args)`` raises. """ await trio.lowlevel.checkpoint_if_cancelled() cancellable = bool(cancellable) # raise early if cancellable.__bool__ raises if limiter is None: limiter = current_default_thread_limiter() # Holds a reference to the task that's blocked in this function waiting # for the result – or None if this function was cancelled and we should # discard the result. task_register = [trio.lowlevel.current_task()] name = f"trio.to_thread.run_sync-{next(_thread_counter)}" placeholder = ThreadPlaceholder(name) # This function gets scheduled into the Trio run loop to deliver the # thread's result. def report_back_in_trio_thread_fn(result): def do_release_then_return_result(): # release_on_behalf_of is an arbitrary user-defined method, so it # might raise an error. If it does, we want that error to # replace the regular return value, and if the regular return was # already an exception then we want them to chain. try: return result.unwrap() finally: limiter.release_on_behalf_of(placeholder) result = outcome.capture(do_release_then_return_result) if task_register[0] is not None: trio.lowlevel.reschedule(task_register[0], result) current_trio_token = trio.lowlevel.current_trio_token() def worker_fn(): current_async_library_cvar.set(None) TOKEN_LOCAL.token = current_trio_token try: ret = sync_fn(*args) if inspect.iscoroutine(ret): # Manually close coroutine to avoid RuntimeWarnings ret.close() raise TypeError( "Trio expected a sync function, but {!r} appears to be " "asynchronous".format(getattr(sync_fn, "__qualname__", sync_fn)) ) return ret finally: del TOKEN_LOCAL.token context = contextvars.copy_context() contextvars_aware_worker_fn = functools.partial(context.run, worker_fn) def deliver_worker_fn_result(result): try: current_trio_token.run_sync_soon(report_back_in_trio_thread_fn, result) except trio.RunFinishedError: # The entire run finished, so the task we're trying to contact is # certainly long gone -- it must have been cancelled and abandoned # us. pass await limiter.acquire_on_behalf_of(placeholder) try: start_thread_soon(contextvars_aware_worker_fn, deliver_worker_fn_result) except: limiter.release_on_behalf_of(placeholder) raise def abort(_): if cancellable: task_register[0] = None return trio.lowlevel.Abort.SUCCEEDED else: return trio.lowlevel.Abort.FAILED return await trio.lowlevel.wait_task_rescheduled(abort) def _run_fn_as_system_task(cb, fn, *args, context, trio_token=None): """Helper function for from_thread.run and from_thread.run_sync. Since this internally uses TrioToken.run_sync_soon, all warnings about raised exceptions canceling all tasks should be noted. """ if trio_token and not isinstance(trio_token, TrioToken): raise RuntimeError("Passed kwarg trio_token is not of type TrioToken") if not trio_token: try: trio_token = TOKEN_LOCAL.token except AttributeError: raise RuntimeError( "this thread wasn't created by Trio, pass kwarg trio_token=..." ) # Avoid deadlock by making sure we're not called from Trio thread try: trio.lowlevel.current_task() except RuntimeError: pass else: raise RuntimeError("this is a blocking function; call it from a thread") q = stdlib_queue.SimpleQueue() trio_token.run_sync_soon(context.run, cb, q, fn, args) return q.get().unwrap() def from_thread_run(afn, *args, trio_token=None): """Run the given async function in the parent Trio thread, blocking until it is complete. Returns: Whatever ``afn(*args)`` returns. Returns or raises whatever the given function returns or raises. It can also raise exceptions of its own: Raises: RunFinishedError: if the corresponding call to :func:`trio.run` has already completed, or if the run has started its final cleanup phase and can no longer spawn new system tasks. Cancelled: if the corresponding call to :func:`trio.run` completes while ``afn(*args)`` is running, then ``afn`` is likely to raise :exc:`trio.Cancelled`, and this will propagate out into RuntimeError: if you try calling this from inside the Trio thread, which would otherwise cause a deadlock. AttributeError: if no ``trio_token`` was provided, and we can't infer one from context. TypeError: if ``afn`` is not an asynchronous function. **Locating a Trio Token**: There are two ways to specify which `trio.run` loop to reenter: - Spawn this thread from `trio.to_thread.run_sync`. Trio will automatically capture the relevant Trio token and use it when you want to re-enter Trio. - Pass a keyword argument, ``trio_token`` specifying a specific `trio.run` loop to re-enter. This is useful in case you have a "foreign" thread, spawned using some other framework, and still want to enter Trio. """ def callback(q, afn, args): @disable_ki_protection async def unprotected_afn(): coro = coroutine_or_error(afn, *args) return await coro async def await_in_trio_thread_task(): q.put_nowait(await outcome.acapture(unprotected_afn)) context = contextvars.copy_context() try: trio.lowlevel.spawn_system_task( await_in_trio_thread_task, name=afn, context=context ) except RuntimeError: # system nursery is closed q.put_nowait( outcome.Error(trio.RunFinishedError("system nursery is closed")) ) context = contextvars.copy_context() context.run(current_async_library_cvar.set, "trio") return _run_fn_as_system_task( callback, afn, *args, context=context, trio_token=trio_token, ) def from_thread_run_sync(fn, *args, trio_token=None): """Run the given sync function in the parent Trio thread, blocking until it is complete. Returns: Whatever ``fn(*args)`` returns. Returns or raises whatever the given function returns or raises. It can also raise exceptions of its own: Raises: RunFinishedError: if the corresponding call to `trio.run` has already completed. RuntimeError: if you try calling this from inside the Trio thread, which would otherwise cause a deadlock. AttributeError: if no ``trio_token`` was provided, and we can't infer one from context. TypeError: if ``fn`` is an async function. **Locating a Trio Token**: There are two ways to specify which `trio.run` loop to reenter: - Spawn this thread from `trio.to_thread.run_sync`. Trio will automatically capture the relevant Trio token and use it when you want to re-enter Trio. - Pass a keyword argument, ``trio_token`` specifying a specific `trio.run` loop to re-enter. This is useful in case you have a "foreign" thread, spawned using some other framework, and still want to enter Trio. """ def callback(q, fn, args): current_async_library_cvar.set("trio") @disable_ki_protection def unprotected_fn(): ret = fn(*args) if inspect.iscoroutine(ret): # Manually close coroutine to avoid RuntimeWarnings ret.close() raise TypeError( "Trio expected a sync function, but {!r} appears to be " "asynchronous".format(getattr(fn, "__qualname__", fn)) ) return ret res = outcome.capture(unprotected_fn) q.put_nowait(res) context = contextvars.copy_context() return _run_fn_as_system_task( callback, fn, *args, context=context, trio_token=trio_token, )