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As noted in the docstring for this function, the timeout is best-effort only -- if the thread is blocked in a syscall, it will not service the exception until it returns. It can also choose to catch and ignore the TimeoutExpired; in either case it will still be running even after the `timeout()` function returns. Raising a vare TimeoutExpired it still somewhat accurate, but obscures that the backend thread may still be running along merrily. Notice such cases, and log a warning about them.
99 lines
3.4 KiB
Python
99 lines
3.4 KiB
Python
import ctypes
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import logging
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import sys
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import threading
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import time
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from types import TracebackType
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from typing import Callable, Optional, Tuple, Type, TypeVar
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# Based on https://code.activestate.com/recipes/483752/
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class TimeoutExpired(Exception):
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"""Exception raised when a function times out."""
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def __str__(self) -> str:
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return "Function call timed out."
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ResultT = TypeVar("ResultT")
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def timeout(timeout: float, func: Callable[[], ResultT]) -> ResultT:
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"""Call the function in a separate thread.
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Return its return value, or raise an exception,
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within approximately 'timeout' seconds.
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The function may receive a TimeoutExpired exception
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anywhere in its code, which could have arbitrary
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unsafe effects (resources not released, etc.).
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It might also fail to receive the exception and
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keep running in the background even though
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timeout() has returned.
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This may also fail to interrupt functions which are
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stuck in a long-running primitive interpreter
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operation."""
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class TimeoutThread(threading.Thread):
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def __init__(self) -> None:
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threading.Thread.__init__(self)
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self.result: Optional[ResultT] = None
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self.exc_info: Tuple[
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Optional[Type[BaseException]],
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Optional[BaseException],
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Optional[TracebackType],
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] = (None, None, None)
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# Don't block the whole program from exiting
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# if this is the only thread left.
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self.daemon = True
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def run(self) -> None:
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try:
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self.result = func()
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except BaseException:
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self.exc_info = sys.exc_info()
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def raise_async_timeout(self) -> None:
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# Called from another thread.
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# Attempt to raise a TimeoutExpired in the thread represented by 'self'.
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assert self.ident is not None # Thread should be running; c_long expects int
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tid = ctypes.c_long(self.ident)
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ctypes.pythonapi.PyThreadState_SetAsyncExc(
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tid, ctypes.py_object(TimeoutExpired)
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)
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thread = TimeoutThread()
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thread.start()
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thread.join(timeout)
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if thread.is_alive():
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# Gamely try to kill the thread, following the dodgy approach from
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# https://stackoverflow.com/a/325528/90777
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#
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# We need to retry, because an async exception received while the
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# thread is in a system call is simply ignored.
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for i in range(10):
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thread.raise_async_timeout()
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time.sleep(0.1)
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if not thread.is_alive():
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break
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if thread.exc_info[1] is not None:
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# Re-raise the exception we sent, if possible, so the
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# stacktrace originates in the slow code
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raise thread.exc_info[1].with_traceback(thread.exc_info[2])
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# If we don't have that for some reason (e.g. we failed to
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# kill it), just raise from here; the thread _may still be
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# running_ because it failed to see any of our exceptions, and
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# we just ignore it.
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if thread.is_alive():
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logging.warning("Failed to time out backend thread")
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raise TimeoutExpired
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if thread.exc_info[1] is not None:
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# Died with some other exception; re-raise it
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raise thread.exc_info[1].with_traceback(thread.exc_info[2])
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assert thread.result is not None # assured if above did not reraise
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return thread.result
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