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| -rw-r--r-- | Doc/library/exceptions.rst | 9 | ||||
| -rw-r--r-- | Doc/library/signal.rst | 70 | ||||
| -rw-r--r-- | Misc/NEWS.d/next/Documentation/2020-11-12-21-26-31.bpo-42340.apumUL.rst | 3 |
3 files changed, 82 insertions, 0 deletions
diff --git a/Doc/library/exceptions.rst b/Doc/library/exceptions.rst index d0b2e62..9e001c8 100644 --- a/Doc/library/exceptions.rst +++ b/Doc/library/exceptions.rst @@ -254,6 +254,15 @@ The following exceptions are the exceptions that are usually raised. accidentally caught by code that catches :exc:`Exception` and thus prevent the interpreter from exiting. + .. note:: + + Catching a :exc:`KeyboardInterrupt` requires special consideration. + Because it can be raised at unpredictable points, it may, in some + circumstances, leave the running program in an inconsistent state. It is + generally best to allow :exc:`KeyboardInterrupt` to end the program as + quickly as possible or avoid raising it entirely. (See + :ref:`handlers-and-exceptions`.) + .. exception:: MemoryError diff --git a/Doc/library/signal.rst b/Doc/library/signal.rst index abc3036..c276b52 100644 --- a/Doc/library/signal.rst +++ b/Doc/library/signal.rst @@ -46,6 +46,9 @@ This has consequences: arbitrary amount of time, regardless of any signals received. The Python signal handlers will be called when the calculation finishes. +* If the handler raises an exception, it will be raised "out of thin air" in + the main thread. See the :ref:`note below <handlers-and-exceptions>` for a + discussion. .. _signals-and-threads: @@ -712,3 +715,70 @@ Do not set :const:`SIGPIPE`'s disposition to :const:`SIG_DFL` in order to avoid :exc:`BrokenPipeError`. Doing that would cause your program to exit unexpectedly also whenever any socket connection is interrupted while your program is still writing to it. + +.. _handlers-and-exceptions: + +Note on Signal Handlers and Exceptions +-------------------------------------- + +If a signal handler raises an exception, the exception will be propagated to +the main thread and may be raised after any :term:`bytecode` instruction. Most +notably, a :exc:`KeyboardInterrupt` may appear at any point during execution. +Most Python code, including the standard library, cannot be made robust against +this, and so a :exc:`KeyboardInterrupt` (or any other exception resulting from +a signal handler) may on rare occasions put the program in an unexpected state. + +To illustrate this issue, consider the following code:: + + class SpamContext: + def __init__(self): + self.lock = threading.Lock() + + def __enter__(self): + # If KeyboardInterrupt occurs here, everything is fine + self.lock.acquire() + # If KeyboardInterrupt occcurs here, __exit__ will not be called + ... + # KeyboardInterrupt could occur just before the function returns + + def __exit__(self, exc_type, exc_val, exc_tb): + ... + self.lock.release() + +For many programs, especially those that merely want to exit on +:exc:`KeyboardInterrupt`, this is not a problem, but applications that are +complex or require high reliability should avoid raising exceptions from signal +handlers. They should also avoid catching :exc:`KeyboardInterrupt` as a means +of gracefully shutting down. Instead, they should install their own +:const:`SIGINT` handler. Below is an example of an HTTP server that avoids +:exc:`KeyboardInterrupt`:: + + import signal + import socket + from selectors import DefaultSelector, EVENT_READ + from http.server import HTTPServer, SimpleHTTPRequestHandler + + interrupt_read, interrupt_write = socket.socketpair() + + def handler(signum, frame): + print('Signal handler called with signal', signum) + interrupt_write.send(b'\0') + signal.signal(signal.SIGINT, handler) + + def serve_forever(httpd): + sel = DefaultSelector() + sel.register(interrupt_read, EVENT_READ) + sel.register(httpd, EVENT_READ) + + while True: + for key, _ in sel.select(): + if key.fileobj == interrupt_read: + interrupt_read.recv(1) + return + if key.fileobj == httpd: + httpd.handle_request() + + print("Serving on port 8000") + httpd = HTTPServer(('', 8000), SimpleHTTPRequestHandler) + serve_forever(httpd) + print("Shutdown...") diff --git a/Misc/NEWS.d/next/Documentation/2020-11-12-21-26-31.bpo-42340.apumUL.rst b/Misc/NEWS.d/next/Documentation/2020-11-12-21-26-31.bpo-42340.apumUL.rst new file mode 100644 index 0000000..aa68574 --- /dev/null +++ b/Misc/NEWS.d/next/Documentation/2020-11-12-21-26-31.bpo-42340.apumUL.rst @@ -0,0 +1,3 @@ +Document that in some circumstances :exc:`KeyboardInterrupt` may cause the +code to enter an inconsistent state. Provided a sample workaround to avoid +it if needed. |