Thank you for your reply. The use case is simple just to offload SQL query to separate thread to not block main thread. The problem is handling situation when thread is blocked due I/O to Database. I am not aware about solution like timeout of query etc. but maybe I have missed something.

Do you have a link to that? Googling king .net threads got me some strange results 😁. What made it particularly sane? On Thu, 20 Jun 2019, 23:59 Michael Foord, wrote:

On Thu, 20 Jun 2019 at 06:15, Matúš Valo wrote:

...

Hi All,

Currently it is not possible to "kill" thread which is blocked. The rationale for this is handling situations when thread is blocked - e.g. when thread is quering DB when lock occurred on Database. In this case, the main thread has no way how to stop the blocked thread. Killing a thread is also popular question - see [1][2].

pthread library and Windows API contains mechanisms for forced termination of threads - see [3] and [4]. It is also simple to use them using ctypes library but after this action one need to "clean" internal data structures which is bad practice:

import time import threading import ctypes

def handler(): # blocked thread handler time.sleep(1000)

t = threading.Thread(name='bar', target=handler) libpt = ctypes.cdll.LoadLibrary("libpthread.so.0")

t.start() libpt.pthread_cancel(ctypes.c_ulong(t.ident)) # This is nasty cleaning of internal python structures del threading._active[t.ident]

Is if feasible to add canceling threads to python threading library? I am willing to help creating a patch (at least for linux).

Back in the day we used the .NET API for king threads from IronPython. It made threads so much saner to work with. It's been an egregious lack in the Python thread API for years and every time it's brought up people rush to say you shouldn't do it as what you really want to do is something else.

Michael

...

[1] https://stackoverflow.com/questions/323972/is-there-any-way-to-kill-a-thread [2] https://www.geeksforgeeks.org/python-different-ways-to-kill-a-thread/ [3] http://man7.org/linux/man-pages/man3/pthread_cancel.3.html [4] https://docs.microsoft.com/en-us/windows/desktop/api/processthreadsapi/nf-pr... _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/ZDERRW... Code of Conduct: http://python.org/psf/codeofconduct/

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On Thu, 20 Jun 2019 at 16:11, Matúš Valo wrote:

I have found out at least this link:

http://www.voidspace.org.uk/ironpython/threading.shtml

There is also example of aborting thread.

Thanks, good reference 😀 It also mentions the same API in Java. Michael

Matus _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/SL7OCS... Code of Conduct: http://python.org/psf/codeofconduct/

-- Michael Foord Python Consultant, Contractor and Trainer https://agileabstractions.com/

On Thu, 20 Jun 2019 at 13:03, Robert Collins wrote:

Do you have a link to that? Googling king .net threads got me some strange results 😁.

Auto-correct. Killing. Thread.abort I believe. I don't think it's very hard to find.

What made it particularly sane?

A straightforward API that works without fuss or excuses. It works by raising an exception in the target thread, which the thread is free to handle (usually for cleanup and then reraise). Michael

On Thu, 20 Jun 2019, 23:59 Michael Foord, wrote:

...

On Thu, 20 Jun 2019 at 06:15, Matúš Valo wrote:

...

Hi All,

Currently it is not possible to "kill" thread which is blocked. The rationale for this is handling situations when thread is blocked - e.g. when thread is quering DB when lock occurred on Database. In this case, the main thread has no way how to stop the blocked thread. Killing a thread is also popular question - see [1][2].

pthread library and Windows API contains mechanisms for forced termination of threads - see [3] and [4]. It is also simple to use them using ctypes library but after this action one need to "clean" internal data structures which is bad practice:

import time import threading import ctypes

def handler(): # blocked thread handler time.sleep(1000)

t = threading.Thread(name='bar', target=handler) libpt = ctypes.cdll.LoadLibrary("libpthread.so.0")

t.start() libpt.pthread_cancel(ctypes.c_ulong(t.ident)) # This is nasty cleaning of internal python structures del threading._active[t.ident]

Is if feasible to add canceling threads to python threading library? I am willing to help creating a patch (at least for linux).

Back in the day we used the .NET API for king threads from IronPython. It made threads so much saner to work with. It's been an egregious lack in the Python thread API for years and every time it's brought up people rush to say you shouldn't do it as what you really want to do is something else.

Michael

...

[1] https://stackoverflow.com/questions/323972/is-there-any-way-to-kill-a-thread [2] https://www.geeksforgeeks.org/python-different-ways-to-kill-a-thread/ [3] http://man7.org/linux/man-pages/man3/pthread_cancel.3.html [4] https://docs.microsoft.com/en-us/windows/desktop/api/processthreadsapi/nf-pr... _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/ZDERRW... Code of Conduct: http://python.org/psf/codeofconduct/

--

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Python Consultant, Contractor and Trainer

https://agileabstractions.com/

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Message archived at

...

https://mail.python.org/archives/list/python-ideas@python.org/message/RFEQKO...

...

Code of Conduct: http://python.org/psf/codeofconduct/

--

Michael Foord Python Consultant, Contractor and Trainer https://agileabstractions.com/

On Thu, 20 Jun 2019 at 16:33, Guido van Rossum wrote:

On Thu, Jun 20, 2019 at 8:21 AM Michael Foord wrote:

...

It works by raising an exception in the target thread, which the thread is free to handle (usually for cleanup and then reraise).

Sure, those are the right semantics. How does it stop blocking I/O though? Suppose the thread is waiting for a server to return a response which just isn't ever going to come, but the connection somehow is kept open by the other side?

Sorry, resending to list as well. It used to be on the CLR getting back control. So it couldn't handle that case. (.NET 1.1). https://jonskeet.uk/csharp/threads/abort.html It has since been improved. It still blocks on the execution of unmanaged code (or computation in a finally block handling the ThreadAbortException), but blocking IO can be interrupted: https://docs.microsoft.com/en-us/dotnet/api/system.threading.thread.abort?vi... If Abort is called on a thread that is blocked or is sleeping, the thread is interrupted and then aborted. This SO question on the topic says: https://stackoverflow.com/questions/365370/proper-way-to-stop-tcplistener Thread.Abort() There are 2 things I see you could do. 1 is that if you have started this TcpListener thread from another you can simply call Thread.Abort instance method on the thread which will cause a threadabortexception to be thrown within the blocking call and walk up the stack. Michael

-- --Guido van Rossum (python.org/~guido) Pronouns: he/him/his (why is my pronoun here?)

-- Michael Foord Python Consultant, Contractor and Trainer https://agileabstractions.com/

On Thu, 27 Jun 2019 at 20:53, Yonatan Zunger wrote:

Generally, threads don't have a notion of non-cooperative thread termination. This is because (unlike processes) threads share address spaces, and an unexpected termination of a thread can leave memory in arbitrary and unexpected states. (For example, what if one thread was holding a mutex when it got killed?)

That's precisely why thread cancellation in managed languages (like Python is) raise an exception to terminate the the thread and honour finally blocks.

The POSIX threading model *does* include the ability to send a signal to a particular thread using pthread_kill(). What that does is cause the process' globally-registered signal handler for that signal to be invoked in that particular thread. With the default handlers, though, these tend to have process-wide effects; e.g., if you pthread_kill(tid, 15) with the default handler, it will simply kill the process. This doesn't make as much sense in the context of Python's signal handling (which is a layer on top of POSIX signal handling) so signals.pthread_kill() isn't a very useful function.

This is not what is being suggested. Read about the semantics of thread killing in C# and Java.

In practice (and I actually had to deal with exactly this problem in my own code just last week!) there are two approaches that tend to work:

Yet again someone who doesn't understand what is being proposed saying it isn't needed. Michael

(1) Cooperative scheduling: Your code has an explicit place within the threads where they check for abort signals and handle them appropriately. For example, if your threads are each running event listeners, you might post a "cancel" event on the main bus; or alternately, you could use a threading.Event to signal everyone to shut down.

(2) Separate address spaces: If (1) isn't possible for some reason, and you need to literally be able to kill off a process noncooperatively -- say, the purpose of your code is to be an intermediate "harness" library which runs someone else's long-running functions in a thread, and it's not possible to require all of that code to obey a cooperative protocol -- then use processes instead of threads.

In many cases, you can achieve this with built-in Python libraries like multiprocessing and ProcessPoolExecutor, and in this case, you can terminate processes simply by calling Popen.send_signal() or the like. In a few cases (e.g., if you're using gRPC, whose client library is incompatible with those for complicated reasons) you'll have to fork() and exec() yourself, typically by using the subprocess.Popen library directly.

Yonatan

On Thu, Jun 20, 2019 at 10:16 AM Michael Foord wrote:

...

On Thu, 20 Jun 2019 at 16:33, Guido van Rossum wrote:

...

...

It works by raising an exception in the target thread, which the

On Thu, Jun 20, 2019 at 8:21 AM Michael Foord wrote: thread is free to handle (usually for cleanup and then reraise).

Sure, those are the right semantics. How does it stop blocking I/O though? Suppose the thread is waiting for a server to return a response which just isn't ever going to come, but the connection somehow is kept open by the other side?

Sorry, resending to list as well.

It used to be on the CLR getting back control. So it couldn't handle that case. (.NET 1.1).

https://jonskeet.uk/csharp/threads/abort.html

It has since been improved. It still blocks on the execution of unmanaged code (or computation in a finally block handling the ThreadAbortException), but blocking IO can be interrupted:

https://docs.microsoft.com/en-us/dotnet/api/system.threading.thread.abort?vi...

If Abort is called on a thread that is blocked or is sleeping, the thread is interrupted and then aborted.

This SO question on the topic says:

https://stackoverflow.com/questions/365370/proper-way-to-stop-tcplistener

Thread.Abort()

There are 2 things I see you could do. 1 is that if you have started this TcpListener thread from another you can simply call Thread.Abort instance method on the thread which will cause a threadabortexception to be thrown within the blocking call and walk up the stack.

Michael

...

-- --Guido van Rossum (python.org/~guido) Pronouns: he/him/his (why is my pronoun here?)

--

Michael Foord

Python Consultant, Contractor and Trainer

https://agileabstractions.com/

_______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/ARIZ3W...

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Code of Conduct: http://python.org/psf/codeofconduct/

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Michael Foord Python Consultant, Contractor and Trainer https://agileabstractions.com/

Although thinking about it more, and in the context of this thread, the idea of an operation to "raise exception X in thread Y" doesn't seem unreasonable. After all, this is basically what happens to the main thread from the signal handler already; the C signal handler caches a bit that gets picked up by PyErr_CheckSignals(), which raises things like KeyboardInterrupt at completely arbitrary points w.r.t. user code. One possible approach (at a very schematic level, not thinking about impl details yet) would be to have a function like Thread.raise(e: BaseException) which stuffs the exception into thread-local storage, which then gets checked at similar times to signals. (Probably not the exact same times since I'm sure there's plenty of code depending implicitly on that function being a noop outside the main thread) Basically, it would be having the Python interpreter doing the cooperative part of cross-thread interruption, so that it would look non-cooperative from the user side. You could even have a Thread.raiseWhen to do the equivalent of setting an alarm. This would definitely be a "use it only if you really mean it" function, but it could be quite useful. Implementing things like watchdog timers is hard right now because of a lack of this sort of feature, but this could open up some nice options. On Thu, Jun 27, 2019 at 2:17 PM Guido van Rossum wrote:

On Thu, Jun 27, 2019 at 1:36 PM Michael Foord wrote:

...

On Thu, 27 Jun 2019 at 20:53, Yonatan Zunger wrote:

...

Generally, threads don't have a notion of non-cooperative thread termination. This is because (unlike processes) threads share address spaces, and an unexpected termination of a thread can leave memory in arbitrary and unexpected states. (For example, what if one thread was holding a mutex when it got killed?)

That's precisely why thread cancellation in managed languages (like Python is) raise an exception to terminate the the thread and honour finally blocks.

There's one issue that Yonatan's post reminded me of though. Currently a thread that's locked in a mutex.acquire() operation cannot be interrupted by a signal at the Python level (unlike I/O, which can be interrupted -- IIRC Victor spent a lot of time making this work). There's a workaround (specify a timeout) but this is still something that would have to be solved for this to be useful.

-- --Guido van Rossum (python.org/~guido) *Pronouns: he/him/his **(why is my pronoun here?)* http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-c...

Sorry for the rude tone of part of that email. Sent from my iPhone

On 27 Jun 2019, at 23:36, Michael Foord wrote:

...

On Thu, 27 Jun 2019 at 20:53, Yonatan Zunger wrote: Generally, threads don't have a notion of non-cooperative thread termination. This is because (unlike processes) threads share address spaces, and an unexpected termination of a thread can leave memory in arbitrary and unexpected states. (For example, what if one thread was holding a mutex when it got killed?)

That's precisely why thread cancellation in managed languages (like Python is) raise an exception to terminate the the thread and honour finally blocks.

...

The POSIX threading model does include the ability to send a signal to a particular thread using pthread_kill(). What that does is cause the process' globally-registered signal handler for that signal to be invoked in that particular thread. With the default handlers, though, these tend to have process-wide effects; e.g., if you pthread_kill(tid, 15) with the default handler, it will simply kill the process. This doesn't make as much sense in the context of Python's signal handling (which is a layer on top of POSIX signal handling) so signals.pthread_kill() isn't a very useful function.

This is not what is being suggested. Read about the semantics of thread killing in C# and Java.

...

In practice (and I actually had to deal with exactly this problem in my own code just last week!) there are two approaches that tend to work:

Yet again someone who doesn't understand what is being proposed saying it isn't needed.

Michael

...

(1) Cooperative scheduling: Your code has an explicit place within the threads where they check for abort signals and handle them appropriately. For example, if your threads are each running event listeners, you might post a "cancel" event on the main bus; or alternately, you could use a threading.Event to signal everyone to shut down.

(2) Separate address spaces: If (1) isn't possible for some reason, and you need to literally be able to kill off a process noncooperatively -- say, the purpose of your code is to be an intermediate "harness" library which runs someone else's long-running functions in a thread, and it's not possible to require all of that code to obey a cooperative protocol -- then use processes instead of threads.

In many cases, you can achieve this with built-in Python libraries like multiprocessing and ProcessPoolExecutor, and in this case, you can terminate processes simply by calling Popen.send_signal() or the like. In a few cases (e.g., if you're using gRPC, whose client library is incompatible with those for complicated reasons) you'll have to fork() and exec() yourself, typically by using the subprocess.Popen library directly.

Yonatan

...

On Thu, Jun 20, 2019 at 10:16 AM Michael Foord wrote:

...

...

On Thu, 20 Jun 2019 at 16:33, Guido van Rossum wrote: On Thu, Jun 20, 2019 at 8:21 AM Michael Foord wrote:

...

It works by raising an exception in the target thread, which the thread is free to handle (usually for cleanup and then reraise).

Sure, those are the right semantics. How does it stop blocking I/O though? Suppose the thread is waiting for a server to return a response which just isn't ever going to come, but the connection somehow is kept open by the other side?

Sorry, resending to list as well.

It used to be on the CLR getting back control. So it couldn't handle that case. (.NET 1.1).

https://jonskeet.uk/csharp/threads/abort.html

It has since been improved. It still blocks on the execution of unmanaged code (or computation in a finally block handling the ThreadAbortException), but blocking IO can be interrupted:

https://docs.microsoft.com/en-us/dotnet/api/system.threading.thread.abort?vi...

If Abort is called on a thread that is blocked or is sleeping, the thread is interrupted and then aborted.

This SO question on the topic says:

https://stackoverflow.com/questions/365370/proper-way-to-stop-tcplistener

Thread.Abort()

There are 2 things I see you could do. 1 is that if you have started this TcpListener thread from another you can simply call Thread.Abort instance method on the thread which will cause a threadabortexception to be thrown within the blocking call and walk up the stack.

Michael

...

-- --Guido van Rossum (python.org/~guido) Pronouns: he/him/his (why is my pronoun here?)

-- Michael Foord Python Consultant, Contractor and Trainer https://agileabstractions.com/

...

_______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/ARIZ3W...

...

Code of Conduct: http://python.org/psf/codeofconduct/

-- Michael Foord Python Consultant, Contractor and Trainer https://agileabstractions.com/