[Haskell-cafe] Help with async library and killing child processes

YueCompl compl.yue at icloud.com
Wed Mar 18 10:06:32 UTC 2020


No, not to cancel a thread, but the business task on that thread's way ahead.  That's what Go's Context is designed for.

About Go context from: https://blog.golang.org/context <https://blog.golang.org/context>
At Google, we require that Go programmers pass a Context parameter as the first argument to every function on the call path between incoming and outgoing requests. This allows Go code developed by many different teams to interoperate well. It provides simple control over timeouts and cancelation and ensures that critical values like security credentials transit Go programs properly.

Server frameworks that want to build on Context should provide implementations of Context to bridge between their packages and those that expect a Context parameter. Their client libraries would then accept a Context from the calling code. By establishing a common interface for request-scoped data and cancelation, Context makes it easier for package developers to share code for creating scalable services.
I (and seems Windows api too) strongly agree that cancelling a thread is problematic:

https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-terminatethread <https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-terminatethread>
TerminateThread is a dangerous function that should only be used in the most extreme cases. You should call TerminateThread only if you know exactly what the target thread is doing, and you control all of the code that the target thread could possibly be running at the time of the termination. For example, TerminateThread can result in the following problems:
If the target thread owns a critical section, the critical section will not be released.
If the target thread is allocating memory from the heap, the heap lock will not be released.
If the target thread is executing certain kernel32 calls when it is terminated, the kernel32 state for the thread's process could be inconsistent.
If the target thread is manipulating the global state of a shared DLL, the state of the DLL could be destroyed, affecting other users of the DLL.

> On 2020-03-18, at 17:16, Branimir Maksimovic <branimir.maksimovic at gmail.com> wrote:
> 
> I'm pretty sure you can't cancel goroutine. Actually I have 
> 
> very bad experience with thread cancelation. Therefore flags
> 
> passed via channels/mvars or shared flags are used to stop
> 
> threads. 
> 
> Greets,
> Branimir.
> 
> On 3/18/20 10:10 AM, YueCompl wrote:
>> Idiomatic resource leakage prevention in short.
>> 
>> An app wants to be as responsive to its users as practically feasible, it would try multiple paths to load-balanced resources, and use the first response that come back, then cancel other paths. Cancellation can help reducing vain computation in such architectures, sometimes largely enough.
>> 
>>> On 2020-03-18, at 16:58, Branimir Maksimovic <branimir.maksimovic at gmail.com <mailto:branimir.maksimovic at gmail.com>> wrote:
>>> 
>>> What's wrong with forkIO?
>>> 
>>> Greets,
>>> Branimir.
>>> 
>>> On 3/18/20 9:37 AM, YueCompl via Haskell-Cafe wrote:
>>>> If not for tight loops or other CPU intensive tasks, you may be interested in Edh, which introduces Go's goroutine to GHC runtime. But Edh threads add much higher overhead on top of GHC threads, so there's a price to pay for simplicity of end programmer's job.
>>>> 
>>>> It's briefly described at https://github.com/e-wrks/edh/tree/master/Tour#program--threading-model <https://github.com/e-wrks/edh/tree/master/Tour#program--threading-model> . This is very new and I'm right now actively working on it for PoC of an STM powered in-memory graph database implementation.
>>>> 
>>>> Best regards,
>>>> Compl
>>>> 
>>>>> On 2020-03-18, at 11:15, Niklas Hamb├╝chen <mail at nh2.me <mailto:mail at nh2.me>> wrote:
>>>>> 
>>>>> I've you're new to async, also check out my recent rework of the docs that talk about this topic specifically:
>>>>> 
>>>>> https://github.com/simonmar/async/pull/105/files <https://github.com/simonmar/async/pull/105/files>
>>>>> 
>>>>> (To be available in the next release of async.)
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