Sending a notification to the main nginx thread

[Maxim Dounin]

Hello!

On Wed, Oct 06, 2021 at 07:53:00PM +0000, Eran Kornblau wrote:

> > 
> > -----Original Message-----
> > From: nginx-devel <nginx-devel-bounces at nginx.org> On Behalf Of Maxim Dounin
> > Sent: Wednesday, 6 October 2021 16:12
> > To: nginx-devel at nginx.org
> > Subject: Re: Sending a notification to the main nginx thread
> > 
> > Hello!
> > 
> > First of all, you may want to take a look at this warning in the development guide:
> > 
> > https://eur02.safelinks.protection.outlook.com/?url=http%3A%2F%2Fnginx.org%2Fen%2Fdocs%2Fdev%2Fdevelopment_guide.html%23threads_pitfalls&data=04%7C01%7C%7Cd0de648d5c294e07aac008d988cae55b%7C0c503748de3f4e2597e26819d53a42b6%7C1%7C1%7C637691227285308685%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=%2BiDgCqNY6Wtl0elz%2FYAG7UvdOmoW%2BsZg9v0i5Oicc%2FY%3D&reserved=0
> > 
> > Quoting it here:
> > 
> > It is recommended to avoid using threads in nginx because it will definitely break things: most nginx functions are not thread-safe. It is expected that a thread will be executing only system calls and thread-safe library functions.
> > If you need to run some code that is not related to client request processing, the proper way is to schedule a timer in the init_process module handler and perform required actions in timer handler. Internally nginx makes use of threads to boost IO-related operations, but this is a special case with a lot of limitations.
> > 
> Thanks Maxim, I completely get that, that is the reason I was looking for way to send a notification to the main thread, and didn't just try to call nginx functions from some other thread.
> In my case, I need to integrate with some 3rd party library that has its own event loop, it would require significant changes to the library to make it run inside nginx's event loop...
> So, my plan is to run it on a side thread, and send notifications between the threads, which would trigger some handler on the main thread whenever new data arrives.
> I can use ngx_notify for this, but if someone will use the module and also nginx's thread pool or some 3rd party module that uses ngx_notify, it will break.
> I think I can live with that, but would be nice to have a solution that is complete.

To re-iterate: inside a "side thread" you are not allowed to call 
any non-thread-safe libc functions.  It is reasonable to assume 
this excludes all 3rd party libraries with their own even loops.

-- 
Maxim Dounin
http://mdounin.ru/