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[Python-Dev] Obtaining stack-frames from co-routine objects

Yury Selivanov yselivanov.ml at gmail.com
Tue Jun 2 06:39:33 CEST 2015 
Hi Ben,

On 2015-05-31 8:35 AM, Ben Leslie wrote:
> Hi Yury,
>
> I'm just starting my exploration into using async/await; all my
> 'real-world' scenarios are currently hypothetical.
>
> One such hypothetical scenario however is that if I have a server
> process running, with some set of concurrent connections, each managed
> by a co-routine. Each co-routine is of some arbitrary complexity e.g:
> some combination of reading files, reading from database, reading from
> peripherals. If I notice one of those co-routines appears stuck and
> not making progress, I'd very much like to debug that, and preferably
> in a way that doesn't necessarily stop the rest of the server (or even
> the co-routine that appears stuck).
>
> The problem with the "if debug: log(...)" approach is that you need
> foreknowledge of the fault state occurring; on a busy server you don't
> want to just be logging every 'switch()'. I guess you could do
> something like "switch_state[outer_coro] = get_current_stack_frames()"
> on each switch. To me double book-keeping something that the
> interpreter already knows seems somewhat wasteful but maybe it isn't
> really too bad.

I guess it all depends on how "switching" is organized in your
framework of choice.  In asyncio, for instance, all the code that
knows about coroutines is in tasks.py.  `Task` class is responsible
for running coroutines, and it's the single place where you would
need to put the "if debug: ..." line for debugging "slow" Futures--
the only thing that coroutines can "stuck" with (the other thing
is accidentally calling blocking code, but your proposal wouldn't
help with that).

Yury
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