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

Guido van Rossum guido at python.org
Fri Jun 12 20:13:23 CEST 2015

On Thu, Jun 11, 2015 at 11:38 PM, Ben Leslie <benno at benno.id.au> wrote:

> On 2 June 2015 at 14:39, Yury Selivanov <yselivanov.ml at gmail.com> wrote:
> > 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).
> I suspect that I haven't properly explained the motivating case.
> My motivating case is being able to debug a relatively large, complex
> system. If the system crashes (through an exception), or in some other
> manner enters an unexpected state (co-routines blocked for too long)
> it would be very nice to be able to debug an arbitrary co-routine, not
> necessarily the one indicating a bad system state, to see exactly what
> it is/was doing at the time the anomalous  behavior occurs.
> So, this is a case of trying to analyse some system wide behavior
> than necessarily one particular task. So until you start
> analysing the rest of the system you don't know which co-routines
> you want to analyse.
> My motivation for this is primarily avoiding double book-keeping. I
> assume that the framework has organised things so that there is
> some data structure to find all the co-routines (or some other object
> wrapping the co-routines) and that all the "switching" occurs in
> one place.
> With that in mind I can have some code that works something like:
> @coroutine
> def switch():
>     coro_stacks[current_coro] =
> inspect.getouterframes(inspect.currentframe())
>     ....
> I think something like this is probably the best approach to achieve
> my desired goals with currently available APIs.
> However I really don't like it as it required this double book-keeping. I'm
> manually retaining this trace back for each coro, which seems like a
> waste of memory, considering the interpreter already has this information
> stored, just unexposed.
> I feel it is desirable, and in-line with existing Python patterns to expose
> the interpreter data structures, rather than making the user do extra work
> to access the same information.

Ben, I suspect that this final paragraph is actually the crux to your
request. You need to understand what the interpreter is doing before you
can propose an API to its data structures. The particular thing to
understand about coroutines is that a coroutine which is suspended at
"yield" or "yield from" has a frame but no stack -- the frame holds the
locals and the suspension point, but it is not connected to any other
frames. Its f_back pointer is literally NULL. (Perhaps you are more used to
threads, where a suspended thread still has a stack.) Moreover, the
interpreter has no bookkeeping that keeps track of suspended frames. So I'm
not sure exactly what information you think the interpreter has stored but
does not expose. Even asyncio/tasks.py does not have this bookkeeping -- it
keeps track of Tasks, which are an asyncio-specific class that wraps
certain coroutines, but not every coroutine is wrapped by a Task (and this
is intentional, as a coroutine is a much more lightweight data structure
than a Task instance).

IOW I don't think that the problem here is that you haven't sufficiently
motivated your use case -- you are asking for information that just isn't
available. (Which is actually where you started the thread -- you can get
to the frame of the coroutine but there's nowhere to go from that frame.)

--Guido van Rossum (python.org/~guido)
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