[Twisted-Python] Strategy for finding source of unhandled error in Deferred

[Jean-Paul Calderone]

On Mon, Mar 26, 2018 at 10:47 AM, Barry Scott <barry.scott at forcepoint.com>
wrote:

> On Sunday, 18 March 2018 15:23:58 BST Jean-Paul Calderone wrote:
> >
> > Try this:
> >
> >     from twisted.internet.defer import setDebugging
> >     setDebugging(True)
> >
> > This gives you creation and callback stacks in the logs.
>
> If you need to track down who put the value into the deferred
> you need to patch the defer.py code to log that info.
>
> There are 2 places that a value enters the Deferred.
> The first is the callback() call. The other is in
>
> defer.py:653 Deffered._runCallbacks()
>
> 653:                        current.result = callback(current.result,
> *args,
> **kw)
>

No, you don't:

exarkun at baryon:/tmp$ cat deferreddebug.py
from twisted.internet.defer import fail, setDebugging
setDebugging(True)
def foo():
    return fail(Exception("boo"))
foo()
exarkun at baryon:/tmp$ python deferreddebug.py
Unhandled error in Deferred:
(debug:  C: Deferred was created:
 C:  File "deferreddebug.py", line 5, in <module>
 C:    foo()
 C:  File "deferreddebug.py", line 4, in foo
 C:    return fail(Exception("boo"))
 C:  File
"/home/exarkun/.local/lib/python2.7/site-packages/twisted/internet/defer.py",
line 106, in fail
 C:    d = Deferred()
 I: First Invoker was:
 I:  File "deferreddebug.py", line 5, in <module>
 I:    foo()
 I:  File "deferreddebug.py", line 4, in foo
 I:    return fail(Exception("boo"))
 I:  File
"/home/exarkun/.local/lib/python2.7/site-packages/twisted/internet/defer.py",
line 107, in fail
 I:    d.errback(result)
)
exarkun at baryon:/tmp$


Although it is true that you only get this behavior for the "Unhandled
error in Deferred" case.  If you explicitly log a Failure from a Deferred,
even with Deferred debugging on, you will not get a callstack for the
creator or invoker.  You'll just get whatever callstack the Failure has
(and the Failure is independent of the Deferred).

Jean-Paul
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