[Twisted-Python] self.channel.writeHeaders(version, code, reason, headers)\nbuiltins.AttributeError: \'NoneType\' object has no attribute \'writeHeaders\'\

[Waqar Khan]

False alarm. Seems like there are some sneaky conditions when I get the
error message.
Like before, I use to get notifyFinish error everytime. Now, it seems that
6/10 times  things are "clean" but then 4/10 times there are notifyFinish
errors.
Wondering on your suggestion. How do I ensure whether the notifyFinish
error deferred has been fired or not.
Could I have like a vanilla "HelloWorld" example?
Thanks

On Mon, Aug 5, 2019 at 2:24 AM Waqar Khan <wk80333 at gmail.com> wrote:

> Hi Glyph,
>     I am not sure I understand.
>
> Is there a method/variable in request which keeps a track whether
> notifyFinish has been fired..
> So, I can do something like.
>
> if not request.hasFiredNotifyFinish:
>        request.finish()
> ??
>
>
> I have sort of able to get around this issue.. though I can't put a finger
> on what actually worked.
> Here is what I did.. First change to 19.7rc01 version.. and just fix the
> channel issue.
>
> Next  self.fetch_response(request).. This is an async def.. So what I did
> was..
>
> async def fetch_response(request):
>         future = {}
>         try:
>            future = await some other async def.. which returns future
>         except asyncio.CancelledError as e:
>            print("Error..", e)
>         return future
>
> I basically ended up doing this everywhere where there is async/await.
> Next, I added this:
> d.addErrback(lambda failure: failure.trap(asyncio.CancelledError))
>
>
> So.. now.. I don't see the notifyFinish error anymore. And I am bit
> terrified not to touch anything.. :-D
>
> But, I want to try out your suggestion as that seems like a more solid way
> to handle the issue. But I am not sure I quite understand, how do I figure
> out whether notifyFinish has been called before calling finish?
>
>
> On Mon, Aug 5, 2019 at 1:56 AM Glyph <glyph at twistedmatrix.com> wrote:
>
>>
>>
>> On Aug 4, 2019, at 9:04 PM, Waqar Khan <wk80333 at gmail.com> wrote:
>>
>> Ah yes.. That is true..
>>
>> If I comment out request.finish() (Here is the doc which I tried to
>> followed:
>> https://twistedmatrix.com/documents/13.0.0/web/howto/web-in-60/interrupted.html
>> )
>> Then actually.. when I try to test out the code... (via curl or like
>> doing requests.get .. to the URI)..
>> it is just stuck..
>>
>>
>> Yep!  This makes sense; if you don't call `.finish()` *at all* then your
>> code will never tell the client that it's done.
>>
>> But if you call `.finish()` *after notifyFinish() has fired* then you
>> get that error.
>>
>> If you change your code to only call `.finish()` if the deferred returned
>> by notifyFinish() has not fired or failed yet, do you get the error?  It's
>> possible that you still do, in which case, there's a bug in Twisted that
>> needs to be fixed.
>>
>> I do want to convey my thanks for the help. Really appreciate it.
>>
>>
>> Thanks for using Twisted :)
>> -g
>>
>> _______________________________________________
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>> Twisted-Python at twistedmatrix.com
>> https://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
>>
>
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