[Python-ideas] re-implementing Twisted for fun and profit

Shane Green shane at umbrellacode.com
Mon Oct 15 04:11:23 CEST 2012

There are definitely bugs and system-dependent behaviour* in these areas.  Just a couple years ago I ran into one that lead to me writing a "handle_expt()" method with this comment before it: 

>     #####
>     #   Semi-crazy method that is working around a sort-of bug within 
>     #   asyncore.  When using select-based I/O multiplexing, the POLLHUP 
>     #   the socket state is indicated by the socket becoming readable, 
>     #   and not by indicating an exceptional event.
>     #   
>     #   When using POLL instead, the flag returned indicates precisely 
>     #   what the state is because "flags & select.POLLHUP" will be true.
>     #   
>     #   In the former case, when using select-based I/O multiplexing, 
>     #   select's indication that the the descriptor has become readable 
>     #   leads to the channel's handle read event method being invoked.  
>     #   Invoking receive on the socket then returns an empty string, 
>     #   which is taken by the channel as an indication that the socket 
>     #   is no longer connected and the channel correctly shuts itself 
>     #   down.
>     #   
>     #   However, asyncore's current implementation of the poll-based 
>     #   I/O multiplex event handling invokes the channel's 
>     #   handle exceptional data event anytime "flags & POLLHUP" is true.  
>     #   While select-based multiplexing would only call this method when 
>     #   OOB or urgent data was detected, it can now be called for POLLHUP 
>     #   events too.
>     #   
>     #   Under most scenarios this is not problematic because poll-based 
>     #   multiplexing also indicates the descriptor is readable and 
>     #   so the handle read event is also called and therefore the 
>     #   channel is properly close, with only an extraneous invocation 
>     #   to handle exceptional event being a side-effect.  Under certain 
>     #   situations, however, the socket is not indicated as being 
>     #   readable, only that it has had an exceptional data event.  I 
>     #   believe this occurs when the attemtp to connect never succeeds, 
>     #   but a POLLHUP does.  Previously this lead to a busy loop, which 
>     #   is what this method fixes.
>     ###

Shane Green 
805-452-9666 | shane at umbrellacode.com

On Oct 14, 2012, at 4:55 PM, Richard Oudkerk <shibturn at gmail.com> wrote:

> On 14/10/2012 11:45pm, Greg Ewing wrote:
>> It does indeed contradict me. It looks like this is
>> implementation-dependent, because I distinctly remember
>> encountering a bug once that I traced back to the fact
>> that I wasn't servicing *all* the fds reported as ready
>> before making another select call.
> Could it have been that some fds were being starved because the earlier ones in the lists were getting priority?  Servicing all fds reported prevents such starvation problems.
> --
> Richard
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