[Twisted-Python] spawnProcess with timeout and same error handling on Unix and Windows

[exarkun at twistedmatrix.com]

On 05:17 pm, adi at roiban.ro wrote:
>On 3 September 2014 16:49, Adi Roiban <adi at roiban.ro> wrote:
>>Hi,
>>
>>If I call spawnProcess with a bad executable, on Unix the process will
>>exit with exit code 1 while on Windows an OSError is raised.
>>
>>I am working on a multi-OS software and to make my life easier I ended
>>up with this hack inspired by _BackRelay
>>
>>https://gist.github.com/adiroiban/bac493f00ce5e94738ce
>>
>>----
>>
>>Is there something already in Twisted doing this? Am I reinventing the 
>>wheel?
>>
>>If not, do you think that it would help to update _BackRelay to
>>support timeout and cross-os error handling?
>
>Searching the Twisted trac I found this ticket
>http://twistedmatrix.com/trac/ticket/4184
>
>Maybe we can leave spawnProcess as the low-level API and then create
>another wrapper which will handle/raise the errors in an consistent
>way.
>
>For the case where command is not found, I prefer the behaviour from
>Windows where an OSError is raise, rather than
>looking at errReceived and trying to guess is execvpe call failed or 
>not.

It is more convenient, certainly.  Unfortunately on POSIX the underlying 
API isn't synchronous - which is why the POSIX implementation doesn't 
deliver the exception synchronously.  So to be uniform we probably need 
to make the Windows behavior asynchronous.

I suggest we can improve the situation by reporting the error more in an 
easier-to-recognize way when we report it asynchronously, though.

For example, instead of forcing the application to parse stderr we could 
deliver a different exception type to processEnded.

A better solution would be to make `spawnProcess` properly asynchronous 
- have it return a `Deferred` that fires with an `IProcessTransport` and 
let that `Deferred` fire with a `Failure` if there is a problem creating 
the new process.

But that's probably more like "introduce an API to replace 
`spawnProcess`" (for which I believe there is also a ticket).

Jean-Paul