[Twisted-Python] Conch - Why does one piece of code "fail" while another works

[Donald Stufft]

On Oct 2, 2013, at 10:00 AM, exarkun at twistedmatrix.com wrote:

> On 01:31 pm, donald at stufft.io wrote:
>> 
>> On Oct 2, 2013, at 9:20 AM, exarkun at twistedmatrix.com wrote:
>>> On 01:21 am, donald at stufft.io wrote:
>>>> I have two pieces of code (see: https://gist.github.com/dstufft/9dc9978dc0af77e82f0c) one "works" (in that it fails verifying the host key like i'd expect) and one doesn't ("it never tries to verify the host key and just exists immediately).
>>>> 
>>>> I don't understand *why* though, why does the connectionReady dance make it work?
>>> 
>>> Where's the second piece of code?
>>> 
>>> Jean-Paul
>> 
>> 
>> Both are in the gist, works.py and fails.py
> 
> Oops, sorry.  The second one disappeared when I clicked the "raw" link I guess.
> 
> In the first version, you `react` until `connectProtocol`'s Deferred fires.  That's immediately upon the TCP connection setup success - before any appreciable SSH happens.
> 
> In the second version, you `react` indefinitely (because `connectionReady` never fires) giving SSH ample time to handshake.

ah ha, So for a real workload instead of a connectionReady you'd probably addCallback a deferred that waits for the SSH to connect and does the work.

> 
> BTW, I'm not sure what you plan to do with this connection, but if you haven't seen `twisted.conch.endpoints` (and <https://tm.tl/6617>) then you probably want to look.

I've seen the endpoints (and even have a version of this working with it). Essentially what i'm trying to do is connect to up to ~200 some SSH hosts and run a bunch of commands on them (and possibly do things like download files). I have this somewhat working with the command endpoint but the snag in that plan is I don't always have direct access to these servers so I need to "gateway" through another SSH host.

I tried to get this working with the command endpoint but I ended up very confused about how the entire thing worked (and I was never able to get the gatewaying working). After some frustration in #twisted I decided to start over and try to grok all the code (and the underlying stuff as well) before just copy/pasting stuff from the examples. Which is where I'm at now.

> 
> Jean-Paul
> 
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-----------------
Donald Stufft
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