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Re: tramp ControlMaster options

From: Thierry Volpiatto
Subject: Re: tramp ControlMaster options
Date: Tue, 21 Jul 2015 07:09:39 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Yuri D'Elia <address@hidden> writes:

> On 20/07/15 20:43, Michael Albinus wrote:
>> Likely you mean `tramp-ssh-controlmaster-options'. This is a variable,
>> and it will be computed only when it is needed. No Tramp ssh connection,
>> no probe of ssh for this.
> I hope I understand how defcustom works(?).
> In emacs 24.5.1, I see this at line 310 of tramp.el:
> (defcustom tramp-ssh-controlmaster-options
>   ...
> whose standard value is computed by calling ssh a few times.
> I don't know what the ;;;###tramp-autoload cookie does though, but no
> miracles I assume?
> Just to confirm, this is the output of "strace -f -e trace=process"
> after a (require 'tramp):
> [pid  2189] vfork(Process 2255 attached
>  <unfinished ...>
> [pid  2255] execve("/usr/bin/ssh", ["/usr/bin/ssh", "-o",
> "ControlMaster"], [/* 73 vars */] <unfinished ...>
> [pid  2189] <... vfork resumed> )       = 2255
> [pid  2255] <... execve resumed> )      = 0
> [pid  2255] arch_prctl(ARCH_SET_FS, 0x7fcdde4cd800) = 0
> ...

So IIUC tramp is calling ssh at load time when evaling its defcustom ?!!

>>> Any reason why we cannot use `ssh -o BatchMode=yes [various flags]
>>>' instead of using a domain name? This would greatly reduce the
>>> issue.
>> The function (!) `tramp-ssh-controlmaster-options' uses "localhost" for
>> test. This shall avoid DNS requests.
> Still connects to a valid host, which might have a daemon running (in
> fact, I have one on localhost for debugging).
> If there's no reason to connect to something, call

Isn't it possible to setup the default value of
`tramp-ssh-controlmaster-options' only when tramp is involved (i.e at
first connection) instead of evaling this at load time ?

If tramp hang forever like it is the case actually user will know why
emacs is hanging.

> I love that tramp detects ControlMaster support in ssh, but calling
> external processes on load, if really needed, should /really/ be quick.

IMHO this should not happen at load time.


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