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Re: [RFC/PATCH] connect: add GIT_SSH_{SEND,RECEIVE}{,_COMMAND} env variables

On Wed, Jan 03 2018, Junio C. Hamano jotted:

> Ævar Arnfjörð Bjarmason  <avarab@xxxxxxxxx> writes:
>> This is useful for talking to systems such as Github or Gitlab that
>> identify user accounts (or deploy keys) by ssh keys. Normally, ssh
>> could do this itself by supplying multiple keys via -i, but that trick
>> doesn't work on these systems as the connection will have already been
>> accepted when the "wrong" key gets rejected.
> You need to explain this a lot better than the above.
> I am sure systems such as Github have more than dozens of users who
> push over ssh and these users identify themselves by which key to
> use when establishing connection just fine (presumably by using a
> "Host" entry for the github URL in ~/.ssh/config), and presumably we
> are not sending "wrong" keys over there.  So there needs to be a lot
> more clear description of the problem you are trying to solve in the
> first place.

Hopefully this is clearer, and depending on how the rest of the
discussion goes I'll submit v2 with something like this in the commit

SSH keys A and B are known to the remote service, and used to identify
two different users.

A can only push to repository X, and B can only fetch from repository Y.

Thus, if you have a script that does:

    GIT_SSH_COMMAND="ssh -i A -i B" git ...

It'll always fail for pulling from X, and pushing to Y. Supply:

    GIT_SSH_COMMAND="ssh -i B -i A" git ...

And now pulling will work, but pushing won't.

If you were to do, where C is a completly unknown key:

    GIT_SSH_COMMAND="ssh -i C -i A" git push X ...

It would work, since ssh wouldn't get far enough in the key negotiation
to drop you into a shell. This is the case you had in mind, but is
unrelated to the problem I'm trying to address.

I tested this on a Gitlab instance, but as far as I know this property
is going to be intrinsic to anything that uses ssh in this way,
i.e. once you get past the step where the server says "this key is OK"
and drops you into a shell, it's not going to retry the whole
negotiation with another key just because the command you ran exited
with non-zero.

So now I just have a GIT_SSH_COMMAND that dispatches to different keys
depending on the operation, as noted in the commit message, and I can
assure you that without that logic it doesn't work.

I thought that use-case might be useful enough to be natively supported,
since right now you either need to hack it up like that, or perform
similar hacks with url/pushurl and ssh host aliases in your config.