ssh: Do not leak ssh sessions on errors

[xh3b4sd]

Hello there. We recently ran into a problem where we were not able to do proper connection handling across ssh tunnels. It turned out that this was partially caused by some buggy code on our side in combination with the things being fixed here in this PR. In case ssh sessions are created, we need to make sure to close them properly even in error cases. The fix proposed here works for us. What do you guys think? Thanks to @zeisss helping out debugging and fixing this issue.

[zeisss]

Some more background infos:

We are using the client/ package over ssh, similar to fleetctl. When our code tries to perform multiple operations in parallel, we ran into forward request denied errors for call 2-10 to ssh.DialCommand(). Afterwards we got administratively prohibited errors for call 11 onwards. This correlates with the default value MaxSessions in sshd. This PR closes the created session if any of the following steps fail.

[antrik]

This looks reasonable at a quick glance. Any chance you could create a functional test case for this?...

[kayrus]

Excellent. This PR actually fixes #1425 issue in proper way. That is why fleet tried to send several agent requests... @antrik functional tests are already written (#1492). What we need to do it to revert my fix workaround (ea415ad) and apply this patch.

P.S. I had feeling that I fixed that wrong, but didn't have enough time to investigate.

[zeisss]

Just rereading my comment I think I missed an info: this PR fixed the administratively prohibited errors for us, and we always now got forward request denied.

We fixed that then by adding a mutex to the RoundTrip() which unlocks when the body gets closed (see giantswarm/inago@7b0d822) - not sure sthg similar needs to be done in fleetctl.

[kayrus]

@zeisss false alarm. my tests succeed because my VM had not enough RAM (256). And functional tests returned positive answer for some reason. My fix should not be reverted. Your and my fixes supplement each other.

[tixxdz]

lgtm.

Thank you!

[xh3b4sd]

Wh00p wh00p.

[jonboulle]

👍 nice