bug#36364: Potential SSH session exhaustion

[Jakob L. Kreuze]

Hello, Guix

In developing a test suite for the internals of 'guix deploy', the
virtual machine I spun up spat out an OpenSSH error that I've never
encountered before.

  Jun 24 16:59:26 localhost sshd[235]: error: no more sessions

This is, initially, quite curious. Creating the SSH session with
#:log-verbosity set to 'protocol gives some insight.

  ;;; [2019/06/24 13:08:36.711847, 2] channel_open: Creating a channel 43 with 
64000 window and 32768 max packet
  ;;; [2019/06/24 13:08:36.711959, 2] ssh_packet_global_request: Received 
SSH_MSG_GLOBAL_REQUEST packet
  ;;; [2019/06/24 13:08:36.711987, 2] ssh_packet_global_request: UNKNOWN 
SSH_MSG_GLOBAL_REQUEST address@hidden 0
  ;;; [2019/06/24 13:08:36.712010, 1] ssh_packet_process: Couldn't do anything 
with packet type 80
  ;;; [2019/06/24 13:08:36.712197, 2] ssh_packet_channel_open_conf: Received a 
CHANNEL_OPEN_CONFIRMATION for channel 43:0
  ;;; [2019/06/24 13:08:36.712219, 2] ssh_packet_channel_open_conf: Remote 
window : 0, maxpacket : 32768
  ;;; [2019/06/24 13:08:36.712618, 2] channel_rcv_change_window: Adding 2097152 
bytes to channel (43:0) (from 0 bytes)
  ;;; [2019/06/24 13:08:36.712651, 2] channel_request: Channel request exec 
success
  ...
  ;;; [2019/06/24 12:59:26.296022, 2] channel_open: Creating a channel 74 with 
64000 window and 32768 max packet
  ;;; [2019/06/24 12:59:26.296262, 2] ssh_packet_channel_open_conf: Received a 
CHANNEL_OPEN_CONFIRMATION for channel 74:8
  ;;; [2019/06/24 12:59:26.296308, 2] ssh_packet_channel_open_conf: Remote 
window : 0, maxpacket : 32768
  ;;; [2019/06/24 12:59:26.296667, 2] channel_rcv_change_window: Adding 2097152 
bytes to channel (74:8) (from 0 bytes)
  ;;; [2019/06/24 12:59:26.296721, 2] channel_request: Channel request exec 
success
  ;;; [2019/06/24 12:59:26.487339, 2] grow_window: growing window (channel 
74:8) to 1280000 bytes
  ;;; [2019/06/24 12:59:26.489031, 2] channel_open: Creating a channel 75 with 
64000 window and 32768 max packet
  ;;; [2019/06/24 12:59:26.489376, 2] ssh_packet_channel_open_conf: Received a 
CHANNEL_OPEN_CONFIRMATION for channel 75:8
  ;;; [2019/06/24 12:59:26.489402, 2] ssh_packet_channel_open_conf: Remote 
window : 0, maxpacket : 32768
  ;;; [2019/06/24 12:59:26.489858, 2] channel_rcv_change_window: Adding 2097152 
bytes to channel (75:8) (from 0 bytes)
  ;;; [2019/06/24 12:59:26.489906, 2] channel_request: Channel request exec 
success
  ;;; [2019/06/24 12:59:26.542586, 2] grow_window: growing window (channel 
75:8) to 1280000 bytes
  ;;; [2019/06/24 12:59:26.546104, 2] channel_open: Creating a channel 76 with 
64000 window and 32768 max packet
  ;;; [2019/06/24 12:59:26.546407, 2] ssh_packet_channel_open_conf: Received a 
CHANNEL_OPEN_CONFIRMATION for channel 76:9
  ;;; [2019/06/24 12:59:26.546438, 2] ssh_packet_channel_open_conf: Remote 
window : 0, maxpacket : 32768
  ;;; [2019/06/24 12:59:26.546839, 2] channel_rcv_change_window: Adding 2097152 
bytes to channel (76:9) (from 0 bytes)
  ;;; [2019/06/24 12:59:26.546865, 2] channel_request: Channel request exec 
success
  ;;; [2019/06/24 12:59:26.630055, 2] grow_window: growing window (channel 
76:9) to 1280000 bytes
  ;;; [2019/06/24 12:59:27.272139, 2] channel_open: Creating a channel 77 with 
64000 window and 32768 max packet
  ;;; [2019/06/24 12:59:27.272846, 1] ssh_packet_channel_open_fail: Channel 
opening failure: channel 77 error (2) open failed

I will admit that my knowledge of the SSH protocol is limited, but the
rising channel number gives me the impression that channels are going
out of scope and aren't being cleaned up. For reference, this occurs
when 'remote-eval' is invoked multiple times -- which, in turn, invokes
'send-files'.

  building path(s) `/gnu/store/3rfkwdbayg7m3wdrydmcljnfqhvrady5-remote-exp.scm'
  sending 1 store item (0 MiB) to 'localhost'...
  exporting path `/gnu/store/3rfkwdbayg7m3wdrydmcljnfqhvrady5-remote-exp.scm'
  building path(s) `/gnu/store/ib59b0pnypsw4vf59f2gwpwhn343xnzb-remote-exp.scm'
  sending 1 store item (0 MiB) to 'localhost'...
  exporting path `/gnu/store/ib59b0pnypsw4vf59f2gwpwhn343xnzb-remote-exp.scm'
  [this is where the "no more sessions" error occurs]

Currently, my code performs all of these 'remote-eval' calls with the
same session. I suppose it would be possible to create a new session for
each call, but I feel I should raise this as it may cause issues down
the line: do we need to do something about cleaning up SSH channels?

If a reproducible example is necessary, you can look to the temporary
'tests/machine.scm' in 64ffe453b0c71f417927737fa808dffe858359cd on my
personal branch.[1]

Regards,
Jakob

[1]: https://git.sr.ht/~jakob/guix/

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