Created attachment 3036 [details]
proposed patch

Reproducer:
1. Start a server:
  /usr/sbin/sshd -D -ddd -p 8022 -o "ClientAliveCountMax=0" -o "ClientAliveInterval=900" -o "RekeyLimit=default 45s" -e

2. Connect with client
  ssh -p 8022 root@localhost "date; while :; do sleep 10; date; done"


Actual result:
No rekey happens, connection is dropped with error

  Timeout, client not responding.


Expected result:
  Rekey happens in 45 seconds, the connection is not dropped


Diagnostics:
The function wait_until_can_do_something() is selecting the timeout for select() function, but sets client_alive_scheduled even if it is scheduled for later than rekey should happen.

Proposed solution:
The client_alive_scheduled variable should be set only if the time is lower than the time for external event (rekeying). The attached patch resolved the problem for me and with openssh-7.4 all tests passed.