Hi Ludo,

Now that I have root access to overdrive1, I could strace the sshd
process (I just did 'strace -p340', noting the process of sshd displayed
with 'herd status sshd'):

--8<---------------cut here---------------start------------->8---
pselect6(87, [3 4], NULL, NULL, NULL, NULL) = 1 (in [3])
accept(3, {sa_family=AF_INET, sin_port=htons(33262), sin_addr=inet_addr("66.158.152.121")}, [128->16]) = 5
fcntl(5, F_GETFL)                       = 0x2 (flags O_RDWR)
pipe2([6, 7], 0)                        = 0
socketpair(AF_UNIX, SOCK_STREAM, 0, [8, 9]) = 0
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0xffff8e0ef0e0) = 644
close(7)                                = 0
close(9)                                = 0
write(8, "\0\0\1\245\0", 5)             = 5
write(8, "\0\0\1\234\nPort 22\nPermitRootLogin no\n"..., 420) = 420
close(8)                                = 0
close(5)                                = 0
getpid()                                = 340
getpid()                                = 340
getpid()                                = 340
getpid()                                = 340
getpid()                                = 340
getpid()                                = 340
getpid()                                = 340
pselect6(87, [3 4 6], NULL, NULL, NULL, NULL) = 1 (in [6])
read(6, "\0", 1)                        = 1
pselect6(87, [3 4 6], NULL, NULL, NULL, NULL) = 1 (in [6])
read(6, "", 1)                          = 0
close(6)                                = 0
pselect6(87, [3 4], NULL, NULL, NULL, NULL) = ? ERESTARTNOHAND (To be restarted if no handler)
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=644, si_uid=0, si_status=255, si_utime=1, si_stime=0} ---
wait4(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 255}], WNOHANG, NULL) = 644
wait4(-1, 0xfffffa4d90e4, WNOHANG, NULL) = 0
rt_sigreturn({mask=[]})                 = -1 EINTR (Interrupted system call)
pselect6(87, [3 4], NULL, NULL, NULL, NULL <detached ...>
--8<---------------cut here---------------end--------------->8---

With the attached v3 patch, the corresponding output (still problematic)
was:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix offload test /etc/guix/machines.scm overdrive1
guix offload: Testing 1 build machines defined in '/etc/guix/machines.scm'...
guix offload: got premature EOF from machine 'overdrive1.guix.gnu.org' from inferior '#<inferior pipe (0 1 1) 7f6ee10d3400>' on port '#<input-output: channel (open) 7f6ee10c5500>'; retrying connection
Backtrace:
In ice-9/boot-9.scm:
  1752:10 10 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In unknown file:
           9 (apply-smob/0 #<thunk 7f6ee5ae9f60>)
In ice-9/boot-9.scm:
    724:2  8 (call-with-prompt _ _ #<procedure default-prompt-handler (k proc)>)
In ice-9/eval.scm:
    619:8  7 (_ #(#(#<directory (guile-user) 7f6ee5ae3c80>)))
In guix/ui.scm:
  2161:12  6 (run-guix-command _ . _)
In ice-9/boot-9.scm:
  1752:10  5 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
  1747:15  4 (with-exception-handler #<procedure 7f6ee25e1480 at ice-9/boot-9.scm:1831:7 (exn)> _ # _ # …)
In ice-9/threads.scm:
   288:21  3 (loop _)
In guix/scripts/offload.scm:
   719:29  2 (_ _)
   719:29  1 (_ _)
In ice-9/boot-9.scm:
  1685:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1685:16: In procedure raise-exception:
Wrong type to apply: 2
--8<---------------cut here---------------end--------------->8---

I haven't tried analyzing the strace output yet.

Maxim