Before this fix using --sftp-ssh with the sftp backend could leave
zombie processes.
This patch fixes the problem that sshClientExternal.session was never
assigned, so Wait() always returned nil without waiting for the SSH
process to exit. This caused zombie processes because the process was
never reaped.
It also ensures that Wait() is only called once on each process.
I gave this issue to Copilot to fix as an experiment. It went off in
the wrong direction to start with and fixed something which wasn't the
problem but still needed fixing. With a bit of a nudge it fixed the
correct problem too.
Co-authored-by: Nick Craig-Wood <nick@craig-wood.com>
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