Why dup()?

David Newall openssh at davidnewall.com
Fri Oct 13 18:24:50 AEDT 2017

On 12/10/17 12:54, Clark Wang wrote:
> How did you hit the problem? "ssh host echo hello | cat" works fine 
> for me.

That's because ssh exited, not because of EOF.

Consider the following:

    ssh localhost 'exec cat > /dev/null' | { read l; echo ${l:-EOF}; }

One should see "EOF" immediately (for values of immediate that include 
startup latency), but that's not what happens.

I did have a better example.  When ssh was run with debug output, it 
showed ssh drain and close output when the remote end closed it's 
output.  Owing to ssh having two copies of the output descriptor, the 
downstream program did not see that EOF (it's not EOF until both file 
descriptors are closed.)  Now, irritatingly, I can't reproduce that.  
Now, the remote program closes its output, the sshd parent closes its 
end of that pipe, but ssh smugly sits there, keeping output (both 
copies) open until the remote program exits.  Clearly there is more 
going on that just two descriptors to the same output pipe.

It's wrong; ssh should not absorb EOF.

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