overeager multios

[Andrew Pimlott <andrew@pimlott.net>]

I just spent several minutes trying to understand a behavior that
turned out to be caused by unexpected triggering of multios.  A
simple example is

    echo hello 3>&1 >&2 | cat

which prints "hello" twice, instead of once as in bash.  (In
reality, "echo hello" would be a program that writes to both fds 1
and 3.)  I think multios is overeager here, as this is a standard
shell idiom, and the only way I know of to achieve this effect.

A fix would be to ignore (for multios purposes) previous opens when
a fd is duped.  the shell would process the above as follows.

1.  fd 1 will be opened to the pipe.  Push that open onto the
    multios stack for fd 1.
2.  fd 1 will be duped to fd 3.  Clear the multios stack.
3.  fd 2 will be duped to fd 1.  Push that open onto the multios
    stack.
4   fd 1 has only one entry in its multios stack, so don't trigger
    multios.

This will ensure that multios only triggers on a command line that
has no useful meaning in sh.

I know I can turn off multios, but I think we can have the best of
both with the above algorithm.

I am using the zsh 4.0.6-13 package on Debian GNU/Linux.

Andrew

PS.  Please Cc: me as I am not subscribed.