Trouble with hDuplicateTo before exec

[Volker Wysk]

Hello.

"hDuplicateTo src tgt" overwrites the tgt handle with the src handle, but it 
doesn't overwrite the file descriptor used by tgt with that used by src. 
Rather, it makes a copy of src's fd and makes the overwritten tgt use that.

That's fine as long as we're in Haskell. But consider "hDuplicateTo h stdin". 
When calling executeFile afterwards, the replaced process expects its 
standard input on handle 0, not on the new fd generated by hDuplicateTo.

A workaround would be to use dupTo instead of hDuplicateTo, if we intend to 
call executeFile afterwards, but I have the case that this isn't known at the 
time. I'm forking a supplied IO action, connecting it with a pipe. The IO 
action may or may not call executeFile.

Regards,
V.W.