Hello:

On systems with  support for open(O_CLOEXEC):

$ zsh -c 'zmodload zsh/system; sysopen -w -o cloexec -u 3 /dev/null; ls -l /proc/self/fd'
total 0
lrwx------ 1 chazelas chazelas 64 May 23 16:35 0 -> /dev/pts/1
lrwx------ 1 chazelas chazelas 64 May 23 16:35 1 -> /dev/pts/1
lrwx------ 1 chazelas chazelas 64 May 23 16:35 2 -> /dev/pts/1
lr-x------ 1 chazelas chazelas 64 May 23 16:35 3 -> /proc/21668/fd

(OK)

$ zsh -c 'zmodload zsh/system; sysopen -w -o cloexec -u 4 /dev/null; ls -l /proc/self/fd'
total 0
lrwx------ 1 chazelas chazelas 64 May 23 16:35 0 -> /dev/pts/1
lrwx------ 1 chazelas chazelas 64 May 23 16:35 1 -> /dev/pts/1
lrwx------ 1 chazelas chazelas 64 May 23 16:35 2 -> /dev/pts/1
lr-x------ 1 chazelas chazelas 64 May 23 16:35 3 -> /proc/21669/fd
l-wx------ 1 chazelas chazelas 64 May 23 16:35 4 -> /dev/null

(not OK: fd 4 passed to ls)

$ zsh -c 'zmodload zsh/system; sysopen -w -o cloexec -u fd /dev/null; ls -l /proc/self/fd'
total 0
lrwx------ 1 chazelas chazelas 64 May 23 16:35 0 -> /dev/pts/1
lrwx------ 1 chazelas chazelas 64 May 23 16:35 1 -> /dev/pts/1
l-wx------ 1 chazelas chazelas 64 May 23 16:35 12 -> /dev/null
lrwx------ 1 chazelas chazelas 64 May 23 16:35 2 -> /dev/pts/1
lr-x------ 1 chazelas chazelas 64 May 23 16:35 3 -> /proc/21674/fd

(not OK: fd 3 passed to ls).

The reason is that in the last two cases, dup()/dup2() was
called on the fd returned by open(). In the first case to make
it the requested "3", and in the second case so it's a fd above
10.

The cloexec flag is one that is attached to the fd, not the open
file description, so it doesn't survive a dup(). So one needs to
reapply the CLOEXEC flag again after the dup.

See attached for a suggested fix. Linux has a dup3() which you
can pass the CLOEXEC flag to, but I don't expect that to  be
very portable.

The code could be simplified so that fcntl() is always called as
it's likely it's going to be called anyway.

-- 
Stephane