Bash and Zsh not POSIX on exec calls

[smondet]

The exec Schism

Specification of exec.

If command is specified, exec shall not return to the shell; rather, the exit
status of the process shall be the exit status of the program implementing
command, which overlaid the shell. If command is not found, the exit status
shall be 127. If command is found, but it is not an executable utility, the
exit status shall be 126. If a redirection error occurs (see
Consequences of Shell Errors),
the shell shall exit with a value in the range 1-125.
Otherwise, exec shall return a zero exit status.

For a given shell, trying:

$shell -c ' exec 4>&3 ; echo "Exec-returns: $?"' ; echo "Shell-returns: $?"

The POSIX ones:

• shell=dash, shell=sh, shell='busbox ash': Shell-returns: 2
• shell=ksh, shell=mksh: Shell-returns: 1

The non-POSIX ones:

• shell=bash, shell=zsh:
  Exec-returns: 1 Shell-returns: 0

[smondet]

It seems that busybox 1.22 returns 2 while 1.27 returns 1 (both are ok with POSIX). This makes the "redirect-fails" test fail.