Re: [PATCH] Fix hang if $OLDPWD points to inaccessible directory

[Eduardo A . Bustamante López]

On Tue, Oct 03, 2017 at 10:29:08PM +0200, Mikulas Patocka wrote:
> If $OLDPWD points to a non-existing directory, 'cd -' will fail.
> But if we clear $OLDPWD, 'cd -' will fail too (with just different message).
[...]

I performed the following tests:


    dualbus@ubuntu:~$ for sh in mksh ksh93 dash zsh posh bash; do echo $sh \| 
$(OLDPWD=/invalid $sh -c 'echo "$OLDPWD | "; cd -; echo " | $?"' 2>&1); done
    mksh | /invalid | mksh: cd: /invalid: No such file or directory | 2
    ksh93 | /invalid | ksh93: cd: /invalid: [No such file or directory] | 1
    dash | /invalid | dash: 1: cd: can't cd to /invalid | 2
    zsh | /home/dualbus | | 0
    posh | /invalid | posh: cd: /invalid - No such file or directory | 1
    bash | | bash: line 0: cd: OLDPWD not set | 1

    dualbus@ubuntu:~$ for sh in mksh ksh93 dash zsh posh bash; do echo $sh \| 
$(OLDPWD=/tmp $sh -c 'echo "$OLDPWD | "; cd -; echo " | $?"' 2>&1); done
    mksh | /tmp | /tmp | 0
    ksh93 | /tmp | /tmp | 0
    dash | /tmp | /tmp | 0
    zsh | /home/dualbus | | 0
    posh | /tmp | /tmp | 0
    bash | /tmp | /tmp | 0


So:

- Bash is the only major shell that performs stat() on OLDPWD to see if it's
  going to import it from the environment or initialize it to an empty
  value.

- Zsh doesn't import OLDPWD from the environment (bash's old behavior)

- The rest of the shells (mksh, ATT ksh, dash, posh) import OLDPWD from
  the environment unconditionally.

- Mikulas already made the argument above of why this change wouldn't
  break existing scripts that rely on bash performing the stat().

- I don't see a way around this behavior using existing bash features.
  The value of OLDPWD can't be reset via BASH_ENV or other
  initialization mechanisms. Sure, `unset OLDPWD; program' might work,
  but I wouldn't call that a solution.


Given the above, I've changed my mind, and I think this patch should be
fine :-)