bug#54384: 29.0.50; shell-resync-dirs failed to work

[Zhiwei Chen]

How to reproduce:
    emacs -Q
    M-x shell RET
    M-x shell-resync-dirs RET

Then Emacs will be unresponsive. Fortunately, C-g can quit the function.

Before quitting, it's able to trace Emacs via perf.

#+begin_src sh
sudo perf trace -p $(pidof emacs)
#+end_src

The output tells Emacs is checking whether a path is directory, but it returns 
false. See the logs below.

#+begin_src text
1898.605 ( 0.003 ms): emacs/889479 openat(dfd: CWD, filename: 0x5569b9323b80, 
flags: RDONLY|CLOEXEC|DIRECTORY|PATH) = -1 ENOENT (No such file or directory)
1898.623 ( 0.003 ms): emacs/889479 openat(dfd: CWD, filename: 0x5569b9323c50, 
flags: RDONLY|CLOEXEC|DIRECTORY|PATH) = -1 ENOENT (No such file or directory)
1898.639 ( 0.002 ms): emacs/889479 openat(dfd: CWD, filename: 0x5569b9323d20, 
flags: RDONLY|CLOEXEC|DIRECTORY|PATH) = -1 ENOENT (No such file or directory)
1898.656 ( 0.003 ms): emacs/889479 openat(dfd: CWD, filename: 0x5569b9323df0, 
flags: RDONLY|CLOEXEC|DIRECTORY|PATH) = -1 ENOENT (No such file or directory)
1898.673 ( 0.002 ms): emacs/889479 openat(dfd: CWD, filename: 0x5569b9323ec0, 
flags: RDONLY|CLOEXEC|DIRECTORY|PATH) = -1 ENOENT (No such file or directory)
1898.689 ( 0.002 ms): emacs/889479 openat(dfd: CWD, filename: 0x5569b9323f90, 
flags: RDONLY|CLOEXEC|DIRECTORY|PATH) = -1 ENOENT (No such file or directory)
#+end_src

Since I'm using zsh, `shell-resync-dirs' will send "dirs -l" to zsh process, 
then grab and parse the output.

* Defect 1

After this while loop 
https://github.com/emacs-mirror/emacs/blob/f8cc706fb3d1ba30715c09aac9654ba08aeeb5c9/lisp/shell.el#L1072-L1074,
 the value of `dlsl` is '("-l" "" "dirs"), which means `shell-resync-dirs'' 
failed to grab/parse the output of `dirs -l'.

* Defect 2

The while loop 
https://github.com/emacs-mirror/emacs/blob/f8cc706fb3d1ba30715c09aac9654ba08aeeb5c9/lisp/shell.el#L1078-L1092
 can be infinite.

When an element of `dlsl' doesn't exist, `file-directory-p' returns false, then 
no element of `dlsl' will be poped. An unintentional looping is happening.

-- 
Zhiwei Chen