bug#49918: 28.0.50; cd function expands CDPATH incorrectly

[Phil Hagelberg]

When using the function `cd' with a relative path and the $CDPATH
environment variable set, it will expand the value of $CDPATH once and
save off that expansion for future use. This causes all calls with
relative paths to fail when run from a different directory.

This reproduces the problem using eshell, but the bug affects any caller
of the `cd' function, not just eshell. However, it does not affect M-x
cd, which will always use absolute paths.

    CDPATH=.:$HOME/src emacs -Q
    M-x eshell
    cd /
    cd tmp
    No such directory found via CDPATH environment variable: tmp

This happens because of this snippet inside the `cd' function:

    (unless cd-path
      (setq cd-path (or (parse-colon-path (getenv "CDPATH"))
                        (list "./"))))

The behavior of the `parse-colon-path' function changed in 2020 in
commit 76098d39c992aa51f5bdb04fb39e40fc5eb409d5:

-    (mapcar (lambda (f)
-             (if (equal "" f) nil
-               (substitute-in-file-name (file-name-as-directory f))))
-           (split-string search-path path-separator))))
+    (let ((spath (substitute-env-vars search-path)))
+      (mapcar (lambda (f)
+                (if (equal "" f) nil
+                  (let ((dir (expand-file-name (file-name-as-directory f))))
+                    ;; Previous implementation used `substitute-in-file-name'
+                    ;; which collapse multiple "/" in front.  Do the same for
+                    ;; backward compatibility.
+                    (if (string-match "\\`/+" dir)
+                        (substring dir (1- (match-end 0))) dir))))
+              (split-string spath path-separator)))))

Previous behavior did not expand relative paths like "." but the new
behavior does. That means that the above call which sets cd-path cannot
use this function as is. Either the old behavior of parse-colon-path
will need to be brought back, or `cd' needs to no longer call that function.

Witness on the current master branch: ($PWD here is ~/src/emacs)

    (parse-colon-path ".:/hey") : -> ("/home/phil/src/emacs/" "/hey/")

Whereas on 27, you get this:

    (parse-colon-path ".:/hey") : -> ("./" "/hey/")

The old behavior is safe to set for the value of `cd-path' but the new
behavior is not.


In GNU Emacs 28.0.50 (build 1, aarch64-unknown-linux-gnu, GTK+ Version 3.24.24, 
cairo version 1.16.0)
 of 2021-08-06 built on reform
Repository revision: 0b049fe71d73f6885a3c81ea31829e3befc18933
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12099001
System Description: Debian GNU/Linux 11 (bullseye)

Configured using:
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