bug#32201: 27.0.50; setenv should not change match-data

[John Shahid]

I have been running into a weird issue lately due to a
'buffer-list-update-hook' that I added in my init.el file.  The hook is
shown below:

    (add-hook 'buffer-list-update-hook
              (lambda ()
                (when (equal major-mode 'go-mode)
                  (unless (boundp 'gopath)
                    (if-let ((root (locate-dominating-file buffer-file-name 
".envrc")))
                        (setq-local gopath (expand-file-name root))
                      (setq-local gopath  nil)))
                  (and gopath
                       (save-match-data
                         (setenv "GOPATH" gopath))))))

The idea of this hook is to set some env variable based on the current
buffer.  Changing process-environment isn't sufficient since some minor
modes I'm using start processes in separate buffers.

The problem seems to happen when all of the suddent 'find-file' will
start openning weird files.  For example, if I'm currently viewing
"~/foo/bar.txt" and use 'C-x C-f' the default file name will be
"~/foo/~jvshahid/foo/bar.txt".  After some debugging it turns out that
the following will happen which cause the match-data to be corrupted:

1. find-file calls abbreviate-file-name
2. abbreviate-file-name calls (expand-file-name "~")
3. expand-file-name runs the buffer-list-update-hook (unknown why)
4. the hook will use setenv which messes up the match-data
5. abbreviate-file-name resumes and use the incorrect match-data and return an 
invalid path

I don't know why '3' is happening.  This usually doesn't happen in a new
Emacs session, only randomly after using Emacs for a while.  May be it
has something to do with file name handlers, the debugger doesn't show
any function calls between 'expand-file-name' and
'run-hooks(buffer-list-update-hook)'.  I also used
'(find-file-name-handler default-directory 'expand-file-name)' but got
back nil.

Usually Emacs gets in a weird state at this point and things like
'buffer-list' starts throwing errors about 'max-lisp-eval-depth'.  I
usually have to set 'buffer-list-update-hook' to 'nil' before being able
to gain control.  It is worth noting that I saw around 1000
*code-conversion-work* buffers in my Emacs session.


In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu)
 of 2018-07-03 built on f388f63bdd5d
Repository revision: 893e62ee7e3630c981adb3efa39ef409500d7657
System Description: Ubuntu 16.04.4 LTS

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