bug#18232: 24.3.92; Filename completion changes the current working directory

[Harald Hanche-Olsen]

C-x C-f (find-file) with filename completion changes the current
working directory (cwd) of emacs.

This is annoying for the following reason:

If you temporarily mount a filesystem and then visit a file in that
filesystem, you cannot unmount the filesystem because it is now the
cwd of a running process.

The obvious workaround is to first visit a file not in this
filesystem, but that is annoying and should be unnecessary.

I have narrowed the cause down to filename completion:

Starting with emacs -Q:

 - Find the PID of the running emacs process.
 - In a root shell, run “lsof -p PID | grep cwd”.
 - Evaluate (read-file-name-default "Filename: ") and don't hit RETURN
 - Navigate to some other directory, and cause filename completion to
   happen. For example, type “/tmp/” and follow up with TAB.
 - Run the lsof command again, and note that the current working
   directory has changed.


In GNU Emacs 24.3.92.2 (x86_64-apple-darwin13.3.0, NS apple-appkit-1265.21)
 of 2014-08-08 on airy
Windowing system distributor `Apple', version 10.3.1265
Configured using:
 `configure --with-ns'

Important settings:
  value of $LC_CTYPE: en_US.UTF-8
  value of $LANG: C
  locale-coding-system: utf-8-unix