bug#49644: 27.2; copy-directory fails when Unix socket file is present in the source directory tree

[Pierre Rouleau]

There is no Emacs crash involved here.  It's just a feature request.

Try `copy-directory' on a directory tree that contains one Unix socket.
Something like "~/elpa/gnupg" which often contains Unix socket files.
This will fail with an error stating that the Unix socket cannot be
copied.

Although its true that Unix sockets cannot be copied, it would be nice
to offer the users the ability to copy the directory tree anyway,
skipping the Unix socket files.

The function `copy-directory' does not provide a mechanism to skip the
copy of such files, so it becomes impossible to copy a directory tree
that contains such a file.  It would be useful to add argument to
provide the ability to skip such files.   Currently, the only way is to
override or advise the function `copy-file' when using `copy-directory'.

The cond clause at the end of `copy-directory' only supports
directories, symbolic links and normal files.  It does not support Unix
socket, which cannot be copied.  In some conditions users may want to
copy the directory tree even in the presence of Unix socket
files. Adding an optional argument for that would increase the
usefulness of the function while allowing user's code to document the
fact that they want (or don't want) to skip things like Unix sockets.

The latest version of files.el code in
https://github.com/emacs-mirror/emacs/blob/master/lisp/files.el#L6148 
still behaves like this.


In GNU Emacs 27.2 (build 1, x86_64-apple-darwin18.7.0)
of 2021-03-25 built on Mojave
System Description:  Mac OS X 10.14.6

Recent messages:
Generating autoloads for ztree-view.el...done
  INFO     Scraping files for pel-bundle-autoloads.el...100% 
Generating autoloads for ztree.el...done
  INFO     Scraping files for pel-bundle-autoloads.el...done
Wrote /Users/roup/.emacs.d/pel-bundle/pel-bundle-autoloads.el [2 times]
Restart Emacs to complete the process!
Type "q" in help window to delete it.
mouse-2, RET: find function's definition
\ is undefined
uncompressing files.el.gz...done

Configured using:
'configure --disable-silent-rules
--enable-locallisppath=/usr/local/share/emacs/site-lisp
--infodir=/usr/local/Cellar/emacs/27.2/share/info/emacs
--prefix=/usr/local/Cellar/emacs/27.2 --with-gnutls --without-x
--with-xml2 --without-dbus --with-modules --without-ns
--without-imagemagick --without-selinux'

Configured features:
NOTIFY KQUEUE ACL GNUTLS LIBXML2 ZLIB MODULES THREADS JSON PDUMPER GMP

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

Major mode: Emacs-Lisp

Minor modes in effect:
  ido-everywhere: t
  which-key-mode: t
  winner-mode: t
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  lispy-mode: t
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  global-anzu-mode: t
  anzu-mode: t
  recentf-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
/Users/roup/.emacs.d/utils/origami hides 
/Users/roup/.emacs.d/elpa/origami-20200331.1019/origami
/Users/roup/.emacs.d/utils/origami-parsers hides 
/Users/roup/.emacs.d/elpa/origami-20200331.1019/origami-parsers
/Users/roup/.emacs.d/utils/sr-speedbar hides 
/Users/roup/.emacs.d/elpa/sr-speedbar-20161025.831/sr-speedbar
/Users/roup/.emacs.d/elpa/lispy-20210121.926/elpa hides 
/Users/roup/.emacs.d/elpa/ivy-20210602.1349/elpa
/Users/roup/.emacs.d/elpa/lfe-mode-20210603.1241/lfe-indent hides 
/usr/local/share/emacs/site-lisp/lfe/lfe-indent
/Users/roup/.emacs.d/elpa/lfe-mode-20210603.1241/lfe-mode hides 
/usr/local/share/emacs/site-lisp/lfe/lfe-mode
/Users/roup/.emacs.d/elpa/lfe-mode-20210603.1241/inferior-lfe hides 
/usr/local/share/emacs/site-lisp/lfe/inferior-lfe
/Users/roup/.emacs.d/elpa/rtags-20210313.1541/rtags hides 
/usr/local/share/emacs/site-lisp/rtags/rtags
/Users/roup/.emacs.d/elpa/rtags-xref-20200310.1909/rtags-xref hides 
/usr/local/share/emacs/site-lisp/rtags/rtags-xref
/Users/roup/.emacs.d/elpa/seq-2.22/seq hides 
/usr/local/Cellar/emacs/27.2/share/emacs/27.2/lisp/emacs-lisp/seq

Features:
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ietf-drums mm-util mail-prsvr mail-utils jka-compr windmove eieio-opt
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xref project edebug backtrace help-fns radix-tree help-mode lispy-tags
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Memory information:
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(symbols 48 29623 1)
(strings 32 142354 5578)
(string-bytes 1 4079491)
(vectors 16 39237)
(vector-slots 8 884160 78722)
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(intervals 56 762 0)
(buffers 1000 16))