bug#25075: 26.0.50; [PATCH] Dired can now dereference symlinks when copying

[Kaushal Modi <kaushal.modi@gmail.com>]

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Copying with symlinks dereferenced is almost always the case for me,
because of the version control system we use. It is a central VCS. So we
need to check out the files before editing them. In checked-in state, all
files are just symlinks to the physical versions in a central cache server.
So almost always "cp -L" is what I want to do when copying files from one
place to another without checking them out. My cp is aliased to "cp -L".

That said, here's the revised patch with updated documentation and
"Revisited Option 1" I mentioned above. This allows the flexibility of
*not* dereferencing symlinks for few selected cases when I would have set
dired-copy-dereference to t by default in my config.

From 28b9a7c41c168daec6645638c06d5333e8b642aa Mon Sep 17 00:00:00 2001
From: Kaushal Modi <kaushal.modi@gmail.com>
Date: Wed, 30 Nov 2016 12:52:42 -0500
Subject: [PATCH] Dired can now dereference symlinks when copying

* lisp/dired.el (dired-copy-dereference): New defcustom, defaults to
nil.

* lisp/dired-aux.el (dired-copy-file-recursive): Add new optional
argument `dereference'.

* lisp/dired-aux.el (dired-copy-file): Use `dired-copy-dereference' as
the `dereference' argument to `dired-copy-file-recursive'.

* lisp/dired-aux.el (dired-do-copy): Invert the value of
  `dired-copy-dereference' in lexical scope when prefix argument is
  '(4). Update function documentation for the new defcustom.

* doc/emacs/dired.texi (Operating on Files): Mention the new defcustom.
---
 doc/emacs/dired.texi |  8 ++++++++
 lisp/dired-aux.el    | 25 +++++++++++++++++++------
 lisp/dired.el        |  6 ++++++
 3 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/doc/emacs/dired.texi b/doc/emacs/dired.texi
index 2cda51a82f..62042c5c1e 100644
--- a/doc/emacs/dired.texi
+++ b/doc/emacs/dired.texi
@@ -638,6 +638,14 @@ Operating on Files
 directories recursively (like @samp{cp -r}).  The default is
 @code{top}, which means to ask before recursively copying a directory.

+@vindex dired-copy-dereference
+@cindex follow symbolic links
+@cindex dereference symbolic links
+The variable @code{dired-copy-dereference} controls whether to copy
+symbolic links as links or after dereferencing (like @samp{cp -L}).
+The default is @code{nil}, which means that the symbolic links are
+copied by creating new ones.
+
 @item D
 @findex dired-do-delete
 @kindex D @r{(Dired)}
diff --git a/lisp/dired-aux.el b/lisp/dired-aux.el
index f94e0537aa..1b9009c744 100644
--- a/lisp/dired-aux.el
+++ b/lisp/dired-aux.el
@@ -1507,12 +1507,13 @@ dired-handle-overwrite
 (defun dired-copy-file (from to ok-flag)
   (dired-handle-overwrite to)
   (dired-copy-file-recursive from to ok-flag dired-copy-preserve-time t
-     dired-recursive-copies))
+     dired-recursive-copies dired-copy-dereference))

 (declare-function make-symbolic-link "fileio.c")

 (defun dired-copy-file-recursive (from to ok-flag &optional
-       preserve-time top recursive)
+       preserve-time top recursive
+                                       dereference)
   (when (and (eq t (car (file-attributes from)))
      (file-in-directory-p to from))
     (error "Cannot copy `%s' into its subdirectory `%s'" from to))
@@ -1524,7 +1525,8 @@ dired-copy-file-recursive
  (copy-directory from to preserve-time)
       (or top (dired-handle-overwrite to))
       (condition-case err
-  (if (stringp (car attrs))
+  (if (and (not dereference)
+                   (stringp (car attrs)))
       ;; It is a symlink
       (make-symbolic-link (car attrs) to ok-flag)
     (copy-file from to ok-flag preserve-time))
@@ -1963,6 +1965,9 @@ dired-copy-how-to-fn
 ;;;###autoload
 (defun dired-do-copy (&optional arg)
   "Copy all marked (or next ARG) files, or copy the current file.
+ARG has to be numeric for above functionality.  See
+`dired-get-marked-files' for more details.
+
 When operating on just the current file, prompt for the new name.

 When operating on multiple or marked files, prompt for a target
@@ -1976,10 +1981,18 @@ dired-do-copy
 the modification time of each old file in the copy, similar to
 the \"-p\" option for the \"cp\" shell command.

-This command copies symbolic links by creating new ones, similar
-to the \"-d\" option for the \"cp\" shell command."
+This command copies symbolic links by creating new ones,
+similar to the \"-d\" option for the \"cp\" shell command.
+But if `dired-copy-dereference' is non-nil, the symbolic
+links are dereferenced and then copied, similar to the \"-L\"
+option for the \"cp\" shell command.  If ARG is a cons with
+element 4 (`\\[universal-argument]'), inverted value of
+`dired-copy-dereference' will be used."
   (interactive "P")
-  (let ((dired-recursive-copies dired-recursive-copies))
+  (let ((dired-recursive-copies dired-recursive-copies)
+        (dired-copy-dereference (if (equal arg '(4))
+                                    (not dired-copy-dereference)
+                                  dired-copy-dereference)))
     (dired-do-create-files 'copy (function dired-copy-file)
    "Copy"
    arg dired-keep-marker-copy
diff --git a/lisp/dired.el b/lisp/dired.el
index daa6d776a9..ca7ab2813b 100644
--- a/lisp/dired.el
+++ b/lisp/dired.el
@@ -191,6 +191,12 @@ dired-copy-preserve-time
   :type 'boolean
   :group 'dired)

+(defcustom dired-copy-dereference nil
+  "If non-nil, Dired dereferences symlinks when copying them.
+This is similar to the \"-L\" option for the \"cp\" shell command."
+  :type 'boolean
+  :group 'dired)
+                                        ;
 ; These variables were deleted and the replacements are on files.el.
 ; We leave aliases behind for back-compatibility.
 (defvaralias 'dired-free-space-program 'directory-free-space-program)
-- 
2.11.0


On Wed, Nov 30, 2016 at 11:12 PM Kaushal Modi <kaushal.modi@gmail.com>
wrote:

> On Wed, Nov 30, 2016 at 7:38 PM Glenn Morris <rgm@gnu.org> wrote:
>
>
> I can't imagine anyone needing this as an option they can set for all
> future uses. Surely what someone might need is a way to make one
> specific copy command dereference? As a prefix argument or a separate
> command, or whatever.
>
>
> Hi Glenn,
>
> I thought of various options:
>
> Option 1: Make a prefix do copy with dereferencing. But for
> `dired-do-copy' (bound to 'C' by default in dired-mode), numeric ARG
> already has a meaning.. M-4 C will copy the next ARG files. Is the
> dired-copy-dereference option special enough for a "C-u" prefix? I don't
> know.
>
> Option 2: Let-bind dired-copy-dereference to t in a separate command and
> have that call `dired-do-copy'. But then the single-letter binding space is
> pretty crowded in dired-mode-map, and probably not worthy for this niche
> command.
>
> Option 3 (is what I did): Have a defcustom available. User can choose to
> either set it to a t by default, or use it in a wrapper command and bind
> that to a binding of their liking.
>
> ---
>
> Re-visiting option 1: Do you think it would be a better idea to have C-u C
> toggle the global value of `dired-copy-dereference' in a let binding and
> then proceed with the copy? So if `dired-copy-dereference' is nil by
> default, plain 'C' will copy symbolic links as links. But "C-u C" will copy
> after dereferencing. Similarly if the user has set `dired-copy-dereference'
> to t, the behavior of "C" and "C-u C" will reverse.
>
> On a separate note, if you look at the dired-do-copy definition, why is
> the global value of dired-recursive-copies assigned to the let-bound
> variable? Does this do anything different than not having that let form
> altogether?
>
>   (let ((dired-recursive-copies dired-recursive-copies))         ;
> <---------------------------------
>     (dired-do-create-files 'copy (function dired-copy-file)
>   "Copy"
>   arg dired-keep-marker-copy
>   nil dired-copy-how-to-fn))
>
> The commit history says:
>
> (dired-do-copy): Bind `dired-recursive-copies' to preserve it.
>     Use dired-copy-how-to-fn as how-to argument to dired-do-create-files.
>
> But I did not understand that.
>
> Here's the vc region history for that line:
>
> commit ba1acd68768ac49d98afbf781851ab95c0263048
> Author: Richard M. Stallman <rms@gnu.org>
> Date:   Thu Sep 16 19:29:30 1999 +0000
>
>     (dired-recursive-copies): New custom variable.
>     (dired-handle-overwrite): Broke a long line.
>     (dired-copy-file): Call `dired-copy-file-recursive' instead of
> `copy-file'.
>     (dired-copy-file-recursive): New function.  Copy directories
> recursively.
>     (dired-do-create-files): Added support for generalized directory
>     target.  How-to function may now return a function.  New fluid
>     variable `dired-one-file'.
>     (dired-copy-how-to-fn): New variable.
>     (dired-do-copy): Bind `dired-recursive-copies' to preserve it.
>     Use dired-copy-how-to-fn as how-to argument to dired-do-create-files.
>     (dired-do-copy-regexp): No recursive copies.
>
> diff --git a/lisp/dired-aux.el b/lisp/dired-aux.el
> --- a/lisp/dired-aux.el
> +++ b/lisp/dired-aux.el
> @@ -1261,3 +1315,1 @@
> -  (dired-do-create-files 'copy (function dired-copy-file)
> -   (if dired-copy-preserve-time "Copy [-p]" "Copy")
> -   arg dired-keep-marker-copy))
> +n  (let ((dired-recursive-copies dired-recursive-copies))
>
> --
>
> Kaushal Modi
>
-- 

Kaushal Modi

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