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


From: Kaushal Modi
Subject: bug#25075: 26.0.50; [PATCH] Dired can now dereference symlinks when copying
Date: Thu, 01 Dec 2016 06:06:21 +0000

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 <address@hidden>
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
address@hidden, which means to ask before recursively copying a directory.
 
address@hidden dired-copy-dereference
address@hidden follow symbolic links
address@hidden 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.
+
address@hidden D
address@hidden dired-do-delete
address@hidden 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 <address@hidden> wrote:
On Wed, Nov 30, 2016 at 7:38 PM Glenn Morris <address@hidden> 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 <address@hidden>
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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