`C-u C-u C-c C-o' forces opening in an external application

Org-mode uses a special setup with a number of different customization
variables to specify how a file should be opened when following a
:file:..." link with `C-c C-o'.  By using a `C-u' prefix, it was
possible to overrule the customized setup and to force opening the
file in Emacs.

Samuel Wales requested to amend this process, so that a double prefix
argument would do the opposite: force opening the application *outside*
of Emacs, using a system default application.  This is what this patch
implements.

Internally this works through a new entry in system specific constants
`org-file-apps-defaults-gnu', org-file-apps-defaults-macosx', and
`org-file-apps-defaults-windowsnt'.  The new entry has the car
`system' and specifies the command that should be used for the double
C-u calls.  As before, the user option `org-file-apps' can overrule
these default settings.

Note that all this only applies to following "file:" links, and does
not make a difference for, for example, "http:" links.

doc/ChangeLog

@@ -1,3 +1,9 @@
+
+2008-11-05  Carsten Dominik  <dominik@science.uva.nl>
+
+	* org.texi (Handling links): Explain the effect of a double prefix
+	arg to `C-c C-o'.
+
 2008-11-02  Charles Sebold  <csebold@gmail.com>
 
 	* org.texi (Org Plot): Add documentation for timefmt option.

doc/org.texi

@@ -2637,7 +2637,8 @@ date.  Furthermore, it will visit text and remote files in @samp{file:} links
 with Emacs and select a suitable application for local non-text files.
 Classification of files is based on file extension only.  See option
 @code{org-file-apps}.  If you want to override the default application and
-visit the file with Emacs, use a @kbd{C-u} prefix.
+visit the file with Emacs, use a @kbd{C-u} prefix.  If you want to avoid
+opening in Emacs, use a @kbd{C-u C-u} prefix.
 @c
 @kindex mouse-2
 @kindex mouse-1

lisp/ChangeLog

@@ -1,3 +1,15 @@
+2008-11-05  Carsten Dominik  <dominik@science.uva.nl>
+
+	* org.el (org-file-apps-defaults-gnu)
+	(org-file-apps-defaults-macosx)
+	(org-file-apps-defaults-windowsnt): Add an entry defining the
+	system command.
+	(org-file-apps): Allow `system' as key and value.
+	(org-open-at-point): Explain the effect of a double prefix arg.
+	(org-open-file): If the argument `in-emacs' is (16),
+	i.e. corresponding to a double prefix argument, try to open the
+	file externally.
+
 2008-11-04  Carsten Dominik  <dominik@science.uva.nl>
 
 	* org.el (org-insert-link): Abbreviate absolute files names in

lisp/org.el

@@ -1117,6 +1117,7 @@ single keystroke rather than having to type \"yes\"."
 
 (defconst org-file-apps-defaults-gnu
   '((remote . emacs)
+    (system . mailcap)
     (t . mailcap))
   "Default file applications on a UNIX or GNU/Linux system.
 See `org-file-apps'.")
@@ -1124,6 +1125,7 @@ See `org-file-apps'.")
 (defconst org-file-apps-defaults-macosx
   '((remote . emacs)
     (t . "open %s")
+    (system . "open %s")
     ("ps.gz"  . "gv %s")
     ("eps.gz" . "gv %s")
     ("dvi"    . "xdvi %s")
@@ -1137,6 +1139,11 @@ See `org-file-apps'.")
   (list
    '(remote . emacs)
    (cons t
+	 (list (if (featurep 'xemacs)
+		   'mswindows-shell-execute
+		 'w32-shell-execute)
+	       "open" 'file))
+   (cons 'system
 	 (list (if (featurep 'xemacs)
 		   'mswindows-shell-execute
 		 'w32-shell-execute)
@@ -1165,11 +1172,15 @@ file identifier are
                Remote files most likely should be visited through Emacs
                because external applications cannot handle such paths.
 `auto-mode'    Matches files that are mached by any entry in `auto-mode-alist',
-               so all files Emacs knows how to handle.  Useing this with
+               so all files Emacs knows how to handle.  Using this with
                command `emacs' will open most files in Emacs.  Beware that this
                will also open html files insite Emacs, unless you add
                (\"html\" . default) to the list as well.
  t             Default for files not matched by any of the other options.
+ `system'      The system command to open files, like `open' on Windows
+               and Mac OS X, and mailcap under GNU/Linux.  This is the command
+               that will be selected if you call `C-c C-o' with a double
+               `C-u C-u' prefix.
 
 Possible values for the command are:
  `emacs'       The file will be visited by the current Emacs process.
@@ -1178,6 +1189,11 @@ Possible values for the command are:
                part.
                This can be used to overrule an unwanted seting in the
                system-specific variable.
+ `system'      Use the system command for opening files, like \"open\".
+               This command is specified by the entry whose car is `system'.
+               Most likely, the system-specific version of this variable
+               does define this command, but you can overrule/replace it
+               here.
  string        A command to be executed by a shell; %s will be replaced
 	       by the path to the file.
  sexp          A Lisp form which will be evaluated.  The file path will
@@ -1190,6 +1206,7 @@ For more examples, see the system specific constants
   :type '(repeat
 	  (cons (choice :value ""
 			(string :tag "Extension")
+			(const :tag "System command to open files" system)
 			(const :tag "Default for unrecognized files" t)
 			(const :tag "Remote file" remote)
 			(const :tag "Links to a directory" directory)
@@ -1197,7 +1214,8 @@ For more examples, see the system specific constants
 			       auto-mode))
 		(choice :value ""
 			(const :tag "Visit with Emacs" emacs)
-			(const :tag "Use system default" default)
+			(const :tag "Use default" default)
+			(const :tag "Use the system command" system)
 			(string :tag "Command")
 			(sexp :tag "Lisp form")))))
 
@@ -6619,7 +6637,9 @@ Org-mode syntax."
 If there is no link at point, this function will search forward up to
 the end of the current subtree.
 Normally, files will be opened by an appropriate application.  If the
-optional argument IN-EMACS is non-nil, Emacs will visit the file."
+optional argument IN-EMACS is non-nil, Emacs will visit the file.
+With a double prefix argument, try to open outside of Emacs, in the
+application the system uses for this file type."
   (interactive "P")
   (org-load-modules-maybe)
   (move-marker org-open-link-marker (point))
@@ -7048,8 +7068,13 @@ onto the ring."
 First, this expands any special file name abbreviations.  Then the
 configuration variable `org-file-apps' is checked if it contains an
 entry for this file type, and if yes, the corresponding command is launched.
+
 If no application is found, Emacs simply visits the file.
-With optional argument IN-EMACS, Emacs will visit the file.
+
+With optional prefix argument IN-EMACS, Emacs will visit the file.
+With a double C-c C-u prefix arg, Org tries to avoid opening in Emacs
+and o use an external application to visit the file.
+
 Optional LINE specifies a line to go to, optional SEARCH a string to
 search for.  If LINE or SEARCH is given, the file will always be
 opened in Emacs.
@@ -7074,14 +7099,19 @@ If the file does not exist, an error is thrown."
 	(setq ext (match-string 1 dfile))
       (if (string-match "^.*\\.\\([a-zA-Z0-9]+\\)$" dfile)
 	  (setq ext (match-string 1 dfile))))
-    (if in-emacs
-	(setq cmd 'emacs)
+    (cond
+     ((equal in-emacs '(16))
+      (setq cmd (cdr (assoc 'system apps))))
+     (in-emacs (setq cmd 'emacs))
+     (t
       (setq cmd (or (and remp (cdr (assoc 'remote apps)))
 		    (and dirp (cdr (assoc 'directory apps)))
 		    (assoc-default dfile (org-apps-regexp-alist apps a-m-a-p)
 				   'string-match)
 		    (cdr (assoc ext apps))
-		    (cdr (assoc t apps)))))
+		    (cdr (assoc t apps))))))
+    (when (eq cmd 'system)
+      (setq cmd (cdr (assoc 'system apps))))
     (when (eq cmd 'default)
       (setq cmd (cdr (assoc t apps))))
     (when (eq cmd 'mailcap)