LaTeX fragments: Better error message when latex/dvipng unavailable

Thomas Morgan writes:

>  I just tried exporting an Org file with LaTeX fragments to HTML
>  on a computer that doesn't have dvipng.  There were error messages
>  in *Messages* ("Failed to create png file..."), but this wasn't
>  obvious to me at first glance because those messages were replaced
>  in the echo area by "Exporting... done" before I could see them.
>
>  So I was wondering, is there a good way to make the user aware of
>  those errors?  Maybe by printing "Exporting... done (with errors)"?

There is now a better error message when either the latex or the
dvipng program does not exist.

lisp/ChangeLog

@@ -1,6 +1,10 @@
 2009-04-30  Carsten Dominik  <carsten.dominik@gmail.com>
 
+	* org-macs.el (org-check-external-command): New defsubst.
+
 	* org.el (org-mode-map): New key for reload.
+	(org-format-latex): Better error message when external programs
+	are not available.
 
 	* org-agenda.el (org-agenda-mode-map): Bind `org-reload'.
 

lisp/org-exp.el

@@ -425,7 +425,10 @@ the first non-white thing on a line.  It will also find the math delimiters
 like $a=b$ and \\( a=b \\) for inline math,  $$a=b$$ and \\[ a=b \\] for
 display math.
 
-This option can also be set with the +OPTIONS line, e.g. \"LaTeX:t\"."
+This option can also be set with the +OPTIONS line, e.g. \"LaTeX:t\".
+
+The default is nil, because this option needs the `dvipng' program which
+is not available on all systems."
   :group 'org-export-translation
   :group 'org-export-latex
   :type 'boolean)

lisp/org-macs.el

@@ -165,6 +165,18 @@ We use a macro so that the test can happen at compilation time."
 	((assoc key option) (cdr (assoc key option)))
 	(t (cdr (assq 'default option)))))
 
+(defsubst org-check-external-command (cmd &optional use no-error)
+  "Check if external progam CMD for USE exists, error if not.
+When the program does exist, return it's path.
+When it does not exist and NO-ERROR is set, return nil.
+Otherwise, throw an error.  The optional argument USE can describe what this
+program is needed for, so that the error message can be more informative."
+  (or (executable-find cmd)
+      (if no-error
+	  nil
+	(error "Can't find `%s'%s" cmd
+	       (if use (format " (%s)" use) "")))))
+
 (defsubst org-inhibit-invisibility ()
   "Modified `buffer-invisibility-spec' for Emacs 21.
 Some ops with invisible text do not work correctly on Emacs 21.  For these

lisp/org.el

@@ -13521,6 +13521,7 @@ The images can be removed again with \\[org-ctrl-c-ctrl-c]."
 	 (matchers (plist-get opt :matchers))
 	 (re-list org-latex-regexps)
 	 (cnt 0) txt link beg end re e checkdir
+	 executables-checked
 	 m n block linkfile movefile ov)
     ;; Check if there are old images files with this prefix, and remove them
     (when (file-directory-p todir)
@@ -13549,6 +13550,14 @@ The images can be removed again with \\[org-ctrl-c-ctrl-c]."
 	    (unless checkdir ; make sure the directory exists
 	      (setq checkdir t)
 	      (or (file-directory-p todir) (make-directory todir)))
+
+	    (unless executables-checked
+	      (org-check-external-command
+	       "latex" "needed to convert LaTeX fragments to images")
+	      (org-check-external-command
+	       "dvipng" "needed to convert LaTeX fragments to images")
+	      (setq executables-checked t))
+
 	    (org-create-formula-image
 	     txt movefile opt forbuffer)
 	    (if overlays