[O] ob: Make src block location available to execution backend

[Vitalie Spinu]

Hi, 

ESS has a visual debugger that depends on the availability of source
references. It would be very nice if the debugger could step directly
through org buffers.

ESS was detecting org-src-edit buffers and redirecting references to
original org file for already quite a while. That because of the
availability of org-edit-src-beg-marker in src-edit
buffers. Unfortunately similar functionality is not available in babel.

I attach a patch that would allow ess-eval-buffer from ob-R.el to access
the location of the currently executed block through a temporary stored
marker in org-babel-current-exec-src-block-head.

    Vitalie

        Modified   lisp/ob-core.el
diff --git a/lisp/ob-core.el b/lisp/ob-core.el
index 8d26c4e..653975a 100644
--- a/lisp/ob-core.el
+++ b/lisp/ob-core.el
@@ -251,7 +251,7 @@ references; a process which could likely result in the 
execution
 of other code blocks.
 
 Returns a list
- (language body header-arguments-alist switches name indent)."
+ (language body header-arguments-alist switches name indent block-head)."
   (let ((case-fold-search t) head info name indent)
     ;; full code block
     (if (setq head (org-babel-where-is-src-block-head))
@@ -274,7 +274,7 @@ Returns a list
     ;; resolve variable references and add summary parameters
     (when (and info (not light))
       (setf (nth 2 info) (org-babel-process-params (nth 2 info))))
-    (when info (append info (list name indent)))))
+    (when info (append info (list name indent head)))))
 
 (defvar org-current-export-file) ; dynamically bound
 (defmacro org-babel-check-confirm-evaluate (info &rest body)
@@ -535,6 +535,8 @@ can not be resolved.")
 
 ;;; functions
 (defvar call-process-region)
+(defvar org-babel-current-exec-src-block-head nil
+  "Marker to the currently processed src block.")
 
 ;;;###autoload
 (defun org-babel-execute-src-block (&optional arg info params)
@@ -562,6 +564,7 @@ block."
       (let* ((params (if params
                         (org-babel-process-params merged-params)
                       (nth 2 info)))
+            (org-babel-current-exec-src-block-head (nth 6 info))
             (cachep (and (not arg) (cdr (assoc :cache params))
                           (string= "yes" (cdr (assoc :cache params)))))
             (new-hash (when cachep (org-babel-sha1-hash info)))
@@ -1592,7 +1595,7 @@ If the point is not on a source block then return nil."
         (< top initial) (< initial bottom)
         (progn (goto-char top) (beginning-of-line 1)
               (looking-at org-babel-src-block-regexp))
-        (point))))))
+        (point-marker))))))
 
 ;;;###autoload
 (defun org-babel-goto-src-block-head ()