babel: :eval header argument takes arguments "never" and "query" to limit evaluation

* lisp/ob.el (org-babel-confirm-evaluate): adding a new :eval header
  argument which can be used to control evaluation

* doc/org.texi (eval): adding documentation for the new :eval header
  argument

doc/org.texi

@@ -11411,13 +11411,17 @@ The following header arguments are defined:
 
 @menu
 * var::				Pass arguments to code blocks
-* results::			Specify the type of results and how they will be collectd and handled
+* results::			Specify the type of results and how they will
+                                be collectd and handled
 * file::			Specify a path for file output
-* dir and remote execution::	Specify the default directory for code block execution
+* dir and remote execution::	Specify the default directory for code block
+                                execution
 * exports::			Export code and/or results
 * tangle::			Toggle tangling and specify file name
-* no-expand::			Turn off variable assignment and noweb expansion during tangling
-* comments::                    Toggle insertion of comments in tangled code files
+* no-expand::			Turn off variable assignment and noweb
+                                expansion during tangling
+* comments::                    Toggle insertion of comments in tangled
+                                code files
 * session::			Preserve the state of code evaluation
 * noweb::			Toggle expansion of noweb references
 * cache::			Avoid re-evaluating unchanged code blocks
@@ -11425,6 +11429,7 @@ The following header arguments are defined:
 * colnames::			Handle column names in tables
 * rownames::			Handle row names in tables
 * shebang::			Make tangled files executable
+* eval::                        Limit evaluation of specific code blocks
 @end menu
 
 @node var, results, Specific header arguments, Specific header arguments
@@ -12003,7 +12008,7 @@ and is then reapplied to the results.
 @end example
 @end itemize
 
-@node shebang,  , rownames, Specific header arguments
+@node shebang, eval, rownames, Specific header arguments
 @subsubsection @code{:shebang}
 
 Setting the @code{:shebang} header argument to a string value
@@ -12011,6 +12016,16 @@ Setting the @code{:shebang} header argument to a string value
 first line of any tangled file holding the code block, and the file
 permissions of the tangled file are set to make it executable.
 
+@node eval, , shebang, Specific header arguments
+@subsubsection @code{:eval}
+The @code{:eval} header argument can be used to limit the evaluation of
+specific code blocks.  @code{:eval} accepts two arguments ``never'' and
+``query''.  @code{:eval never} will ensure that a code block is never
+evaluated, this can be useful for protecting against the evaluation of
+dangerous code blocks.  @code{:eval query} will require a query for every
+execution of a code block regardless of the value of the
+@code{org-confirm-babel-evaluate} variable.
+
 @node Results of evaluation, Noweb reference syntax, Header arguments, Working With Source Code
 @section Results of evaluation
 @cindex code block, results of evaluation

lisp/ob.el

@@ -171,14 +171,20 @@ any confirmation from the user.
 
 Note disabling confirmation may result in accidental evaluation
 of potentially harmful code."
-  (unless (or (not (if (functionp org-confirm-babel-evaluate)
-		       (funcall org-confirm-babel-evaluate
-				(nth 0 info) (nth 1 info))
-		     org-confirm-babel-evaluate))
-	      (yes-or-no-p
-	       (format "Evaluate this%scode on your system?"
-		       (if info (format " %s " (nth 0 info)) " "))))
-    (error "evaluation aborted")))
+  (let ((eval (cdr (assoc :eval (nth 2 info)))))
+    (when (or (equal eval "never")
+	      (and (equal eval "query")
+		   (not (yes-or-no-p
+			(format "Evaluate this%scode on your system?"
+				(if info (format " %s " (nth 0 info)) " ")))))
+	      (and (or (and (functionp org-confirm-babel-evaluate)
+			    (funcall org-confirm-babel-evaluate
+				     (nth 0 info) (nth 1 info)))
+		       org-confirm-babel-evaluate)
+		   (not (yes-or-no-p
+			 (format "Evaluate this%scode on your system?"
+				 (if info (format " %s " (nth 0 info)) " "))))))
+      (error "evaluation aborted"))))
 
 ;;;###autoload
 (defun org-babel-execute-src-block-maybe ()