From 9b690462a319e4268f15949bba0fe024dca77182 Mon Sep 17 00:00:00 2001
From: Jonas Bernoulli <jonas@bernoul.li>
Date: Thu, 6 Oct 2022 13:11:06 +0200
Subject: [PATCH] Allow returning empty list from post-processing block

* lisp/ob-ref.el (org-babel-ref-resolve): When the result an empty
list, then treat it as a list, not as the symbol nil.
* testing/lisp/test-ob.el (test-ob/post-header-arguments): Add new
test.
* etc/ORG-NEWS (Post-processing code blocks can return an empty list):
Document change in behavior.
---
 etc/ORG-NEWS            |  6 ++++++
 lisp/ob-ref.el          |  2 +-
 testing/lisp/test-ob.el | 24 ++++++++++++++++++++++++
 3 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/etc/ORG-NEWS b/etc/ORG-NEWS
index 4728528f8..74095d101 100644
--- a/etc/ORG-NEWS
+++ b/etc/ORG-NEWS
@@ -425,6 +425,12 @@ The new variable name is =org-plantuml-args=.  It now applies to both
 jar PlantUML file and executable.
 
 ** Miscellaneous
+*** Post-processing code blocks can return an empty list
+
+When the result of a regular code block is nil, then that was already
+treated as an empty list.  Now that is also the case for code blocks
+that post-process the result of another block.
+
 *** Styles are customizable in ~biblatex~ citation processor
 
 It is now possible to add new styles or modify old ones in ~biblatex~
diff --git a/lisp/ob-ref.el b/lisp/ob-ref.el
index b79e47900..2b4a16aea 100644
--- a/lisp/ob-ref.el
+++ b/lisp/ob-ref.el
@@ -199,7 +199,7 @@ Emacs Lisp representation of the value of the variable."
 				(org-babel-execute-src-block nil info params))))
 		     (error "Reference `%s' not found in this buffer" ref))))
 	     (cond
-	      ((symbolp result) (format "%S" result))
+	      ((and result (symbolp result)) (format "%S" result))
 	      ((and index (listp result))
 	       (org-babel-ref-index-list index result))
 	      (t result)))))))))
diff --git a/testing/lisp/test-ob.el b/testing/lisp/test-ob.el
index 99912fe63..9051a642d 100644
--- a/testing/lisp/test-ob.el
+++ b/testing/lisp/test-ob.el
@@ -136,6 +136,30 @@ should still return the link."
     (let ((info (org-babel-get-src-block-info)))
       (should (string= "no" (cdr (assq :tangle (nth 2 info))))))))
 
+(ert-deftest test-ob/post-header-arguments ()
+  "When the result of a post-processing source block is an empty
+list, then it should be treated as such; not as the symbol nil."
+  (should
+   (let ((default-directory temporary-file-directory))
+     (org-test-with-temp-text
+         "
+#+name: addheader
+#+header: :var rows=\"\"
+#+begin_src elisp :hlines yes
+  '()
+#+end_src
+#+header: :post addheader(*this*)
+#+<point>begin_src emacs-lisp :results table
+#+end_src
+#+RESULTS:
+: nil"
+       (org-babel-execute-src-block)
+       (goto-char (1- (point-max)))
+       (equal (buffer-substring-no-properties
+               (line-beginning-position)
+               (line-end-position))
+              "#+RESULTS:")))))
+
 (ert-deftest test-ob/elisp-in-header-arguments ()
   "Test execution of elisp forms in header arguments."
   (org-test-with-temp-text-in-file "