[PATCH] org-babel-tangle: Do not allow tangling into self

[Ihor Radchenko <yantar92@posteo.net>]

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Hello,

We currently allow tangle to shoot its own foot:

file.org:

#+begin_src org :tangle file.org
Overwrite me.
#+end_src

The attached patch makes Org throw an error in such scenario.

I am almost sure that it is not going to break anyone's workflow.
Or will it?..


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From e2b0913930119737bac69d17738d0ebb44f999f7 Mon Sep 17 00:00:00 2001
Message-ID: <e2b0913930119737bac69d17738d0ebb44f999f7.1704729564.git.yantar92@posteo.net>
From: Ihor Radchenko <yantar92@posteo.net>
Date: Mon, 8 Jan 2024 16:58:24 +0100
Subject: [PATCH] org-babel-tangle: Do not allow tangling into self

* lisp/ob-tangle.el (org-babel-tangle): Throw an error when trying to
tangle into the org file we tangle from.
* etc/ORG-NEWS (It is no longer allowed to tangle into the same file
as Org source): Document the breaking change.
---
 etc/ORG-NEWS      | 12 ++++++++++++
 lisp/ob-tangle.el | 10 +++++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/etc/ORG-NEWS b/etc/ORG-NEWS
index b808357d8..a8d14673c 100644
--- a/etc/ORG-NEWS
+++ b/etc/ORG-NEWS
@@ -13,6 +13,18 @@ Please send Org bug reports to mailto:emacs-orgmode@gnu.org.
 
 * Version 9.7 (not released yet)
 ** Important announcements and breaking changes
+*** It is no longer allowed to tangle into the same file as Org source
+
+Previously, =file.org= with the following contents
+
+: #+begin_src org :tangle file.org
+: Text
+: #+end_src
+
+would overwrite itself.
+
+Now, an error is thrown.
+
 *** ~org-agenda-search-headline-for-time~ now ignores all the timestamp in headings
 
 Previously, ~org-agenda-search-headline-for-time~ made Org agenda
diff --git a/lisp/ob-tangle.el b/lisp/ob-tangle.el
index 933a3eba1..914f4b7b6 100644
--- a/lisp/ob-tangle.el
+++ b/lisp/ob-tangle.el
@@ -256,7 +256,8 @@ (defun org-babel-tangle (&optional arg target-file lang-re)
 	     (when (equal arg '(16))
 	       (or (cdr (assq :tangle (nth 2 (org-babel-get-src-block-info 'no-eval))))
 		   (user-error "Point is not in a source code block"))))
-	    path-collector)
+	    path-collector
+            (source-file buffer-file-name))
 	(mapc ;; map over file-names
 	 (lambda (by-fn)
 	   (let ((file-name (car by-fn)))
@@ -313,6 +314,13 @@ (defun org-babel-tangle (&optional arg target-file lang-re)
                                       (compare-buffer-substrings
                                        nil nil nil
                                        tangle-buf nil nil)))))))
+                     (when (equal (if (file-name-absolute-p file-name)
+                                      file-name
+                                    (expand-file-name file-name))
+                                  (if (file-name-absolute-p source-file)
+                                      source-file
+                                    (expand-file-name source-file)))
+                       (error "Not allowed to tangle into the same file as self"))
                      ;; We do not erase, but overwrite previous file
                      ;; to preserve any existing symlinks.
 		     (write-region nil nil file-name)
-- 
2.42.0


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-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>