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.
* testing/lisp/test-ob-tangle.el (ob-tangle/tangle-to-self): New test.

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.
+
 *** iCalendar export now supports multiline =SUMMARY=, =LOCATION=, and =DESCRIPTION= properties
 
 Previously, it was not possible to specify multi-line location,

lisp/ob-tangle.el

@@ -256,7 +256,8 @@ matching a regular expression."
 	     (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 @@ matching a regular expression."
                                       (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)

testing/lisp/test-ob-tangle.el

@@ -553,6 +553,20 @@ another block
                     (org-split-string (buffer-string))))
               (delete-file file))))))
 
+(ert-deftest ob-tangle/tangle-to-self ()
+  "Do not allow tangling into self."
+  (let ((file (make-temp-file "org-tangle-" nil ".org")))
+    (unwind-protect
+        (with-current-buffer (find-file-noselect file)
+          (insert
+           (format "
+#+begin_src elisp :tangle %s
+2
+#+end_src
+" file))
+          (should-error (org-babel-tangle)))
+      (delete-file file))))
+
 (ert-deftest ob-tangle/detangle-false-positive ()
   "Test handling of false positive link during detangle."
   (let (buffer)