ob-core.el/babel: Special handling for attachment links in src block

* ob-core.el (org-babel-merge-params): Specifying the symbol 'attach`
or string "'attach" as the value of the `:dir' header now functions as
":dir (org-attach-dir nil t) :mkdirp t".
(org-babel-result-to-file): Optional TYPE argument accepts symbol
'attachment to fixup up paths under `(org-attach-dir)' and use the
link type "attachment:" when that is detected.
(org-babel-insert-result): Pass symbol `attachment' as TYPE to
`org-babel-result-to-file'.
* org-attach.el (org-attach-dir): Added autoload header to simplify
dependencies necessary to support this feature (called in
`org-babel-merge-params').
* test-ob.el (test-ob-core/dir-attach): Added unit test for the new
attach feature.

doc/org-manual.org

@@ -17542,6 +17542,13 @@ directory with {{{kbd(M-x cd RET DIRECTORY)}}}, and then not setting
 variable ~default-directory~.  Setting =mkdirp= header argument to
 a non-~nil~ value creates the directory, if necessary.
 
+Setting =dir= to the symbol ~attach~ or the string ~"'attach"~ will
+set =dir= to the directory returned by ~(org-attach-dir)~, set =:mkdir
+yes=, and insert any file paths, as when using =:results file=, which
+are under the node's attachment directory using =attachment:= links
+instead of the usual =file:= links.  Any returned path outside of the
+attachment directory will use =file:= links as per usual.
+
 For example, to save the plot file in the =Work/= folder of the home
 directory---notice tilde is expanded:
 

etc/ORG-NEWS

@@ -798,6 +798,13 @@ Finally, the closures are only evaluated if they're not overridden for
 a source block. This improves efficiency in cases where the result of
 a compute-expensive closure would otherwise be discarded.
 
+*** New special value ~'attach~ for src block =:dir= option
+
+Passing the symbol ~attach~ or string ="'attach"= (with quotes) to the =:dir=
+option of a src block is now equivalent to =:dir (org-attach-dir) :mkdir yes=
+and any file results with a path descended from the attachment directory will
+use =attachment:= style links instead of the standard =file:= link type.
+
 ** Miscellaneous
 *** =org-bibtex= includes =doi= and =url= entries when exporting to BiBTeX
 =doi= and =url= entries have been made optional for some publication

lisp/ob-core.el

@@ -801,7 +801,8 @@ block."
 		    (let ((*this* (if (not file) result
 				    (org-babel-result-to-file
 				     file
-				     (org-babel--file-desc params result)))))
+				     (org-babel--file-desc params result)
+                                     'attachment))))
 		      (setq result (org-babel-ref-resolve post))
 		      (when file
 			(setq result-params (remove "file" result-params))))))
@@ -2298,11 +2299,14 @@ INFO may provide the values of these header arguments (in the
   (cond ((stringp result)
 	 (setq result (org-no-properties result))
 	 (when (member "file" result-params)
-	   (setq result (org-babel-result-to-file
-			 result
-			 (org-babel--file-desc (nth 2 info) result)))))
+	   (setq result
+                 (org-babel-result-to-file
+		  result
+		  (org-babel--file-desc (nth 2 info) result)
+                  'attachment))))
 	((listp result))
 	(t (setq result (format "%S" result))))
+
   (if (and result-params (member "silent" result-params))
       (progn (message (replace-regexp-in-string "%" "%%" (format "%S" result)))
 	     result)
@@ -2605,27 +2609,49 @@ in the buffer."
 		 (line-beginning-position 2))
 	     (point))))))
 
-(defun org-babel-result-to-file (result &optional description)
+(defun org-babel-result-to-file (result &optional description type)
   "Convert RESULT into an Org link with optional DESCRIPTION.
 If the `default-directory' is different from the containing
-file's directory then expand relative links."
+file's directory then expand relative links.
+
+If the optional TYPE is passed as 'attachment` and the path is a
+descendant of the DEFAULT-DIRECTORY, the generated link will be
+specified as an an \"attachment:\" style link."
   (when (stringp result)
-    (let ((same-directory?
-	   (and (buffer-file-name (buffer-base-buffer))
-		(not (string= (expand-file-name default-directory)
-			      (expand-file-name
-			       (file-name-directory
-			        (buffer-file-name (buffer-base-buffer)))))))))
-      (format "[[file:%s]%s]"
-	      (if (and default-directory
-		       (buffer-file-name (buffer-base-buffer)) same-directory?)
-		  (if (eq org-link-file-path-type 'adaptive)
-		      (file-relative-name
-		       (expand-file-name result default-directory)
-		       (file-name-directory
-			(buffer-file-name (buffer-base-buffer))))
-		    (expand-file-name result default-directory))
-		result)
+    (let* ((result-file-name (expand-file-name result))
+           (base-file-name (buffer-file-name (buffer-base-buffer)))
+           (base-directory (and buffer-file-name
+                                (file-name-directory base-file-name)))
+           (same-directory?
+	    (and base-file-name
+	         (not (string= (expand-file-name default-directory)
+			       (expand-file-name
+			        base-directory)))))
+           (request-attachment (eq type 'attachment))
+           (attach-dir (let* ((default-directory base-directory)
+                              (dir (org-attach-dir nil t)))
+                         (when dir
+                           (expand-file-name dir))))
+           (in-attach-dir (and request-attachment
+                               attach-dir
+                               (string-prefix-p
+                                attach-dir
+                                result-file-name))))
+      (format "[[%s:%s]%s]"
+              (pcase type
+                ((and 'attachment (guard in-attach-dir)) "attachment")
+                (_ "file"))
+              (if (and request-attachment in-attach-dir)
+                  (file-relative-name result-file-name)
+	        (if (and default-directory
+		         base-file-name same-directory?)
+		    (if (eq org-link-file-path-type 'adaptive)
+		        (file-relative-name
+		         result-file-name
+                         (file-name-directory
+			  base-file-name))
+		      result-file-name)
+		  result))
 	      (if description (concat "[" description "]") "")))))
 
 (defun org-babel-examplify-region (beg end &optional results-switches inline)
@@ -2756,10 +2782,17 @@ parameters when merging lists."
 	   (setq exports (funcall merge
 				  exports-exclusive-groups
 				  exports
-				  (split-string
+                                  (split-string
                                    (cond ((and value (functionp value)) (funcall value))
                                          (value value)
                                          (t ""))))))
+          ((or '(:dir . attach) '(:dir . "'attach"))
+           (unless (org-attach-dir nil t)
+             (error "No attachment directory for element (add :ID: or :DIR: property)"))
+           (setq params (append
+                         `((:dir . ,(org-attach-dir nil t))
+                           (:mkdirp . "yes"))
+                         (assq-delete-all :dir (assq-delete-all :mkdir params)))))
 	  ;; Regular keywords: any value overwrites the previous one.
 	  (_ (setq params (cons pair (assq-delete-all (car pair) params)))))))
     ;; Handle `:var' and clear out colnames and rownames for replaced

lisp/org-attach.el

@@ -324,6 +324,7 @@ Shows a list of commands and prompts for another key to execute a command."
 	    (command-execute command)
 	  (error "No such attachment command: %c" c))))))
 
+;;;###autoload
 (defun org-attach-dir (&optional create-if-not-exists-p no-fs-check)
   "Return the directory associated with the current outline node.
 First check for DIR property, then ID property.

testing/lisp/test-ob.el

@@ -1770,6 +1770,72 @@ nil
                 (file-modes "t.sh")
               (delete-file "t.sh"))))))
 
+(ert-deftest test-ob-core/dir-attach ()
+  "Test :dir header using special 'attach value"
+  (should
+   (org-test-with-temp-text-in-file
+    "* 'attach Symbol
+<point>#+begin_src elisp :dir 'attach :results file
+(with-temp-file \"test.txt\" (insert \"attachment testing\n\"))
+\"test.txt\"
+#+end_src"
+    (org-id-get-create)
+    (org-babel-execute-src-block)
+    (goto-char (org-babel-where-is-src-block-result))
+    (forward-line)
+    (and
+     (file-exists-p (format "%s/test.txt" (org-attach-dir nil t)))
+     (string= (buffer-substring-no-properties (point) (line-end-position))
+              "[[attachment:test.txt]]"))))
+  (should
+   (org-test-with-temp-text-in-file
+    "* 'attach String
+<point>#+begin_src elisp :dir \"'attach\" :results file
+(with-temp-file \"test.txt\" (insert \"attachment testing\n\"))
+\"test.txt\"
+#+end_src"
+    (org-id-get-create)
+    (org-babel-execute-src-block)
+    (goto-char (org-babel-where-is-src-block-result))
+    (forward-line)
+    (and
+     (file-exists-p (format "%s/test.txt" (org-attach-dir nil t)))
+     (string= (buffer-substring-no-properties (point) (line-end-position))
+              "[[attachment:test.txt]]"))))
+  (should
+   (org-test-with-temp-text-in-file
+    "* 'attach with Existing DIR property
+:PROPERTIES:
+:DIR:      custom-attach-dir
+:END:
+
+<point>#+begin_src elisp :dir 'attach :results file
+(with-temp-file \"test.txt\" (insert \"attachment testing\n\"))
+\"test.txt\"
+#+end_src"
+    (message "DIR: %s" (org-attach-dir t))
+    (org-babel-execute-src-block)
+    (goto-char (org-babel-where-is-src-block-result))
+    (forward-line)
+    (and
+     (file-exists-p (format "%s/test.txt" (org-attach-dir nil t)))
+     (string= (buffer-substring-no-properties (point) (line-end-position))
+              "[[attachment:test.txt]]"))))
+  (should-error
+   (org-test-with-temp-text-in-file
+    "* 'attach with no ID or DIR
+<point>#+begin_src elisp :dir 'attach :results file
+(with-temp-file \"test.txt\" (insert \"attachment testing\n\"))
+\"test.txt\"
+#+end_src"
+    (org-babel-execute-src-block)
+    (goto-char (org-babel-where-is-src-block-result))
+    (forward-line)
+    (and
+     (file-exists-p (format "%s/test.txt" (org-attach-dir nil t)))
+     (string= (buffer-substring-no-properties (point) (line-end-position))
+              "[[attachment:test.txt]]")))))
+
 (ert-deftest test-ob-core/dir-mkdirp ()
   "Test :mkdirp with :dir header combination."
   (should-not