fix(plantuml): {org-,}plantuml-jar-path resolution

- Ensures `org-plantuml-jar-path` is less likely to be overwritten if the
  user changes it.
- Sets `org-plantuml-jar-path` and `plantuml-default-exec-mode` after
  calling `plantuml-download-jar`, so things Just Work™️ without
  needing the user to know to update these variables.

Close: #7992

modules/lang/plantuml/config.el

@@ -3,15 +3,24 @@
 (use-package! plantuml-mode
   :commands plantuml-download-jar
   :init
-  (setq plantuml-jar-path (concat doom-data-dir "plantuml.jar")
+  (setq plantuml-jar-path (concat doom-data-dir "plantuml.jar"))
-        org-plantuml-jar-path plantuml-jar-path)
   :config
   (set-popup-rule! "^\\*PLANTUML" :size 0.4 :select nil :ttl 0)
-
   (setq plantuml-default-exec-mode
         (cond ((file-exists-p plantuml-jar-path) 'jar)
-              ((executable-find "plantuml") 'executable)
+              ((executable-find plantuml-executable-path) 'executable)
-              (plantuml-default-exec-mode))))
+              (plantuml-default-exec-mode)))
+  ;; HACK: If plantuml-jar-path is missing at startup, then plantuml-mode will
+  ;;   operate in another `plantuml-default-exec-mode'. After using
+  ;;   plantuml-download-jar, we change it to `jar' just for this session.
+  (defadvice! +plantuml--use-downloaded-jar-a (fn &rest args)
+    "Configure plantuml-mode to use the downloaded jar for this session."
+    :around #'plantuml-download-jar
+    (let ((downloaded? (not (file-exists-p plantuml-jar-path))))
+      (prog1 (apply fn args)
+        (when (and downloaded? (file-exists-p plantuml-jar-path))
+          (setq org-plantuml-jar-path plantuml-jar-path
+                plantuml-default-exec-mode 'jar))))))
 
 
 (use-package! flycheck-plantuml
@@ -25,8 +34,14 @@
 
 
 (after! ob-plantuml
-  ;; HACK Force ob-plantuml to use `plantuml-mode''s building mechanism, which
+  ;; The nested `after!' is needed to ensure `org-plantuml-jar-path's new
-  ;;      is more sophisticated.
+  ;; default without overwriting any user config.
+  (after! plantuml-mode
+    (when (equal org-plantuml-jar-path "")
+      (setq org-plantuml-jar-path plantuml-jar-path)))
+
+  ;; HACK: Force ob-plantuml to use `plantuml-mode''s building mechanism, which
+  ;;   is more sophisticated.
   (advice-add #'org-babel-execute:plantuml
               :override #'+plantuml-org-babel-execute:plantuml-a)
   (add-to-list 'org-babel-default-header-args:plantuml