org-element: Implement lazy cache synchronization

* lisp/org-element.el (org-element-cache-sync-idle-time): Change
  default value.
(org-element-cache-sync-duration, org-element-cache-sync-break,
org-element--cache-sync-requests, org-element--cache-sync-timer,
org-element--cache-sync-keys, org-element--cache-default-key,
org-element--cache-change-warning): New variables.
(org-element-cache-merge-changes-threshold,
org-element--cache-status): Removed variables.
(org-element--cache-key, org-element--cache-generate-key,
org-element--cache-key-less-p, org-element--cache-find,
org-element--cache-set-timer, org-element--cache-process-request,
org-element--cache-submit-request, org-element--parse-to,
org-element--cache-interrupt-p, org-element--cache-put,
org-element--cache-active-p): New functions.
(org-element--cache-compare): Adapt to new keys in AVL tree.
(org-element--cache-pending-changes-p,
org-element--cache-cancel-changes, org-element--cache-mapc,
org-element-cache-get, org-element-cache-put): Removed functions.
(org-element--cache-before-change): Use new variables.
(org-element--cache-after-change): Renamed from
`org-element--cache-record-change'.
(org-element-cache-get): Change signature.
(org-element-cache-put): Rewrite function.  Use new tools.
(org-element-cache-reset): Adapt to new variables.
(org-element--cache-sync): Rewrite function.

* lisp/ox.el (org-export--generate-copy-script): Do not copy through
  new cache-related variables.
(org-export-ignored-local-variables): New variable.

* testing/lisp/test-org-element.el (test-org-element/cache): New test.

Now only the part of the cache that needs to be accessed is updated
synchronously.  Otherwise, it happens on idle time.

lisp/ox.el

@@ -263,6 +263,16 @@ whose extension is either \"png\", \"jpeg\", \"jpg\", \"gif\",
 See `org-export-inline-image-p' for more information about
 rules.")
 
+(defconst org-export-ignored-local-variables
+  '(org-font-lock-keywords
+    org-element--cache org-element--cache-objects org-element--cache-sync-keys
+    org-element--cache-sync-requests org-element--cache-sync-timer)
+  "List of variables not copied through upon buffer duplication.
+Export process takes place on a copy of the original buffer.
+When this copy is created, all Org related local variables not in
+this list are copied to the new buffer.  Variables with an
+unreadable value are also ignored.")
+
 (defvar org-export-async-debug nil
   "Non-nil means asynchronous export process should leave data behind.
 
@@ -2964,11 +2974,7 @@ The function assumes BUFFER's major mode is `org-mode'."
 	       (when (consp entry)
 		 (let ((var (car entry))
 		       (val (cdr entry)))
-		   (and (not (memq var '(org-font-lock-keywords
-					 ;; Do not share cache across
-					 ;; buffers as values are
-					 ;; modified by side effect.
-					 org-element--cache)))
+		   (and (not (memq var org-export-ignored-local-variables))
 			(or (memq var
 				  '(default-directory
 				     buffer-file-name

testing/lisp/test-org-element.el

@@ -3036,5 +3036,116 @@ Paragraph \\alpha."
 	   (org-element-type (org-element-context (org-element-at-point))))))))
 
 
+
+;;; Test Cache.
+
+(ert-deftest test-org-element/cache ()
+  "Test basic expectations and common pitfalls for cache."
+  ;; Shift positions.
+  (should
+   (equal '(18 . 23)
+	  (org-test-with-temp-text "para1\n\npara2\n\npara3"
+	    (let ((org-element-use-cache t))
+	      (save-excursion (goto-char (point-max)) (org-element-at-point))
+	      (insert "add")
+	      (forward-line 4)
+	      (let ((element (org-element-at-point)))
+		(cons (org-element-property :begin element)
+		      (org-element-property :end element)))))))
+  ;; Partial shifting: when the contents of a greater element are
+  ;; modified, only shift ending positions.
+  (should
+   (org-test-with-temp-text
+       "#+BEGIN_CENTER\nPara1\n\nPara2\n\nPara3\n#+END_CENTER"
+     (let ((org-element-use-cache t))
+       (save-excursion (search-forward "3") (org-element-at-point))
+       (search-forward "Para2")
+       (insert " ")
+       (let ((element (org-element-property :parent (org-element-at-point))))
+	 (equal (cons (org-element-property :begin element)
+		      (org-element-property :end element))
+		(cons (point-min) (point-max)))))))
+  ;; Re-parent shifted elements.
+  (should
+   (eq 'item
+       (org-test-with-temp-text "- item\n\n\n  para1\n  para2"
+	 (let ((org-element-use-cache t))
+	   (end-of-line)
+	   (org-element-at-point)
+	   (save-excursion (goto-char (point-max)) (org-element-at-point))
+	   (forward-line)
+	   (delete-char 1)
+	   (goto-char (point-max))
+	   (org-element-type
+	    (org-element-property :parent (org-element-at-point)))))))
+  ;; Modifying the last line of an element alters the element below.
+  (should
+   (org-test-with-temp-text "para1\n\npara2"
+     (let ((org-element-use-cache t))
+       (goto-char (point-max))
+       (org-element-at-point)
+       (forward-line -1)
+       (insert "merge")
+       (let ((element (org-element-at-point)))
+	 (equal (cons (org-element-property :begin element)
+		      (org-element-property :end element))
+		(cons (point-min) (point-max)))))))
+  ;; Modifying the first line of an element alters the element above.
+  (should
+   (org-test-with-temp-text ": fixed-width\n:not-fixed-width"
+     (let ((org-element-use-cache t))
+       (goto-char (point-max))
+       (org-element-at-point)
+       (search-backward ":")
+       (forward-char)
+       (insert " ")
+       (let ((element (org-element-at-point)))
+	 (equal (cons (org-element-property :begin element)
+		      (org-element-property :end element))
+		(cons (point-min) (point-max)))))))
+  ;; Sensitive change: adding a line alters document structure both
+  ;; above and below.
+  (should
+   (eq 'example-block
+       (org-test-with-temp-text "#+BEGIN_EXAMPLE\nPara1\n\nPara2\n"
+	 (let ((org-element-use-cache t))
+	   (goto-char (point-max))
+	   (org-element-at-point)
+	   (insert "#+END_EXAMPLE")
+	   (search-backward "Para1")
+	   (org-element-type (org-element-at-point))))))
+  (should
+   (eq 'example-block
+       (org-test-with-temp-text "Para1\n\nPara2\n#+END_EXAMPLE"
+	 (let ((org-element-use-cache t))
+	   (save-excursion (goto-char (point-max)) (org-element-at-point))
+	   (insert "#+BEGIN_EXAMPLE\n")
+	   (search-forward "Para2")
+	   (org-element-type (org-element-at-point))))))
+  ;; Sensitive change: removing a line alters document structure both
+  ;; above and below.
+  (should
+   (eq 'example-block
+       (org-test-with-temp-text
+	   "# +BEGIN_EXAMPLE\nPara1\n\nPara2\n#+END_EXAMPLE"
+	 (let ((org-element-use-cache t))
+	   (save-excursion (goto-char (point-max)) (org-element-at-point))
+	   (forward-char)
+	   (delete-char 1)
+	   (search-forward "Para2")
+	   (org-element-type (org-element-at-point))))))
+  (should
+   (eq 'example-block
+       (org-test-with-temp-text
+	   "#+BEGIN_EXAMPLE\nPara1\n\nPara2\n# +END_EXAMPLE"
+	 (let ((org-element-use-cache t))
+	   (save-excursion (goto-char (point-max)) (org-element-at-point))
+	   (search-forward "# ")
+	   (delete-char -1)
+	   (search-backward "Para1")
+	   (org-element-type (org-element-at-point)))))))
+
+
 (provide 'test-org-element)
+
 ;;; test-org-element.el ends here