Fix `org-backward-paragraph' on block opening line

* lisp/org.el (org-backward-paragraph): Do not error when called from
  a block opening line.
* testing/lisp/test-org.el (test-org/backward-paragraph): Add tests.

Reported-by: Omar Antolin <omar.antolin@gmail.com>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-10/msg00051.html>

lisp/org.el

@@ -24205,16 +24205,25 @@ convenience:
 
   - On an affiliated keyword, jump to the first one.
   - On a table or a property drawer, move to its beginning.
-  - On a verse or source block, stop before blank lines."
+  - On comment, example, export, src and verse blocks, stop
+    before blank lines."
   (interactive)
   (unless (bobp)
     (let* ((deactivate-mark nil)
 	   (element (org-element-at-point))
 	   (type (org-element-type element))
-	   (contents-begin (org-element-property :contents-begin element))
 	   (contents-end (org-element-property :contents-end element))
 	   (post-affiliated (org-element-property :post-affiliated element))
-	   (begin (org-element-property :begin element)))
+	   (begin (org-element-property :begin element))
+	   (special?			;blocks handled specially
+	    (memq type '(comment-block example-block export-block src-block
+				       verse-block)))
+	   (contents-begin
+	    (if special?
+		;; These types have no proper contents.  Fake line
+		;; below the block opening line as contents beginning.
+		(save-excursion (goto-char begin) (line-beginning-position 2))
+	      (org-element-property :contents-begin element))))
       (cond
        ((not element) (goto-char (point-min)))
        ((= (point) begin)
@@ -24225,11 +24234,8 @@ convenience:
 	(goto-char (org-element-property
 		    :post-affiliated (org-element-property :parent element))))
        ((memq type '(property-drawer table)) (goto-char begin))
-       ((memq type '(src-block verse-block))
-	(when (eq type 'src-block)
-	  (setq contents-begin
-		(save-excursion (goto-char begin) (forward-line) (point))))
-	(if (= (point) contents-begin) (goto-char post-affiliated)
+       (special?
+	(if (<= (point) contents-begin) (goto-char post-affiliated)
 	  ;; Inside a verse block, see blank lines as paragraph
 	  ;; separators.
 	  (let ((origin (point)))
@@ -24238,7 +24244,6 @@ convenience:
 	      (skip-chars-forward " \r\t\n" origin)
 	      (if (= (point) origin) (goto-char contents-begin)
 		(beginning-of-line))))))
-       ((not contents-begin) (goto-char (or post-affiliated begin)))
        ((eq type 'paragraph)
 	(goto-char contents-begin)
 	;; When at first paragraph in an item or a footnote definition,

testing/lisp/test-org.el

@@ -3536,23 +3536,18 @@ SCHEDULED: <2017-05-06 Sat>
      t))
   ;; Regular test.
   (should
-   (org-test-with-temp-text "P1\n\nP2\n\nP3"
-     (goto-char (point-max))
+   (org-test-with-temp-text "P1\n\nP2\n\nP3<point>"
      (org-backward-paragraph)
      (looking-at "P3")))
   (should
-   (org-test-with-temp-text "P1\n\nP2\n\nP3"
-     (goto-char (point-max))
-     (beginning-of-line)
+   (org-test-with-temp-text "P1\n\nP2\n\n<point>P3"
      (org-backward-paragraph)
-     (looking-at "P2")))
+     (looking-at-p "P2")))
   ;; Ignore depth.
   (should
-   (org-test-with-temp-text "P1\n\n#+BEGIN_CENTER\nP2\n#+END_CENTER\nP3"
-     (goto-char (point-max))
-     (beginning-of-line)
+   (org-test-with-temp-text "P1\n\n#+BEGIN_CENTER\nP2\n#+END_CENTER\n<point>P3"
      (org-backward-paragraph)
-     (looking-at "P2")))
+     (looking-at-p "P2")))
   ;; Ignore invisible elements.
   (should
    (org-test-with-temp-text "* H1\n  P1\n* H2"
@@ -3563,49 +3558,56 @@ SCHEDULED: <2017-05-06 Sat>
      (bobp)))
   ;; On an affiliated keyword, jump to the first one.
   (should
-   (org-test-with-temp-text "P1\n#+name: n\n#+caption: c1\n#+caption: c2\nP2"
-     (search-forward "c2")
+   (org-test-with-temp-text
+       "P1\n#+name: n\n#+caption: c1\n#+caption: <point>c2\nP2"
      (org-backward-paragraph)
-     (looking-at "#\\+name")))
+     (looking-at-p "#\\+name")))
   ;; On the second element in an item or a footnote definition, jump
   ;; to item or the definition.
   (should
-   (org-test-with-temp-text "- line1\n\n  line2"
-     (goto-char (point-max))
-     (beginning-of-line)
+   (org-test-with-temp-text "- line1\n\n<point>  line2"
      (org-backward-paragraph)
-     (looking-at "- line1")))
+     (looking-at-p "- line1")))
   (should
-   (org-test-with-temp-text "[fn:1] line1\n\n  line2"
-     (goto-char (point-max))
-     (beginning-of-line)
+   (org-test-with-temp-text "[fn:1] line1\n\n<point>  line2"
      (org-backward-paragraph)
-     (looking-at "\\[fn:1\\] line1")))
+     (looking-at-p "\\[fn:1\\] line1")))
   ;; On a table (resp. a property drawer), ignore table rows
   ;; (resp. node properties).
   (should
-   (org-test-with-temp-text "| a | b |\n| c | d |\nP1"
-     (goto-char (point-max))
-     (beginning-of-line)
+   (org-test-with-temp-text "| a | b |\n| c | d |\n<point>P1"
      (org-backward-paragraph)
      (bobp)))
   (should
    (org-test-with-temp-text "* H\n:PROPERTIES:\n:prop: value\n:END:\n<point>P1"
      (org-backward-paragraph)
-     (looking-at ":PROPERTIES:")))
-  ;; On a source or verse block, stop before blank lines.
+     (looking-at-p ":PROPERTIES:")))
+  ;; On a comment, example, src and verse blocks, stop before blank
+  ;; lines.
   (should
-   (org-test-with-temp-text "#+BEGIN_VERSE\nL1\n\nL2\n\nL3\n#+END_VERSE"
-     (search-forward "L3")
-     (beginning-of-line)
+   (org-test-with-temp-text "#+BEGIN_VERSE\nL1\n\nL2\n\n<point>L3\n#+END_VERSE"
      (org-backward-paragraph)
-     (looking-at "L2")))
+     (looking-at-p "L2")))
   (should
-   (org-test-with-temp-text "#+BEGIN_SRC\nL1\n\nL2\n\nL3#+END_SRC"
-     (search-forward "L3")
-     (beginning-of-line)
+   (org-test-with-temp-text "#+BEGIN_SRC\nL1\n\nL2\n\n<point>L3#+END_SRC"
      (org-backward-paragraph)
-     (looking-at "L2"))))
+     (looking-at-p "L2")))
+  ;; In comment, example, export, src and verse blocks, stop below
+  ;; opening line when called from within the block.
+  (should
+   (org-test-with-temp-text "#+BEGIN_VERSE\nL1\nL2<point>\n#+END_VERSE"
+     (org-backward-paragraph)
+     (looking-at-p "L1")))
+  (should
+   (org-test-with-temp-text "#+BEGIN_EXAMPLE\nL1\nL2<point>\n#+END_EXAMPLE"
+     (org-backward-paragraph)
+     (looking-at-p "L1")))
+  ;; When called from the opening line itself, however, move to
+  ;; beginning of block.
+  (should
+   (org-test-with-temp-text "#+BEGIN_<point>EXAMPLE\nL1\n#+END_EXAMPLE"
+     (org-backward-paragraph)
+     (bobp))))
 
 (ert-deftest test-org/forward-element ()
   "Test `org-forward-element' specifications."