ox: Throw an error on unresolved id links

* lisp/ox.el (org-export-get-environment): Properly find file
  associated to id link.
(org-export-resolve-id-link): Throw an error on unresolved id links.

Use `org-id-find' instead of `org-id-find-id-file' as the latter may
returns the file attached to current buffer on search failure.

Reported-by: Jacob Gerlach <jacobgerlach@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/96020>

lisp/ox.el

@@ -1371,7 +1371,7 @@ inferior to file-local settings."
 	 (let ((link (org-element-context)))
 	   (when (eq (org-element-type link) 'link)
 	     (let* ((id (org-element-property :path link))
-		    (file (org-id-find-id-file id)))
+		    (file (car (org-id-find id))))
 	       (when file
 		 (push (cons id (file-relative-name file)) alist)))))))
       alist))))
@@ -4105,18 +4105,19 @@ significant."
 INFO is a plist used as a communication channel.
 
 Return value can be the headline element matched in current parse
-tree, a file name or nil.  Assume LINK type is either \"id\" or
-\"custom-id\"."
+tree or a file name.  Assume LINK type is either \"id\" or
+\"custom-id\".  Throw an error if no match is found."
   (let ((id (org-element-property :path link)))
     ;; First check if id is within the current parse tree.
     (or (org-element-map (plist-get info :parse-tree) 'headline
 	  (lambda (headline)
-	    (when (or (string= (org-element-property :ID headline) id)
-		      (string= (org-element-property :CUSTOM_ID headline) id))
+	    (when (or (equal (org-element-property :ID headline) id)
+		      (equal (org-element-property :CUSTOM_ID headline) id))
 	      headline))
 	  info 'first-match)
 	;; Otherwise, look for external files.
-	(cdr (assoc id (plist-get info :id-alist))))))
+	(cdr (assoc id (plist-get info :id-alist)))
+	(user-error "Unable to resolve ID \"%s\"" id))))
 
 (defun org-export-resolve-radio-link (link info)
   "Return radio-target object referenced as LINK destination.

testing/lisp/test-ox.el

@@ -2518,42 +2518,50 @@ Another text. (ref:text)
 
 (ert-deftest test-org-export/resolve-id-link ()
   "Test `org-export-resolve-id-link' specifications."
-  ;; 1. Regular test for custom-id link.
-  (org-test-with-parsed-data "* Headline1
+  ;; Regular test for custom-id link.
+  (should
+   (equal '("Headline1")
+	  (org-test-with-parsed-data "* Headline1
 :PROPERTIES:
 :CUSTOM_ID: test
 :END:
 * Headline 2
 \[[#test]]"
-    (should
-     (org-export-resolve-id-link
-      (org-element-map tree 'link 'identity info t) info)))
-  ;; 2. Failing test for custom-id link.
-  (org-test-with-parsed-data "* Headline1
+	    (org-element-property
+	     :title
+	     (org-export-resolve-id-link
+	      (org-element-map tree 'link 'identity info t) info)))))
+  ;; Throw an error on failing searches.
+  (should-error
+   (org-test-with-parsed-data "* Headline1
 :PROPERTIES:
 :CUSTOM_ID: test
 :END:
 * Headline 2
 \[[#no-match]]"
-    (should-not
      (org-export-resolve-id-link
       (org-element-map tree 'link 'identity info t) info)))
-  ;; 3. Test for internal id target.
-  (org-test-with-parsed-data "* Headline1
+  ;; Test for internal id target.
+  (should
+   (equal '("Headline1")
+	  (org-test-with-parsed-data "* Headline1
 :PROPERTIES:
 :ID: aaaa
 :END:
 * Headline 2
 \[[id:aaaa]]"
-    (should
-     (org-export-resolve-id-link
-      (org-element-map tree 'link 'identity info t) info)))
-  ;; 4. Test for external id target.
-  (org-test-with-parsed-data "[[id:aaaa]]"
-    (should
-     (org-export-resolve-id-link
-      (org-element-map tree 'link 'identity info t)
-      (org-combine-plists info '(:id-alist (("aaaa" . "external-file"))))))))
+	    (org-element-property
+	     :title
+	     (org-export-resolve-id-link
+	      (org-element-map tree 'link 'identity info t) info)))))
+  ;; Test for external id target.
+  (should
+   (equal
+    "external-file"
+    (org-test-with-parsed-data "[[id:aaaa]]"
+      (org-export-resolve-id-link
+       (org-element-map tree 'link 'identity info t)
+       (org-combine-plists info '(:id-alist (("aaaa" . "external-file")))))))))
 
 (ert-deftest test-org-export/resolve-radio-link ()
   "Test `org-export-resolve-radio-link' specifications."