Fix infinite loop on XHP in comments

`syntax-ppss` moves point to the position given, so ensure that we
wrap calls with `save-excursion` when the position is specified.

Fixes #31

Author: Wilfred

hack-mode.el

@@ -79,7 +79,7 @@
 (defun hack--propertize-xhp ()
   "Put syntax properties on XHP blocks."
   (let* ((start-pos (match-beginning 1))
-         (ppss (syntax-ppss start-pos))
+         (ppss (save-excursion (syntax-ppss start-pos)))
          (in-comment (nth 4 ppss)))
     (unless in-comment
       (hack--forward-parse-xhp start-pos nil)
@@ -198,11 +198,14 @@ If we find one, move point to its end, and set match data."
     (save-excursion
       (while (and (not end-pos)
                   (re-search-forward hack-xhp-start-regex limit t))
-        ;; Ignore XHP in comments.
-        (unless (nth 4 (syntax-ppss (match-beginning 1)))
-          (setq start-pos (match-beginning 1))
-          (hack--forward-parse-xhp start-pos limit t)
-          (setq end-pos (point)))))
+        (let* ((ppss-start
+                (save-excursion (syntax-ppss (match-beginning 1))))
+               (in-comment (nth 4 ppss-start)))
+          ;; Ignore XHP in comments.
+          (unless in-comment
+            (setq start-pos (match-beginning 1))
+            (hack--forward-parse-xhp start-pos limit t)
+            (setq end-pos (point))))))
     (when end-pos
       (set-match-data (list start-pos end-pos))
       (goto-char end-pos))))

test/hack-unit-test.el

@@ -862,3 +862,9 @@ baz()"
 <
 "))
 
+(ert-deftest hack-hang-on-xhp-in-comment ()
+  "Regression test for font-lock hanging on XHP inside comments."
+  (with-hack-buffer "/*
+<foo></foo>
+*/"))
+