Fix comment detection on open parens

Characters having both open paren syntax and comment start syntax were
being detected as open parens even when they should have been part a
comment starter (Bug#24870).

* src/syntax.c (in_2char_comment_start): New function, extracted from
`scan_sexps_forward'.
(scan_sexps_forward): Add check for a 2-char comment starter before the
loop.  Inside the loop, do that check after incrementing the 'from'
character index.  Move the single char comment syntax cases into the
switch instead of special casing them before.
* test/src/syntax-tests.el (parse-partial-sexp-paren-comments):
(parse-partial-sexp-continue-over-comment-marker): New tests.

2 files changed, 147 insertions, 55 deletions

diff --git a/src/syntax.c b/src/syntax.c
index 84147a2dc15..0ee1c746ec3 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -3092,6 +3092,36 @@ the prefix syntax flag (p).  */)
   return Qnil;
 }
 
+
+/* If the character at FROM_BYTE is the second part of a 2-character
+   comment opener based on PREV_FROM_SYNTAX, update STATE and return
+   true.  */
+static bool
+in_2char_comment_start (struct lisp_parse_state *state,
+                        int prev_from_syntax,
+                        ptrdiff_t prev_from,
+                        ptrdiff_t from_byte)
+{
+  int c1, syntax;
+  if (SYNTAX_FLAGS_COMSTART_FIRST (prev_from_syntax)
+      && (c1 = FETCH_CHAR_AS_MULTIBYTE (from_byte),
+          syntax = SYNTAX_WITH_FLAGS (c1),
+          SYNTAX_FLAGS_COMSTART_SECOND (syntax)))
+    {
+      /* Record the comment style we have entered so that only
+         the comment-end sequence of the same style actually
+         terminates the comment section.  */
+      state->comstyle
+        = SYNTAX_FLAGS_COMMENT_STYLE (syntax, prev_from_syntax);
+      bool comnested = (SYNTAX_FLAGS_COMMENT_NESTED (prev_from_syntax)
+                        | SYNTAX_FLAGS_COMMENT_NESTED (syntax));
+      state->incomment = comnested ? 1 : -1;
+      state->comstr_start = prev_from;
+      return true;
+    }
+  return false;
+}
+
 /* Parse forward from FROM / FROM_BYTE to END,
    assuming that FROM has state STATE,
    and return a description of the state of the parse at END.
@@ -3107,8 +3137,6 @@ scan_sexps_forward (struct lisp_parse_state *state,
                     int commentstop)
 {
   enum syntaxcode code;
-  int c1;
-  bool comnested;
   struct level { ptrdiff_t last, prev; };
   struct level levelstart[100];
   struct level *curlevel = levelstart;
@@ -3122,7 +3150,6 @@ scan_sexps_forward (struct lisp_parse_state *state,
   ptrdiff_t prev_from;          /* Keep one character before FROM.  */
   ptrdiff_t prev_from_byte;
   int prev_from_syntax, prev_prev_from_syntax;
-  int syntax;
   bool boundary_stop = commentstop == -1;
   bool nofence;
   bool found;
@@ -3187,53 +3214,31 @@ do { prev_from = from;				\
     }
   else if (start_quoted)
     goto startquoted;
+  else if ((from < end)
+           && (in_2char_comment_start (state, prev_from_syntax,
+                                       prev_from, from_byte)))
+    {
+      INC_FROM;
+      prev_from_syntax = Smax; /* the syntax has already been "used up". */
+      goto atcomment;
+    }
 
   while (from < end)
     {
-      if (SYNTAX_FLAGS_COMSTART_FIRST (prev_from_syntax)
-          && (c1 = FETCH_CHAR (from_byte),
-              syntax = SYNTAX_WITH_FLAGS (c1),
-              SYNTAX_FLAGS_COMSTART_SECOND (syntax)))
-        {
-          /* Record the comment style we have entered so that only
-             the comment-end sequence of the same style actually
-             terminates the comment section.  */
-          state->comstyle
-            = SYNTAX_FLAGS_COMMENT_STYLE (syntax, prev_from_syntax);
-          comnested = (SYNTAX_FLAGS_COMMENT_NESTED (prev_from_syntax)
-                       | SYNTAX_FLAGS_COMMENT_NESTED (syntax));
-          state->incomment = comnested ? 1 : -1;
-          state->comstr_start = prev_from;
-          INC_FROM;
-          prev_from_syntax = Smax; /* the syntax has already been
-                                      "used up". */
-          code = Scomment;
-        }
-      else
+      INC_FROM;
+
+      if ((from < end)
+          && (in_2char_comment_start (state, prev_from_syntax,
+                                      prev_from, from_byte)))
         {
           INC_FROM;
-          code = prev_from_syntax & 0xff;
-          if (code == Scomment_fence)
-            {
-              /* Record the comment style we have entered so that only
-                 the comment-end sequence of the same style actually
-                 terminates the comment section.  */
-              state->comstyle = ST_COMMENT_STYLE;
-              state->incomment = -1;
-              state->comstr_start = prev_from;
-              code = Scomment;
-            }
-          else if (code == Scomment)
-            {
-              state->comstyle = SYNTAX_FLAGS_COMMENT_STYLE (prev_from_syntax, 0);
-              state->incomment = (SYNTAX_FLAGS_COMMENT_NESTED (prev_from_syntax) ?
-                                 1 : -1);
-              state->comstr_start = prev_from;
-            }
+          prev_from_syntax = Smax; /* the syntax has already been "used up". */
+          goto atcomment;
         }
 
       if (SYNTAX_FLAGS_PREFIX (prev_from_syntax))
         continue;
+      code = prev_from_syntax & 0xff;
       switch (code)
         {
         case Sescape:
@@ -3252,24 +3257,15 @@ do { prev_from = from;				\
         symstarted:
           while (from < end)
             {
-              int symchar = FETCH_CHAR_AS_MULTIBYTE (from_byte);
-
-              if (SYNTAX_FLAGS_COMSTART_FIRST (prev_from_syntax)
-                  && (syntax = SYNTAX_WITH_FLAGS (symchar),
-                      SYNTAX_FLAGS_COMSTART_SECOND (syntax)))
+              if (in_2char_comment_start (state, prev_from_syntax,
+                                          prev_from, from_byte))
                 {
-                  state->comstyle
-                    = SYNTAX_FLAGS_COMMENT_STYLE (syntax, prev_from_syntax);
-                  comnested = (SYNTAX_FLAGS_COMMENT_NESTED (prev_from_syntax)
-                               | SYNTAX_FLAGS_COMMENT_NESTED (syntax));
-                  state->incomment = comnested ? 1 : -1;
-                  state->comstr_start = prev_from;
                   INC_FROM;
-                  prev_from_syntax = Smax;
-                  code = Scomment;
+                  prev_from_syntax = Smax; /* the syntax has already been "used up". */
                   goto atcomment;
                 }
 
+              int symchar = FETCH_CHAR_AS_MULTIBYTE (from_byte);
               switch (SYNTAX (symchar))
                 {
                 case Scharquote:
@@ -3290,8 +3286,19 @@ do { prev_from = from;				\
           curlevel->prev = curlevel->last;
           break;
 
-        case Scomment_fence: /* Can't happen because it's handled above.  */
+        case Scomment_fence:
+          /* Record the comment style we have entered so that only
+             the comment-end sequence of the same style actually
+             terminates the comment section.  */
+          state->comstyle = ST_COMMENT_STYLE;
+          state->incomment = -1;
+          state->comstr_start = prev_from;
+          goto atcomment;
         case Scomment:
+          state->comstyle = SYNTAX_FLAGS_COMMENT_STYLE (prev_from_syntax, 0);
+          state->incomment = (SYNTAX_FLAGS_COMMENT_NESTED (prev_from_syntax) ?
+                              1 : -1);
+          state->comstr_start = prev_from;
         atcomment:
           if (commentstop || boundary_stop) goto done;
         startincomment:
diff --git a/test/src/syntax-tests.el b/test/src/syntax-tests.el
new file mode 100644
index 00000000000..6edde0b137b
--- /dev/null
+++ b/test/src/syntax-tests.el
@@ -0,0 +1,85 @@
+;;; syntax-tests.el --- tests for syntax.c functions -*- lexical-binding: t -*-
+
+;; Copyright (C) 2017 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+
+(ert-deftest parse-partial-sexp-continue-over-comment-marker ()
+  "Continue a parse that stopped in the middle of a comment marker."
+  (with-temp-buffer
+    (let ((table (make-syntax-table)))
+      (modify-syntax-entry ?/ ". 124")
+      (modify-syntax-entry ?* ". 23b")
+      (set-syntax-table table))
+    (insert "/*C*/\nX")
+    (goto-char (point-min))
+    (let* ((pointC (progn (search-forward "C") (1- (point))))
+           (preC (1- pointC))
+           (pointX (progn (search-forward "X") (1- (point))))
+           (aftC (+ 2 pointC))
+           (ppsC (parse-partial-sexp (point-min) pointC))
+           (pps-preC (parse-partial-sexp (point-min) preC))
+           (pps-aftC (parse-partial-sexp (point-min) aftC))
+           (ppsX (parse-partial-sexp (point-min) pointX)))
+      ;; C should be inside comment.
+      (should (= (nth 0 ppsC) 0))
+      (should (eq (nth 4 ppsC) t))
+      (should (= (nth 8 ppsC) (- pointC 2)))
+      ;; X should not be in comment or list.
+      (should (= (nth 0 ppsX) 0))
+      (should-not (nth 4 ppsX))
+      ;; Try using OLDSTATE.
+      (should (equal (parse-partial-sexp preC pointC nil nil pps-preC)
+                     ppsC))
+      (should (equal (parse-partial-sexp pointC aftC nil nil ppsC)
+                     pps-aftC))
+      (should (equal (parse-partial-sexp preC aftC nil nil pps-preC)
+                     pps-aftC))
+      (should (equal (parse-partial-sexp aftC pointX nil nil pps-aftC)
+                     ppsX)))))
+
+(ert-deftest parse-partial-sexp-paren-comments ()
+  "Test syntax parsing with paren comment markers.
+Specifically, where the first character of the comment marker is
+also has open paren syntax (see Bug#24870)."
+  (with-temp-buffer
+    (let ((table (make-syntax-table)))
+      (modify-syntax-entry ?\{  "(}1nb" table)
+      (modify-syntax-entry ?\}  "){4nb" table)
+      (modify-syntax-entry ?-  ". 123" table)
+      (set-syntax-table table))
+    (insert "{-C-}\nX")
+    (goto-char (point-min))
+    (let* ((pointC (progn (search-forward "C") (1- (point))))
+           (pointX (progn (search-forward "X") (1- (point))))
+           (ppsC (parse-partial-sexp (point-min) pointC))
+           (ppsX (parse-partial-sexp (point-min) pointX)))
+      ;; C should be inside nestable comment, not list.
+      (should (= (nth 0 ppsC) 0))
+      (should (= (nth 4 ppsC) 1))
+      (should (= (nth 8 ppsC) (- pointC 2)))
+      ;; X should not be in comment or list.
+      (should (= (nth 0 ppsX) 0))
+      (should-not (nth 4 ppsX))
+      ;; Try using OLDSTATE.
+      (should (equal (parse-partial-sexp pointC pointX nil nil ppsC)
+                     ppsX)))))
+
+;;; syntax-tests.el ends here