Timestamp fixes for undo.

* doc/lispref/text.texi (Undo):
Document (t . 0) and (t . -1) in buffer-undo-list.
* etc/NEWS: Changes to visited-file-modtime, set-visited-file-modtime.
* lisp/files.el (clear-visited-file-modtime): Move here from fileio.c.
* src/atimer.c (schedule_atimer):
* src/fileio.c (Ffile_newer_than_file_p):
Minor cleanup: use EMACS_TIME_LT so that we can remove EMACS_TIME_GT.
* src/buffer.c (buffer-undo-list): Document (t . 0) and (t . -1).
* src/fileio.c (Fclear_visited_file_modtime): Move to lisp/files.el.
(syms_of_fileio): Remove Sclear_visited_file_name.
(Fvisited_file_modtime): Return -1, not (-1 ...), when the visited
file doesn't exist; this avoids an ambiguity with negative timestamps.
(Fset_visited_file_modtime): Accept -1 and 0 as time-list arg.
* src/systime.h (make_emacs_time, invalid_emacs_time):
Don't assume struct timespec layout; POSIX doesn't guarantee it.
(EMACS_TIME_NE, EMACS_TIME_GT, EMACS_TIME_GE): Remove.
* src/undo.c (record_first_change): Push (visited-file-modtime) onto
undo list rather than reimplementing it by hand, incorrectly.

Fixes: debbugs:14824

6 files changed, 52 insertions, 49 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index a96f1153ef0..049c917eddd 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,21 @@
+2013-07-10  Paul Eggert  <eggert@cs.ucla.edu>
+
+        Timestamp fixes for undo (Bug#14824).
+        * atimer.c (schedule_atimer):
+        * fileio.c (Ffile_newer_than_file_p):
+        Minor cleanup: use EMACS_TIME_LT so that we can remove EMACS_TIME_GT.
+        * buffer.c (buffer-undo-list): Document (t . 0) and (t . -1).
+        * fileio.c (Fclear_visited_file_modtime): Move to lisp/files.el.
+        (syms_of_fileio): Remove Sclear_visited_file_name.
+        (Fvisited_file_modtime): Return -1, not (-1 ...), when the visited
+        file doesn't exist; this avoids an ambiguity with negative timestamps.
+        (Fset_visited_file_modtime): Accept -1 and 0 as time-list arg.
+        * systime.h (make_emacs_time, invalid_emacs_time):
+        Don't assume struct timespec layout; POSIX doesn't guarantee it.
+        (EMACS_TIME_NE, EMACS_TIME_GT, EMACS_TIME_GE): Remove.
+        * undo.c (record_first_change): Push (visited-file-modtime) onto
+        undo list rather than reimplementing it by hand, incorrectly.
+
 2013-07-09  Ken Brown  <kbrown@cornell.edu>
 
         * sheap.c (STATIC_HEAP_SIZE) [__x86_64__]: Increase to 18MB.
diff --git a/src/atimer.c b/src/atimer.c
index 73c7aa5686b..bb5294670d3 100644
--- a/src/atimer.c
+++ b/src/atimer.c
@@ -336,7 +336,7 @@ schedule_atimer (struct atimer *t)
   struct atimer *a = atimers, *prev = NULL;
 
   /* Look for the first atimer that is ripe after T.  */
-  while (a && EMACS_TIME_GT (t->expiration, a->expiration))
+  while (a && EMACS_TIME_LT (a->expiration, t->expiration))
     prev = a, a = a->next;
 
   /* Insert T in front of the atimer found, if any.  */
diff --git a/src/buffer.c b/src/buffer.c
index 94104ef535c..81768849a4b 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -6095,6 +6095,11 @@ and is the visited file's modification time, as of that time.  If the
 modification time of the most recent save is different, this entry is
 obsolete.
 
+An entry (t . 0) means means the buffer was previously unmodified but
+its time stamp was unknown because it was not associated with a file.
+An entry (t . -1) is similar, except that it means the buffer's visited
+file did not exist.
+
 An entry (nil PROPERTY VALUE BEG . END) indicates that a text property
 was modified between BEG and END.  PROPERTY is the property name,
 and VALUE is the old value.
diff --git a/src/fileio.c b/src/fileio.c
index d030c78c422..cb863410ccf 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -3345,7 +3345,7 @@ otherwise, if FILE2 does not exist, the answer is t.  */)
   if (stat (SSDATA (absname2), &st2) < 0)
     return Qt;
 
-  return (EMACS_TIME_GT (get_stat_mtime (&st1), get_stat_mtime (&st2))
+  return (EMACS_TIME_LT (get_stat_mtime (&st2), get_stat_mtime (&st1))
           ? Qt : Qnil);
 }
 
@@ -5375,36 +5375,19 @@ See Info node `(elisp)Modification Time' for more details.  */)
   return Qnil;
 }
 
-DEFUN ("clear-visited-file-modtime", Fclear_visited_file_modtime,
-       Sclear_visited_file_modtime, 0, 0, 0,
-       doc: /* Clear out records of last mod time of visited file.
-Next attempt to save will certainly not complain of a discrepancy.  */)
-  (void)
-{
-  current_buffer->modtime = make_emacs_time (0, UNKNOWN_MODTIME_NSECS);
-  current_buffer->modtime_size = -1;
-  return Qnil;
-}
-
 DEFUN ("visited-file-modtime", Fvisited_file_modtime,
        Svisited_file_modtime, 0, 0, 0,
        doc: /* Return the current buffer's recorded visited file modification time.
 The value is a list of the form (HIGH LOW USEC PSEC), like the time values that
 `file-attributes' returns.  If the current buffer has no recorded file
 modification time, this function returns 0.  If the visited file
-doesn't exist, HIGH will be -1.
+doesn't exist, return -1.
 See Info node `(elisp)Modification Time' for more details.  */)
   (void)
 {
-  if (EMACS_NSECS (current_buffer->modtime) < 0)
-    {
-      if (EMACS_NSECS (current_buffer->modtime) == NONEXISTENT_MODTIME_NSECS)
-        {
-          /* make_lisp_time won't work here if time_t is unsigned.  */
-          return list4i (-1, 65535, 0, 0);
-        }
-      return make_number (0);
-    }
+  int ns = EMACS_NSECS (current_buffer->modtime);
+  if (ns < 0)
+    return make_number (UNKNOWN_MODTIME_NSECS - ns);
   return make_lisp_time (current_buffer->modtime);
 }
 
@@ -5415,12 +5398,22 @@ Useful if the buffer was not read from the file normally
 or if the file itself has been changed for some known benign reason.
 An argument specifies the modification time value to use
 \(instead of that of the visited file), in the form of a list
-\(HIGH LOW USEC PSEC) as returned by `current-time'.  */)
-  (Lisp_Object time_list)
+\(HIGH LOW USEC PSEC) or an integer flag as returned by
+`visited-file-modtime'.  */)
+  (Lisp_Object time_flag)
 {
-  if (!NILP (time_list))
+  if (!NILP (time_flag))
     {
-      current_buffer->modtime = lisp_time_argument (time_list);
+      EMACS_TIME mtime;
+      if (INTEGERP (time_flag))
+        {
+          CHECK_RANGED_INTEGER (time_flag, -1, 0);
+          mtime = make_emacs_time (0, UNKNOWN_MODTIME_NSECS - XINT (time_flag));
+        }
+      else
+        mtime = lisp_time_argument (time_flag);
+
+      current_buffer->modtime = mtime;
       current_buffer->modtime_size = -1;
     }
   else
@@ -6121,7 +6114,6 @@ This includes interactive calls to `delete-file' and
   defsubr (&Swrite_region);
   defsubr (&Scar_less_than_car);
   defsubr (&Sverify_visited_file_modtime);
-  defsubr (&Sclear_visited_file_modtime);
   defsubr (&Svisited_file_modtime);
   defsubr (&Sset_visited_file_modtime);
   defsubr (&Sdo_auto_save);
diff --git a/src/systime.h b/src/systime.h
index c3bc00c1479..df733b290c3 100644
--- a/src/systime.h
+++ b/src/systime.h
@@ -67,7 +67,9 @@ SYSTIME_INLINE time_t *emacs_secs_addr (EMACS_TIME *t) { return &t->tv_sec; }
 SYSTIME_INLINE EMACS_TIME
 make_emacs_time (time_t s, int ns)
 {
-  EMACS_TIME r = { s, ns };
+  EMACS_TIME r;
+  r.tv_sec = s;
+  r.tv_nsec = ns;
   return r;
 }
 
@@ -75,7 +77,9 @@ make_emacs_time (time_t s, int ns)
 SYSTIME_INLINE EMACS_TIME
 invalid_emacs_time (void)
 {
-  EMACS_TIME r = { 0, -1 };
+  EMACS_TIME r;
+  r.tv_sec = 0;
+  r.tv_nsec = -1;
   return r;
 }
 
@@ -166,21 +170,6 @@ EMACS_TIME_EQ (EMACS_TIME t1, EMACS_TIME t2)
   return timespec_cmp (t1, t2) == 0;
 }
 SYSTIME_INLINE int
-EMACS_TIME_NE (EMACS_TIME t1, EMACS_TIME t2)
-{
-  return timespec_cmp (t1, t2) != 0;
-}
-SYSTIME_INLINE int
-EMACS_TIME_GT (EMACS_TIME t1, EMACS_TIME t2)
-{
-  return timespec_cmp (t1, t2) > 0;
-}
-SYSTIME_INLINE int
-EMACS_TIME_GE (EMACS_TIME t1, EMACS_TIME t2)
-{
-  return timespec_cmp (t1, t2) >= 0;
-}
-SYSTIME_INLINE int
 EMACS_TIME_LT (EMACS_TIME t1, EMACS_TIME t2)
 {
   return timespec_cmp (t1, t2) < 0;
diff --git a/src/undo.c b/src/undo.c
index d8711882fbf..234b8510f0a 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -229,10 +229,9 @@ record_first_change (void)
   if (base_buffer->base_buffer)
     base_buffer = base_buffer->base_buffer;
 
-  bset_undo_list
-    (current_buffer,
-     Fcons (Fcons (Qt, make_lisp_time (base_buffer->modtime)),
-            BVAR (current_buffer, undo_list)));
+  bset_undo_list (current_buffer,
+                  Fcons (Fcons (Qt, Fvisited_file_modtime ()),
+                         BVAR (current_buffer, undo_list)));
 }
 
 /* Record a change in property PROP (whose old value was VAL)