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

2 files changed, 11 insertions, 0 deletions

diff --git a/etc/ChangeLog b/etc/ChangeLog
index 843f176a6a8..9ed1b6e0df7 100644
--- a/etc/ChangeLog
+++ b/etc/ChangeLog
@@ -1,3 +1,8 @@
+2013-07-10  Paul Eggert  <eggert@cs.ucla.edu>
+
+        Timestamp fixes for undo (Bug#14824).
+        * NEWS: Changes to visited-file-modtime, set-visited-file-modtime.
+
 2013-07-08  Jan Djärv  <jan.h.d@swipnet.se>
 
         * NEWS: NS can be build with ImageMagick.
diff --git a/etc/NEWS b/etc/NEWS
index 1c81c8329aa..6dd89fcd4d7 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -537,6 +537,9 @@ alist of extended attributes as returned by the new function
 `file-extended-attributes'.  The attributes can be applied to another
 file using `set-file-extended-attributes'.
 
+** `visited-file-modtime' now returns -1 for nonexistent files.
+Formerly it returned a list (-1 LOW USEC PSEC), but this was ambiguous
+in the presence of files with negative time stamps.
 
 * Lisp Changes in Emacs 24.4
 
@@ -613,6 +616,9 @@ Emacs uses `image-default-frame-delay'.
 *** New functions `image-current-frame' and `image-show-frame' for getting
 and setting the current frame of a multi-frame image.
 
+** The function `set-visited-file-modtime' now accepts a 0 or -1 argument
+with the same interpretation as the returned value of `visited-file-modtime'.
+
 ** time-to-seconds is not obsolete any more.
 ** New function special-form-p.
 ** Docstrings can be made dynamic by adding a `dynamic-docstring-function'