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

1 files changed, 5 insertions, 0 deletions

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 modification time of the most recent save is different, this entry is
 obsolete.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.