Change current-time back to list form

Change current-time and related functions back to using the
traditional list form.  Also, add a new boolean variable
current-time-list that lets people try out (TICKS . HZ) form,
with the goal of smoothing the transition.
* src/timefns.c (CURRENT_TIME_LIST): Change default back to true.
(current-time-list): New boolean Lisp variable, which defaults to
CURRENT_TIME_LIST.  All uses of CURRENT_TIME_LIST changed to
use current_time_list, and all documentation changed.

6 files changed, 87 insertions, 42 deletions

diff --git a/doc/lispintro/emacs-lisp-intro.texi b/doc/lispintro/emacs-lisp-intro.texi
index afaed10cdfe..049c8a65a8f 100644
--- a/doc/lispintro/emacs-lisp-intro.texi
+++ b/doc/lispintro/emacs-lisp-intro.texi
@@ -15343,9 +15343,12 @@ nil
 100
 @end group
 @group
-(1351051674579989697 . 1000000000)
-(1173477761000000000 . 1000000000)
-(1351050967734791805 . 1000000000)
+(20615 27034 579989 697000)
+(17905 55681 0 0)
+(20615 26327 734791 805000)@footnote{If @code{current-time-list} is
+@code{nil} the three timestamps are @code{(1351051674579989697
+. 1000000000)}, @code{(1173477761000000000 . 1000000000)}, and
+@code{(1351050967734791805 . 1000000000)}, respectively.}
 13188
 "-rw-r--r--"
 @end group
diff --git a/doc/lispref/files.texi b/doc/lispref/files.texi
index 4394f64a320..75905658e64 100644
--- a/doc/lispref/files.texi
+++ b/doc/lispref/files.texi
@@ -1423,9 +1423,9 @@ For example, here are the file attributes for @file{files.texi}:
 @group
 (file-attributes "files.texi" 'string)
      @result{}  (nil 1 "lh" "users"
-          (1351023123050040152 . 1000000000)
-          (1310720023000000000 . 1000000000)
-          (1351023659902289872 . 1000000000)
+          (20614 64019 50040 152000)
+          (20000 23 0 0)
+          (20614 64555 902289 872000)
           122295 "-rw-rw-rw-"
           t 6473924464520138
           1014478468)
@@ -1449,14 +1449,20 @@ is owned by the user with name @samp{lh}.
 @item "users"
 is in the group with name @samp{users}.
 
-@item (1351023123050040152 . 1000000000)
-was last accessed on October 23, 2012, at 20:12:03.050040152 UTC.
+@item (20614 64019 50040 152000)
+was last accessed on October 23, 2012, at 20:12:03.050040152 UTC@.
+(This timestamp is @code{(1351023123050040152 . 1000000000)}
+if @code{current-time-list} is @code{nil}.)
 
-@item (1310720023000000000 . 1000000000)
-was last modified on July 15, 2001, at 08:53:43.000000000 UTC.
+@item (20000 23 0 0)
+was last modified on July 15, 2001, at 08:53:43.000000000 UTC@.
+(This timestamp is @code{(1310720023000000000 . 1000000000)}
+if @code{current-time-list} is @code{nil}.)
 
-@item (1351023659902289872 . 1000000000)
-last had its status changed on October 23, 2012, at 20:20:59.902289872 UTC.
+@item (20614 64555 902289 872000)
+last had its status changed on October 23, 2012, at 20:20:59.902289872 UTC@.
+(This timestamp is @code{(1351023659902289872 . 1000000000)}
+if @code{current-time-list} is @code{nil}.)
 
 @item 122295
 is 122295 bytes long.  (It may not contain 122295 characters, though,
diff --git a/doc/lispref/intro.texi b/doc/lispref/intro.texi
index d1a3fef7a4b..975215d6976 100644
--- a/doc/lispref/intro.texi
+++ b/doc/lispref/intro.texi
@@ -503,9 +503,11 @@ if the information is not available.
 @example
 @group
 emacs-build-time
-     @result{} (1650228902637038831 . 1000000000)
+     @result{} (25194 55894 8547 617000)
 @end group
 @end example
+(This timestamp is @code{(1651169878008547617 . 1000000000)}
+if @code{current-time-list} was @code{nil} when Emacs was built.)
 @end defvar
 
 @defvar emacs-version
diff --git a/doc/lispref/os.texi b/doc/lispref/os.texi
index 89ddf164a17..20b6c1cec63 100644
--- a/doc/lispref/os.texi
+++ b/doc/lispref/os.texi
@@ -1359,6 +1359,10 @@ This represents the number of seconds using the formula:
 @tex
 $high \times 2^{16} + low + micro \times 10^{-6} + pico \times 10^{-12}$.
 @end tex
+If @code{current-time-list} is @code{t},
+some functions may default to returning two- or
+three-element lists, with omitted @var{micro} and @var{pico}
+components defaulting to zero.
 On all current machines @var{pico} is a multiple of 1000, but this
 may change as higher-resolution clocks become available.
 @end itemize
@@ -1410,15 +1414,28 @@ The operating system limits the range of time and zone values.
 @end example
 @end defun
 
+@defvar current-time-list
+This boolean variable is a transition aid.  If @code{t},
+@code{current-time} and related functions return timestamps in list
+form, typically @code{(@var{high} @var{low} @var{micro} @var{pico})};
+otherwise, they use @code{(@var{ticks} . @var{hz})} form.  Currently
+this variable defaults to @code{t}, for behavior compatible with
+previous Emacs versions.  Developers are encourage to test
+timestamp-related code with this variable set to @code{nil}, as it
+will default to @code{nil} in a future Emacs version, and will be
+removed in some version after that.
+@end defvar
+
 @defun current-time
 This function returns the current time as a Lisp timestamp.
-The timestamp has the form @code{(@var{ticks} . @var{hz})} where
+If @code{current-time-list} is @code{nil},
+the timestamp has the form @code{(@var{ticks} . @var{hz})} where
 @var{ticks} counts clock ticks and @var{hz} is the clock ticks per second.
-
-In Emacs 28 and earlier, the returned timestamp had the list form
-@code{(@var{high} @var{low} @var{usec} @var{psec})}.  You can use
-@code{(time-convert nil 'list)} to return the current time in this
-older form.  @xref{Time Conversion}.
+Otherwise, the timestamp has the list form
+@code{(@var{high} @var{low} @var{usec} @var{psec})}.
+You can use @code{(time-convert nil t)} or @code{(time-convert nil 'list)}
+to obtain a particular form regardless of the value of
+@code{current-time-list}.  @xref{Time Conversion}.
 @end defun
 
 @defun float-time &optional time
diff --git a/etc/NEWS b/etc/NEWS
index 70087f2629e..9f93c4067f7 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -203,12 +203,15 @@ use the new 'tamil-itrans-digits' and 'tamil-inscript-digits' input
 methods instead.
 
 +++
-** current-time and related functions now yield (TICKS . HZ) timestamps.
-Previously they yielded timestamps of the forms (HI LO US PS), (HI LO
-US) or (HI LO), which were less regular and less efficient and which
-lacked information about clock resolution.  This long-planned change
-was documented in Emacs 27.  To convert a timestamp X to the old
-4-element list form, you can use (time-convert X 'list).
+** New variable current-time-list governing default timestamp form.
+Functions like current-time now yield (TICKS . HZ) timestamps if this
+new variable is nil.  The variable defaults to t, which means these
+functions default to timestamps of the forms (HI LO US PS), (HI LO US)
+or (HI LO), which are less regular and less efficient.  This is part
+of a long-planned change first documented in Emacs 27.  Developers are
+encourage to test timestamp-related code with this variable set to
+nil, as it will default to nil in a future Emacs version and will be
+removed some time after that.
 
 
 * Changes in Emacs 29.1
diff --git a/src/timefns.c b/src/timefns.c
index 651e0760e83..bca9a566e08 100644
--- a/src/timefns.c
+++ b/src/timefns.c
@@ -69,11 +69,11 @@ enum { TM_YEAR_BASE = 1900 };
 # define FASTER_TIMEFNS 1
 #endif
 
-/* current-time etc. generate (TICKS . HZ) timestamps.
-   To change that to the old 4-element list format (HI LO US PS),
-   compile with -DCURRENT_TIME_LIST=1.  */
+/* current-time-list defaults to t, typically generating (HI LO US PS)
+   timestamps.  To change the default to nil, generating (TICKS . HZ)
+   timestamps, compile with -DCURRENT_TIME_LIST=0.  */
 #ifndef CURRENT_TIME_LIST
-enum { CURRENT_TIME_LIST = false };
+enum { CURRENT_TIME_LIST = true };
 #endif
 
 #if FIXNUM_OVERFLOW_P (1000000000)
@@ -568,7 +568,7 @@ lisp_time_seconds (struct lisp_time t)
 Lisp_Object
 make_lisp_time (struct timespec t)
 {
-  if (CURRENT_TIME_LIST)
+  if (current_time_list)
     {
       time_t s = t.tv_sec;
       int ns = t.tv_nsec;
@@ -1171,13 +1171,13 @@ time_arith (Lisp_Object a, Lisp_Object b, bool subtract)
     }
 
   /* Return an integer if the timestamp resolution is 1,
-     otherwise the (TICKS . HZ) form if !CURRENT_TIME_LIST or if
+     otherwise the (TICKS . HZ) form if !current_time_list or if
      either input used (TICKS . HZ) form or the result can't be expressed
      exactly in (HI LO US PS) form, otherwise the (HI LO US PS) form
      for backward compatibility.  */
   return (EQ (hz, make_fixnum (1))
 	  ? ticks
-	  : (!CURRENT_TIME_LIST
+	  : (!current_time_list
 	     || aform == TIMEFORM_TICKS_HZ
 	     || bform == TIMEFORM_TICKS_HZ
 	     || !trillion_factor (hz))
@@ -1716,7 +1716,7 @@ usage: (encode-time TIME &rest OBSOLESCENT-ARGUMENTS)  */)
     time_error (mktime_errno);
 
   if (EQ (hz, make_fixnum (1)))
-    return (CURRENT_TIME_LIST
+    return (current_time_list
 	    ? list2 (hi_time (value), lo_time (value))
 	    : INT_TO_INTEGER (value));
   else
@@ -1747,7 +1747,7 @@ bits, and USEC and PSEC are the microsecond and picosecond counts.  */)
   struct lisp_time t;
   enum timeform input_form = decode_lisp_time (time, false, &t, 0);
   if (NILP (form))
-    form = CURRENT_TIME_LIST ? Qlist : Qt;
+    form = current_time_list ? Qlist : Qt;
   if (EQ (form, Qlist))
     return ticks_hz_list4 (t.ticks, t.hz);
   if (EQ (form, Qinteger))
@@ -1762,14 +1762,15 @@ bits, and USEC and PSEC are the microsecond and picosecond counts.  */)
 
 DEFUN ("current-time", Fcurrent_time, Scurrent_time, 0, 0, 0,
        doc: /* Return the current time, as the number of seconds since 1970-01-01 00:00:00.
-The time is returned as a pair of integers (TICKS . HZ), where TICKS
-counts clock ticks and HZ is the clock ticks per second.
-
-In Emacs 28 and earlier, the returned timestamp had the form (HIGH LOW
-USEC PSEC), where HIGH is the most significant bits of the seconds,
-LOW the least significant 16 bits, and USEC and PSEC are the
-microsecond and picosecond counts.  Use \(time-convert nil \\='list)
-if you need this older timestamp form.  */)
+If the variable `current-time-list' is nil, the time is returned as a
+pair of integers (TICKS . HZ), where TICKS counts clock ticks and HZ
+is the clock ticks per second.  Otherwise, the time is returned as a
+list of integers (HIGH LOW USEC PSEC) where HIGH has the most
+significant bits of the seconds, LOW has the least significant 16
+bits, and USEC and PSEC are the microsecond and picosecond counts.
+
+You can use `time-convert' to get a particular timestamp form
+regardless of the value of `current-time-list'.  */)
   (void)
 {
   return make_lisp_time (current_timespec ());
@@ -2025,6 +2026,19 @@ syms_of_timefns (void)
 
   DEFSYM (Qencode_time, "encode-time");
 
+  DEFVAR_BOOL ("current-time-list", current_time_list,
+	       doc: /* Whether `current-time' should return list or (TICKS . HZ) form.
+
+This boolean variable is a transition aid.  If t, `current-time' and
+related functions return timestamps in list form, typically
+\(HIGH LOW USEC PSEC); otherwise, they use (TICKS . HZ) form.
+Currently this variable defaults to t, for behavior compatible with
+previous Emacs versions.  Developers are encourage to test
+timestamp-related code with this variable set to nil, as it will
+default to nil in a future Emacs version, and will be removed in some
+version after that.  */);
+  current_time_list = CURRENT_TIME_LIST;
+
   defsubr (&Scurrent_time);
 #ifdef CLOCKS_PER_SEC
   defsubr (&Scurrent_cpu_time);