zic: work around GNOME bug by refusing to go back before the Big Bang

This works around Gnome bug 730332
<https://bugzilla.gnome.org/show_bug.cgi?id=730332>.
zic 2014c introduced the idea of generating a transition at the
minimum time value -2**63, to avoid ambiguities about what to do
before the first real transition.  Through Glib release 2.40, the
interval_end function of Glib's gtimezone.c subtracts one from
this, to find the end time of the zeroth interval (i.e., the
interval containing all the "early" time stamps); this subtraction
typically overflows and wraps around to 2**63 - 1, which causes
Glib to go off the rails and assume that all time stamps are
"early".  For example, Glib computes Sao Paulo time stamps as if
Brazil's circa-1913 rules were still in effect.
(Thanks to Leonardo Chiquitto for reporting the bug.)
Work around the bug by not generating time stamps equal to -2**63.
Come to think of it, time stamps before the Big Bang are physically
suspect anyway, so don't generate time stamps before the Big Bang.
* Makefile (GCC_DEBUG_FLAGS): Add comment re BIG_BANG.
* NEWS, zic.8 (NOTES): Document this.
* zic.c (BIG_BANG): New macro.
(big_bang_time): New constant.
(writezone, outzone, addtt): Compare to big_bang_time, not to
min_time, when deciding whether to output a time stamp.

Makefile

@@ -100,6 +100,7 @@ YEARISTYPE=	./yearistype
 LDLIBS=
 
 # Add the following to the end of the "CFLAGS=" line as needed.
+#  -DBIG_BANG=-9999999LL if the Big Bang occurred at time -9999999 (see zic.c)
 #  -DHAVE_ADJTIME=0 if `adjtime' does not exist (SVR0?)
 #  -DHAVE_DOS_FILE_NAMES if file names have drive specifiers etc. (MS-DOS)
 #  -DHAVE_GETTEXT=1 if `gettext' works (GNU, Linux, Solaris); also see LDLIBS

NEWS

@@ -1,6 +1,15 @@
 News for the tz database
 
 
+Unreleased, experimental changes
+
+  Changes affecting code
+
+    zic no longer generates files containing time stamps before the Big Bang.
+    This works around GNOME bug 730332
+    <https://bugzilla.gnome.org/show_bug.cgi?id=730332>.
+
+
 Release 2014c - 2014-05-13 07:44:13 -0700
 
   Changes affecting near-future time stamps

zic.8

@@ -519,6 +519,12 @@ produces a single transition to daylight saving at the new UT offset
 To get separate transitions
 use multiple zone continuation lines
 specifying transition instants using universal time.
+.PP
+Time stamps before the Big Bang are silently omitted from the output.
+This works around bugs in software that mishandles large negative time
+stamps.  Call it sour grapes, but pre-Big-Bang time stamps are
+physically suspect anyway.  The estimated time of the Big Bang is
+approximate and may change in future versions.
 .SH FILE
 /usr/local/etc/zoneinfo	standard directory used for created files
 .SH "SEE ALSO"

zic.c

@@ -723,6 +723,34 @@ warning(_("hard link failed, symbolic link used"));
 static const zic_t min_time = (zic_t) -1 << (TIME_T_BITS_IN_FILE - 1);
 static const zic_t max_time = -1 - ((zic_t) -1 << (TIME_T_BITS_IN_FILE - 1));
 
+/* Estimated time of the Big Bang, in seconds since the POSIX epoch.
+   zic does not output time stamps before this, partly because they
+   are physically suspect, and partly because GNOME mishandles them; see
+   GNOME bug 730332 <https://bugzilla.gnome.org/show_bug.cgi?id=730332>.
+
+   The following estimate for the Big Bang time is taken from:
+
+   Ade PAR, Aghanim N, Armitage-Caplan C et al.  Planck 2013 results.
+   I. Overview of products and scientific results.
+   arXiv:1303.5062 2013-03-20 20:10:01 UTC
+   <http://arxiv.org/pdf/1303.5062v1> [PDF]
+
+   Page 36, Table 9, row Age/Gyr, column Planck+WP+highL+BAO best fit,
+   gives the value 13.7965.  Multiplying this by 1000000000 and then
+   by 31557600 (the number of seconds in an astronomical year), yields
+   435384428400000000.  This estimate intentionally ignores the
+   difference between the POSIX epoch and the paper's publication
+   date, as being beneath the paper's precision.
+
+   This estimate is approximate, and may change in future versions.
+   Please do not rely on its exact value.  */
+
+#ifndef BIG_BANG
+#define BIG_BANG (-435384428400000000LL)
+#endif
+
+static const zic_t big_bang_time = BIG_BANG;
+
 static int
 itsdir(const char *const name)
 {
@@ -1478,7 +1506,7 @@ writezone(const char *const name, const char *const string, char version)
 
 		toi = 0;
 		fromi = 0;
-		while (fromi < timecnt && attypes[fromi].at < min_time)
+		while (fromi < timecnt && attypes[fromi].at < big_bang_time)
 			++fromi;
 		for ( ; fromi < timecnt; ++fromi) {
 			if (toi > 1 && ((attypes[fromi].at +
@@ -2167,9 +2195,9 @@ outzone(const struct zone * const zpfirst, const int zonecount)
 		*/
 		stdoff = 0;
 		zp = &zpfirst[i];
-		usestart = i > 0 && (zp - 1)->z_untiltime > min_time;
+		usestart = i > 0 && (zp - 1)->z_untiltime > big_bang_time;
 		useuntil = i < (zonecount - 1);
-		if (useuntil && zp->z_untiltime <= min_time)
+		if (useuntil && zp->z_untiltime <= big_bang_time)
 			continue;
 		gmtoff = zp->z_gmtoff;
 		eat(zp->z_filename, zp->z_linenum);
@@ -2185,7 +2213,7 @@ outzone(const struct zone * const zpfirst, const int zonecount)
 			if (usestart) {
 				addtt(starttime, type);
 				usestart = FALSE;
-			} else	addtt(min_time, type);
+			} else	addtt(big_bang_time, type);
 		} else for (year = min_year; year <= max_year; ++year) {
 			if (useuntil && year > zp->z_untilrule.r_hiyear)
 				break;
@@ -2362,8 +2390,8 @@ error(_("can't determine time zone abbreviation to use just after until time"));
 static void
 addtt(const zic_t starttime, int type)
 {
-	if (starttime <= min_time ||
-		(timecnt == 1 && attypes[0].at < min_time)) {
+	if (starttime <= big_bang_time ||
+		(timecnt == 1 && attypes[0].at < big_bang_time)) {
 		gmtoffs[0] = gmtoffs[type];
 		isdsts[0] = isdsts[type];
 		ttisstds[0] = ttisstds[type];