From ab884fffe3fc82a710bea66ad651720d71c938b8 Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Tue, 30 Apr 2024 09:52:13 +0100
Subject: [PATCH] libstdc++: Fix std::chrono::tzdb to work with vanguard format

I found some issues in the std::chrono::tzdb parser by testing the
tzdata "vanguard" format, which uses new features that aren't enabled in
the "main" and "rearguard" data formats.

Since 2024a the keyword "minimum" is no longer valid for the FROM and TO
fields in a Rule line, which means that "m" is now a valid abbreviation
for "maximum". Previously we expected either "mi" or "ma". For backwards
compatibility, a FROM field beginning with "mi" is still supported and
is treated as 1900. The "maximum" keyword is only allowed in TO now,
because it makes no sense in FROM. To support these changes the
minmax_year and minmax_year2 classes for parsing FROM and TO are
replaced with a single years_from_to class that reads both fields.

The vanguard format makes use of %z in Zone FORMAT fields, which caused
an exception to be thrown from ZoneInfo::set_abbrev because no % or /
characters were expected when a Zone doesn't use a named Rule. The
ZoneInfo::to(sys_info&) function now uses format_abbrev_str to replace
any %z with the current offset. Although format_abbrev_str also checks
for %s and STD/DST formats, those only make sense when a named Rule is
in effect, so won't occur when ZoneInfo::to(sys_info&) is used.

Since making this change on trunk, the tzdata-2024b release started
using %z in the main format, not just vanguard. This makes a backport to
release branches necessary (see PR 116657).

This change also implements a feature that has always been missing from
time_zone::_M_get_sys_info: finding the Rule that is active before the
specified time point, so that we can correctly handle %s in the FORMAT
for the first new sys_info that gets created. This requires implementing
a poorly documented feature of zic, to get the LETTERS field from a
later transition, as described at
https://mm.icann.org/pipermail/tz/2024-April/058891.html
In order for this to work we need to be able to distinguish an empty
letters field (as used by CE%sT where the variable part is either empty
or "S") from "the letters field is not known for this transition". The
tzdata file uses "-" for an empty letters field, which libstdc++ was
previously replacing with "" when the Rule was parsed. Instead, we now
preserve the "-" in the Rule object, so that "" can be used for the case
where we don't know the letters (and so need to decide it).

(cherry picked from commit 0ca8d56f2085715f27ee536c6c344bc47af49cdd)

Upstream-Status: Backport [https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=5ceea2ac106d6dd1aa8175670b15a801316cf1c9]

Signed-off-by: Markus Volk <f_l_k@t-online.de>
---
 libstdc++-v3/src/c++20/tzdb.cc                | 265 +++++++++++-------
 .../std/time/time_zone/sys_info_abbrev.cc     | 106 +++++++
 libstdc++-v3/testsuite/std/time/tzdb/1.cc     |   6 +-
 3 files changed, 274 insertions(+), 103 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/std/time/time_zone/sys_info_abbrev.cc

diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index c7c7cc9deee6..7e8cce7ce8cf 100644
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -342,51 +342,103 @@ namespace std::chrono
       friend istream& operator>>(istream&, on_day&);
     };
 
-    // Wrapper for chrono::year that reads a year, or one of the keywords
-    // "minimum" or "maximum", or an unambiguous prefix of a keyword.
-    struct minmax_year
+    // Wrapper for two chrono::year values, which reads the FROM and TO
+    // fields of a Rule line. The FROM field is a year and TO is a year or
+    // one of the keywords "maximum" or "only" (or an abbreviation of those).
+    // For backwards compatibility, the keyword "minimum" is recognized
+    // for FROM and interpreted as 1900.
+    struct years_from_to
     {
-      year& y;
+      year& from;
+      year& to;
 
-      friend istream& operator>>(istream& in, minmax_year&& y)
+      friend istream& operator>>(istream& in, years_from_to&& yy)
       {
-	if (ws(in).peek() == 'm') // keywords "minimum" or "maximum"
+	string s;
+	auto c = ws(in).peek();
+	if (c == 'm') [[unlikely]] // keyword "minimum"
 	  {
-	    string s;
-	    in >> s; // extract the rest of the word, but only look at s[1]
-	    if (s[1] == 'a')
-	      y.y = year::max();
-	    else if (s[1] == 'i')
-	      y.y = year::min();
-	    else
-	      in.setstate(ios::failbit);
+	    in >> s; // extract the rest of the word
+	    yy.from = year(1900);
+	  }
+	else if (int num = 0; in >> num) [[likely]]
+	  yy.from = year{num};
+
+	c = ws(in).peek();
+	if (c == 'm') // keyword "maximum"
+	  {
+	    in >> s; // extract the rest of the word
+	    yy.to = year::max();
+	  }
+	else if (c == 'o') // keyword "only"
+	  {
+	    in >> s; // extract the rest of the word
+	    yy.to = yy.from;
 	  }
 	else if (int num = 0; in >> num)
-	  y.y = year{num};
+	  yy.to = year{num};
+
 	return in;
       }
     };
 
-    // As above for minmax_year, but also supports the keyword "only",
-    // meaning that the TO year is the same as the FROM year.
-    struct minmax_year2
+    bool
+    select_std_or_dst_abbrev(string& abbrev, minutes save)
     {
-      minmax_year to;
-      year from;
+      if (size_t pos = abbrev.find('/'); pos != string::npos)
+	{
+	  // Select one of "STD/DST" for standard or daylight.
+	  if (save == 0min)
+	    abbrev.erase(pos);
+	  else
+	    abbrev.erase(0, pos + 1);
+	  return true;
+	}
+      return false;
+    }
 
-      friend istream& operator>>(istream& in, minmax_year2&& y)
-      {
-	if (ws(in).peek() == 'o') // keyword "only"
-	  {
-	    string s;
-	    in >> s; // extract the whole keyword
-	    y.to.y = y.from;
-	  }
-	else
-	  in >> std::move(y.to);
-	return in;
-      }
-    };
+    // Set the sys_info::abbrev string by expanding any placeholders.
+    void
+    format_abbrev_str(sys_info& info, string_view letters = {})
+    {
+      if (size_t pos = info.abbrev.find('%'); pos != string::npos)
+	{
+	  if (info.abbrev[pos + 1] == 's')
+	    {
+	      // Expand "%s" to the variable part, given by Rule::letters.
+	      if (letters == "-")
+		info.abbrev.erase(pos, 2);
+	      else
+		info.abbrev.replace(pos, 2, letters);
+	    }
+	  else if (info.abbrev[pos + 1] == 'z')
+	    {
+	      // Expand "%z" to the UT offset as +/-hh, +/-hhmm, or +/-hhmmss.
+	      hh_mm_ss<seconds> t(info.offset);
+	      string z(1, "+-"[t.is_negative()]);
+	      long val = t.hours().count();
+	      int digits = 2;
+	      if (int m = t.minutes().count())
+		{
+		  digits = 4;
+		  val *= 100;
+		  val += m;
+		  if (int s = t.seconds().count())
+		    {
+		      digits = 6;
+		      val *= 100;
+		      val += s;
+		    }
+		}
+	      auto sval = std::to_string(val);
+	      z += string(digits - sval.size(), '0');
+	      z += sval;
+	      info.abbrev.replace(pos, 2, z);
+	    }
+	}
+      else
+	select_std_or_dst_abbrev(info.abbrev, info.save);
+    }
 
     // A time zone information record.
     // Zone  NAME        STDOFF  RULES   FORMAT  [UNTIL]
@@ -462,6 +514,7 @@ namespace std::chrono
 	info.offset = offset();
 	info.save = minutes(m_save);
 	info.abbrev = format();
+	format_abbrev_str(info); // expand %z
 	return true;
       }
 
@@ -469,12 +522,9 @@ namespace std::chrono
       friend class time_zone;
 
       void
-      set_abbrev(const string& abbrev)
+      set_abbrev(string abbrev)
       {
-	// In practice, the FORMAT field never needs expanding here.
-	if (abbrev.find_first_of("/%") != abbrev.npos)
-	  __throw_runtime_error("std::chrono::time_zone: invalid data");
-	m_buf = abbrev;
+	m_buf = std::move(abbrev);
 	m_pos = 0;
 	m_expanded = true;
       }
@@ -544,9 +594,7 @@ namespace std::chrono
 
 	// Rule  NAME  FROM  TO  TYPE  IN  ON  AT  SAVE  LETTER/S
 
-	in >> quoted(rule.name)
-	   >> minmax_year{rule.from}
-	   >> minmax_year2{rule.to, rule.from};
+	in >> quoted(rule.name) >> years_from_to{rule.from, rule.to};
 
 	if (char type; in >> type && type != '-')
 	  in.setstate(ios::failbit);
@@ -557,7 +605,7 @@ namespace std::chrono
 	if (save_time.indicator != at_time::Wall)
 	  {
 	    // We don't actually store the save_time.indicator, because we
-	    // assume that it's always deducable from the actual offset value.
+	    // assume that it's always deducible from the offset value.
 	    auto expected = save_time.time == 0s
 			      ? at_time::Standard
 			      : at_time::Daylight;
@@ -567,8 +615,6 @@ namespace std::chrono
 	rule.save = save_time.time;
 
 	in >> rule.letters;
-	if (rule.letters == "-")
-	  rule.letters.clear();
 	return in;
       }
 
@@ -719,58 +765,6 @@ namespace std::chrono
 #endif // TZDB_DISABLED
   };
 
-#ifndef TZDB_DISABLED
-  namespace
-  {
-    bool
-    select_std_or_dst_abbrev(string& abbrev, minutes save)
-    {
-      if (size_t pos = abbrev.find('/'); pos != string::npos)
-	{
-	  // Select one of "STD/DST" for standard or daylight.
-	  if (save == 0min)
-	    abbrev.erase(pos);
-	  else
-	    abbrev.erase(0, pos + 1);
-	  return true;
-	}
-      return false;
-    }
-
-    // Set the sys_info::abbrev string by expanding any placeholders.
-    void
-    format_abbrev_str(sys_info& info, string_view letters = {})
-    {
-      if (size_t pos = info.abbrev.find("%s"); pos != string::npos)
-	{
-	  // Expand "%s" to the variable part, given by Rule::letters.
-	  info.abbrev.replace(pos, 2, letters);
-	}
-      else if (size_t pos = info.abbrev.find("%z"); pos != string::npos)
-	{
-	  // Expand "%z" to the UT offset as +/-hh, +/-hhmm, or +/-hhmmss.
-	  hh_mm_ss<seconds> t(info.offset);
-	  string z(1, "+-"[t.is_negative()]);
-	  long val = t.hours().count();
-	  if (minutes m = t.minutes(); m != m.zero())
-	    {
-	      val *= 100;
-	      val += m.count();
-	      if (seconds s = t.seconds(); s != s.zero())
-		{
-		  val *= 100;
-		  val += s.count();
-		}
-	    }
-	  z += std::to_string(val);
-	  info.abbrev.replace(pos, 2, z);
-	}
-      else
-	select_std_or_dst_abbrev(info.abbrev, info.save);
-    }
-  }
-#endif // TZDB_DISABLED
-
   // Implementation of std::chrono::time_zone::get_info(const sys_time<D>&)
   sys_info
   time_zone::_M_get_sys_info(sys_seconds tp) const
@@ -839,12 +833,72 @@ namespace std::chrono
     info.abbrev = ri.format();
 
     string_view letters;
-    if (i != infos.begin())
+    if (i != infos.begin() && i[-1].expanded())
+      letters = i[-1].next_letters();
+
+    if (letters.empty())
       {
-	if (i[-1].expanded())
-	  letters = i[-1].next_letters();
-	// XXX else need to find Rule active before this time and use it
-	// to know the initial offset, save, and letters.
+	sys_seconds t = info.begin - seconds(1);
+	const year_month_day date(chrono::floor<days>(t));
+
+	// Try to find a Rule active before this time, to get initial
+	// SAVE and LETTERS values. There may not be a Rule for the period
+	// before the first DST transition, so find the earliest DST->STD
+	// transition and use the LETTERS from that.
+	const Rule* active_rule = nullptr;
+	sys_seconds active_rule_start = sys_seconds::min();
+	const Rule* first_std = nullptr;
+	for (const auto& rule : rules)
+	  {
+	    if (rule.save == minutes(0))
+	      {
+		if (!first_std)
+		  first_std = &rule;
+		else if (rule.from < first_std->from)
+		  first_std = &rule;
+		else if (rule.from == first_std->from)
+		  {
+		    if (rule.start_time(rule.from, {})
+			  < first_std->start_time(first_std->from, {}))
+		      first_std = &rule;
+		  }
+	      }
+
+	    year y = date.year();
+
+	    if (y > rule.to) // rule no longer applies at time t
+	      continue;
+	    if (y < rule.from) // rule doesn't apply yet at time t
+	      continue;
+
+	    sys_seconds rule_start;
+
+	    seconds offset{}; // appropriate for at_time::Universal
+	    if (rule.when.indicator == at_time::Wall)
+	      offset = info.offset;
+	    else if (rule.when.indicator == at_time::Standard)
+	      offset = ri.offset();
+
+	    // Time the rule takes effect this year:
+	    rule_start = rule.start_time(y, offset);
+
+	    if (rule_start >= t && rule.from < y)
+	      {
+		// Try this rule in the previous year.
+		rule_start = rule.start_time(--y, offset);
+	      }
+
+	    if (active_rule_start < rule_start && rule_start < t)
+	      {
+		active_rule_start = rule_start;
+		active_rule = &rule;
+	      }
+	  }
+
+	if (active_rule)
+	  letters = active_rule->letters;
+	else if (first_std)
+	  letters = first_std->letters;
       }
 
     const Rule* curr_rule = nullptr;
@@ -2069,9 +2123,11 @@ namespace std::chrono
 	      istringstream in2(std::move(rules));
 	      in2 >> rules_time;
 	      inf.m_save = duration_cast<minutes>(rules_time.time);
+	      // If the FORMAT is "STD/DST" then we can choose the right one
+	      // now, so that we store a shorter string.
 	      select_std_or_dst_abbrev(fmt, inf.m_save);
 	    }
-	  inf.set_abbrev(fmt);
+	  inf.set_abbrev(std::move(fmt));
 	}
 
       // YEAR [MONTH [DAY [TIME]]]
@@ -2082,7 +2138,12 @@ namespace std::chrono
 	  abbrev_month m{January};
 	  int d = 1;
 	  at_time t{};
+	  // XXX DAY should support ON format, e.g. lastSun or Sun>=8
 	  in >> m >> d >> t;
+	  // XXX UNTIL field should be interpreted
+	  // "using the rules in effect just before the transition"
+	  // so might need to store as year_month_day and hh_mm_ss and only
+	  // convert to a sys_time once we know the offset in effect.
 	  inf.m_until = sys_days(year(y)/m.m/day(d)) + seconds(t.time);
 	}
       else
diff --git a/libstdc++-v3/testsuite/std/time/time_zone/sys_info_abbrev.cc b/libstdc++-v3/testsuite/std/time/time_zone/sys_info_abbrev.cc
new file mode 100644
index 000000000000..f1a8fff02f58
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/time/time_zone/sys_info_abbrev.cc
@@ -0,0 +1,106 @@
+// { dg-do run { target c++20 } }
+// { dg-require-effective-target tzdb }
+// { dg-require-effective-target cxx11_abi }
+// { dg-xfail-run-if "no weak override on AIX" { powerpc-ibm-aix* } }
+
+#include <chrono>
+#include <fstream>
+#include <testsuite_hooks.h>
+
+static bool override_used = false;
+
+namespace __gnu_cxx
+{
+  const char* zoneinfo_dir_override() {
+    override_used = true;
+    return "./";
+  }
+}
+
+using namespace std::chrono;
+
+void
+test_format()
+{
+  std::ofstream("tzdata.zi") << R"(# version test_1
+Zone Africa/Bissau -1:2:20 - LMT 1912 Ja 1 1u
+                   -1      - %z  1975
+                   0       - GMT
+Zon Some/Zone 1:2:3   - %z 1900
+              1:23:45 - %z 1950
+Zo Another/Zone 1:2:3 -     AZ0     1901
+                1     Roolz A%sZ    2000
+                1     Roolz SAZ/DAZ 2005
+                1     Roolz %z
+Rule Roolz 1950 max - April 1 2 1 D
+Rul  Roolz 1950 max - Oct   1 1 0 S
+Z Strange/Zone 1       - X%sX    1980
+               1       - FOO/BAR 1990
+               2:00    - %zzz    1995
+               0:9     - %zzz    1996
+               0:8:7   - %zzz    1997
+               0:6:5.5 - %zzz    1998
+)";
+
+  const auto& db = reload_tzdb();
+  VERIFY( override_used ); // If this fails then XFAIL for the target.
+  VERIFY( db.version == "test_1" );
+
+  // Test formatting %z as
+  auto tz = locate_zone("Africa/Bissau");
+  auto inf = tz->get_info(sys_days(1974y/1/1));
+  VERIFY( inf.abbrev == "-01" );
+
+  tz = locate_zone("Some/Zone");
+  inf = tz->get_info(sys_days(1899y/1/1));
+  VERIFY( inf.abbrev == "+010203" );
+  inf = tz->get_info(sys_days(1955y/1/1));
+  VERIFY( inf.abbrev == "+012345" );
+
+  tz = locate_zone("Another/Zone");
+  // Test formatting %s as the LETTER/S field from the active Rule.
+  inf = tz->get_info(sys_days(1910y/January/1));
+  VERIFY( inf.abbrev == "ASZ" );
+  inf = tz->get_info(sys_days(1950y/January/1));
+  VERIFY( inf.abbrev == "ASZ" );
+  inf = tz->get_info(sys_days(1950y/June/1));
+  VERIFY( inf.abbrev == "ADZ" );
+  inf = tz->get_info(sys_days(1999y/January/1));
+  VERIFY( inf.abbrev == "ASZ" );
+  inf = tz->get_info(sys_days(1999y/July/1));
+  VERIFY( inf.abbrev == "ADZ" );
+  // Test formatting STD/DST according to the active Rule.
+  inf = tz->get_info(sys_days(2000y/January/2));
+  VERIFY( inf.abbrev == "SAZ" );
+  inf = tz->get_info(sys_days(2001y/January/1));
+  VERIFY( inf.abbrev == "SAZ" );
+  inf = tz->get_info(sys_days(2001y/July/1));
+  VERIFY( inf.abbrev == "DAZ" );
+  // Test formatting %z as the offset determined by the active Rule.
+  inf = tz->get_info(sys_days(2005y/January/2));
+  VERIFY( inf.abbrev == "+01" );
+  inf = tz->get_info(sys_days(2006y/January/1));
+  VERIFY( inf.abbrev == "+01" );
+  inf = tz->get_info(sys_days(2006y/July/1));
+  VERIFY( inf.abbrev == "+02" );
+
+  // Test formatting %z, %s and S/D for a Zone with no associated Rules.
+  tz = locate_zone("Strange/Zone");
+  inf = tz->get_info(sys_days(1979y/January/1));
+  VERIFY( inf.abbrev == "XX" ); // No Rule means nothing to use for %s.
+  inf = tz->get_info(sys_days(1981y/July/1));
+  VERIFY( inf.abbrev == "FOO" ); // Always standard time means first string.
+  inf = tz->get_info(sys_days(1994y/July/1));
+  VERIFY( inf.abbrev == "+02zz" );
+  inf = tz->get_info(sys_days(1995y/July/1));
+  VERIFY( inf.abbrev == "+0009zz" );
+  inf = tz->get_info(sys_days(1996y/July/1));
+  VERIFY( inf.abbrev == "+000807zz" );
+  inf = tz->get_info(sys_days(1997y/July/1));
+  VERIFY( inf.abbrev == "+000606zz" );
+}
+
+int main()
+{
+  test_format();
+}
diff --git a/libstdc++-v3/testsuite/std/time/tzdb/1.cc b/libstdc++-v3/testsuite/std/time/tzdb/1.cc
index 796f3a8b4256..7a31c1c20ba7 100644
--- a/libstdc++-v3/testsuite/std/time/tzdb/1.cc
+++ b/libstdc++-v3/testsuite/std/time/tzdb/1.cc
@@ -39,11 +39,15 @@ test_locate()
   const tzdb& db = get_tzdb();
   const time_zone* tz = db.locate_zone("GMT");
   VERIFY( tz != nullptr );
-  VERIFY( tz->name() == "Etc/GMT" );
   VERIFY( tz == std::chrono::locate_zone("GMT") );
   VERIFY( tz == db.locate_zone("Etc/GMT") );
   VERIFY( tz == db.locate_zone("Etc/GMT+0") );
 
+  // Since 2022f GMT is now a Zone and Etc/GMT a link instead of vice versa,
+  // but only when using the vanguard format. As of 2024a, the main and
+  // rearguard formats still have Etc/GMT as a Zone and GMT as a link.
+  VERIFY( tz->name() == "GMT" || tz->name() == "Etc/GMT" );
+
   VERIFY( db.locate_zone(db.current_zone()->name()) == db.current_zone() );
 }
 
-- 
2.43.5

