Stop throwing exception in TimeZoneInfo POSIX parsing (#27969)

IsDaylightSavingTime_CasablancaMultiYearDaylightSavings fails on rhel.8

When parsing the tzdata POSIX string that contains an 'n' Julian date, we are currently throwing an exception, and then falling back to a TimeZoneInfo without DST enabled. However, this is a mistake because there are other DST transitions that were read from the tzdata file that are valid and usable. We shouldn't be throwing that information away.

So instead, we now skip the POSIX string if we detect an unsupported 'n' Julian date, and just use the last transition as the AdjustmentRule for all the DateTimes in the future. This way we can still make DST determinations correctly for some DateTimes.

Fix https://github.com/dotnet/corefx/issues/42192

Author: eerhardt

src/System.Private.CoreLib/Resources/Strings.resx

@@ -2704,9 +2704,6 @@
   <data name="InvalidTimeZone_InvalidJulianDay" xml:space="preserve">
     <value>Invalid Julian day in POSIX strings.</value>
   </data>
-  <data name="InvalidTimeZone_NJulianDayNotSupported" xml:space="preserve">
-    <value>Julian n day in POSIX strings is not supported.</value>
-  </data>
   <data name="InvalidTimeZone_NoTTInfoStructures" xml:space="preserve">
     <value>There are no ttinfo structures in the tzfile.  At least one ttinfo structure is required in order to construct a TimeZoneInfo object.</value>
   </data>

src/System.Private.CoreLib/shared/System/TimeZoneInfo.Unix.cs

@@ -985,21 +985,11 @@ private static void TZif_GenerateAdjustmentRule(ref int index, TimeSpan timeZone
                 // NOTE: index == dts.Length
                 DateTime startTransitionDate = dts[index - 1];
 
-                if (!string.IsNullOrEmpty(futureTransitionsPosixFormat))
-                {
-                    AdjustmentRule? r = TZif_CreateAdjustmentRuleForPosixFormat(futureTransitionsPosixFormat, startTransitionDate, timeZoneBaseUtcOffset);
-
-                    if (r != null)
-                    {
-                        if (!IsValidAdjustmentRuleOffest(timeZoneBaseUtcOffset, r))
-                        {
-                            NormalizeAdjustmentRuleOffset(timeZoneBaseUtcOffset, ref r);
-                        }
+                AdjustmentRule? r = !string.IsNullOrEmpty(futureTransitionsPosixFormat) ?
+                    TZif_CreateAdjustmentRuleForPosixFormat(futureTransitionsPosixFormat, startTransitionDate, timeZoneBaseUtcOffset) :
+                    null;
 
-                        rulesList.Add(r);
-                    }
-                }
-                else
+                if (r == null)
                 {
                     // just use the last transition as the rule which will be used until the end of time
 
@@ -1008,22 +998,22 @@ private static void TZif_GenerateAdjustmentRule(ref int index, TimeSpan timeZone
                     TimeSpan daylightDelta = transitionType.IsDst ? transitionOffset : TimeSpan.Zero;
                     TimeSpan baseUtcDelta = transitionType.IsDst ? TimeSpan.Zero : transitionOffset;
 
-                    AdjustmentRule r = AdjustmentRule.CreateAdjustmentRule(
+                    r = AdjustmentRule.CreateAdjustmentRule(
                         startTransitionDate,
                         DateTime.MaxValue,
                         daylightDelta,
-                        default(TransitionTime),
-                        default(TransitionTime),
+                        default,
+                        default,
                         baseUtcDelta,
                         noDaylightTransitions: true);
+                }
 
-                    if (!IsValidAdjustmentRuleOffest(timeZoneBaseUtcOffset, r))
-                    {
-                        NormalizeAdjustmentRuleOffset(timeZoneBaseUtcOffset, ref r);
-                    }
-
-                    rulesList.Add(r);
+                if (!IsValidAdjustmentRuleOffest(timeZoneBaseUtcOffset, r))
+                {
+                    NormalizeAdjustmentRuleOffset(timeZoneBaseUtcOffset, ref r);
                 }
+
+                rulesList.Add(r);
             }
 
             index++;
@@ -1115,15 +1105,20 @@ private static TZifType TZif_GetEarlyDateTransitionType(TZifType[] transitionTyp
                             daylightSavingsTimeSpan = TZif_CalculateTransitionOffsetFromBase(daylightSavingsTimeSpan, baseOffset);
                         }
 
-                        TransitionTime dstStart = TZif_CreateTransitionTimeFromPosixRule(start, startTime);
-                        TransitionTime dstEnd = TZif_CreateTransitionTimeFromPosixRule(end, endTime);
+                        TransitionTime? dstStart = TZif_CreateTransitionTimeFromPosixRule(start, startTime);
+                        TransitionTime? dstEnd = TZif_CreateTransitionTimeFromPosixRule(end, endTime);
+
+                        if (dstStart == null || dstEnd == null)
+                        {
+                            return null;
+                        }
 
                         return AdjustmentRule.CreateAdjustmentRule(
                             startTransitionDate,
                             DateTime.MaxValue,
                             daylightSavingsTimeSpan,
-                            dstStart,
-                            dstEnd,
+                            dstStart.GetValueOrDefault(),
+                            dstEnd.GetValueOrDefault(),
                             baseOffset,
                             noDaylightTransitions: false);
                     }
@@ -1214,11 +1209,11 @@ private static DateTime ParseTimeOfDay(ReadOnlySpan<char> time)
             return timeOfDay;
         }
 
-        private static TransitionTime TZif_CreateTransitionTimeFromPosixRule(ReadOnlySpan<char> date, ReadOnlySpan<char> time)
+        private static TransitionTime? TZif_CreateTransitionTimeFromPosixRule(ReadOnlySpan<char> date, ReadOnlySpan<char> time)
         {
             if (date.IsEmpty)
             {
-                return default(TransitionTime);
+                return null;
             }
 
             if (date[0] == 'M')
@@ -1264,7 +1259,8 @@ private static TransitionTime TZif_CreateTransitionTimeFromPosixRule(ReadOnlySpa
                     // 
                     // If we need to support n format, we'll have to have a floating adjustment rule support this case.
 
-                    throw new InvalidTimeZoneException(SR.InvalidTimeZone_NJulianDayNotSupported);
+                    // Since we can't support this rule, return null to indicate to skip the POSIX rule.
+                    return null;
                 }
 
                 // Julian day