[release/6.0-maui] Android remove backward timezones (#64228)

Fixes #63693

It was discovered that Android produces duplicate TimeZone DisplayNames among all timezone IDs in GetSystemTimeZones. These duplicate DisplayNames occur across TimeZone IDs that are aliases, where all except one are backward timezone IDs.

If a name is changed, put its old spelling in the 'backward' file

From the Android TimeZone data file tzdata, it isn't obvious which TimeZone IDs are backward (I find it strange that they're included in the first place), however we discovered that on some versions of Android, there is an adjacent file tzlookup.xml that can aid us in determining which TimeZone IDs are "current" (not backward).

This PR aims to utilize tzlookup.xml when it exists and post-filter's the Populated TimeZone IDs in the AndroidTzData instance by removing IDs and their associated information (byteoffset and length) from the AndroidTzData instance if it is not found in tzlookup.xml. This is using the assumption that all non-backward TimeZone IDs make it to the tzlookup.xml file.

This PR also adds a new TimeZoneInfo Test to check whether or not there are duplicate DisplayNames in GetSystemTimeZones

Author: github-actions[bot]

src/libraries/System.Private.CoreLib/src/System/TimeZoneInfo.Unix.Android.cs

@@ -193,6 +193,7 @@ private sealed class AndroidTzData
             private string[] _ids;
             private int[] _byteOffsets;
             private int[] _lengths;
+            private bool[] _isBackwards;
             private string _tzFileDir;
             private string _tzFilePath;
 
@@ -230,7 +231,7 @@ public AndroidTzData()
                 foreach (var tzFileDir in tzFileDirList)
                 {
                     string tzFilePath = Path.Combine(tzFileDir, TimeZoneFileName);
-                    if (LoadData(tzFilePath))
+                    if (LoadData(tzFileDir, tzFilePath))
                     {
                         _tzFileDir = tzFileDir;
                         _tzFilePath = tzFilePath;
@@ -241,10 +242,62 @@ public AndroidTzData()
                 throw new TimeZoneNotFoundException(SR.TimeZoneNotFound_ValidTimeZoneFileMissing);
             }
 
+            // On some versions of Android, the tzdata file may still contain backward timezone ids.
+            // We attempt to use tzlookup.xml, which is available on some versions of Android to help
+            // validate non-backward timezone ids
+            // tzlookup.xml is an autogenerated file that contains timezone ids in this form:
+            //
+            // <timezones ianaversion="2019b">
+            //   <countryzones>
+            //     <country code="au" default="Australia/Sydney" everutc="n">
+            //       <id alts="Australia/ACT,Australia/Canberra,Australia/NSW">Australia/Sydney</id>
+            //       ...
+            //       ...
+            //       <id>Australia/Eucla</id>
+            //     </country>
+            //     <country ...>
+            //       ...
+            //       ...
+            //       ...
+            //     </country>
+            //   </countryzones>
+            // </timezones>
+            //
+            // Once the timezone cache is populated with the IDs, we reference tzlookup id tags
+            // to determine if an id is backwards and label it as such if they are.
+            private void FilterBackwardIDs(string tzFileDir, out HashSet<string> tzLookupIDs)
+            {
+                tzLookupIDs = new HashSet<string>();
+                try
+                {
+                    using (StreamReader sr = File.OpenText(Path.Combine(tzFileDir, "tzlookup.xml")))
+                    {
+                        string? tzLookupLine;
+                        while (sr.Peek() >= 0)
+                        {
+                            if (!(tzLookupLine = sr.ReadLine())!.AsSpan().TrimStart().StartsWith("<id", StringComparison.Ordinal))
+                                continue;
+
+                            int idStart = tzLookupLine!.IndexOf('>') + 1;
+                            int idLength = tzLookupLine.LastIndexOf("</", StringComparison.Ordinal) - idStart;
+                            if (idStart <= 0 || idLength < 0)
+                            {
+                                // Either the start tag <id ... > or the end tag </id> are not found
+                                continue;
+                            }
+                            string id = tzLookupLine.Substring(idStart, idLength);
+                            tzLookupIDs.Add(id);
+                        }
+                    }
+                }
+                catch {}
+            }
+
             [MemberNotNullWhen(true, nameof(_ids))]
             [MemberNotNullWhen(true, nameof(_byteOffsets))]
             [MemberNotNullWhen(true, nameof(_lengths))]
-            private bool LoadData(string path)
+            [MemberNotNullWhen(true, nameof(_isBackwards))]
+            private bool LoadData(string tzFileDir, string path)
             {
                 if (!File.Exists(path))
                 {
@@ -254,7 +307,7 @@ private bool LoadData(string path)
                 {
                     using (FileStream fs = File.OpenRead(path))
                     {
-                        LoadTzFile(fs);
+                        LoadTzFile(tzFileDir, fs);
                     }
                     return true;
                 }
@@ -266,15 +319,16 @@ private bool LoadData(string path)
             [MemberNotNull(nameof(_ids))]
             [MemberNotNull(nameof(_byteOffsets))]
             [MemberNotNull(nameof(_lengths))]
-            private void LoadTzFile(Stream fs)
+            [MemberNotNull(nameof(_isBackwards))]
+            private void LoadTzFile(string tzFileDir, Stream fs)
             {
                 const int HeaderSize = 24;
                 Span<byte> buffer = stackalloc byte[HeaderSize];
 
                 ReadTzDataIntoBuffer(fs, 0, buffer);
 
                 LoadHeader(buffer, out int indexOffset, out int dataOffset);
-                ReadIndex(fs, indexOffset, dataOffset);
+                ReadIndex(tzFileDir, fs, indexOffset, dataOffset);
             }
 
             private void LoadHeader(Span<byte> buffer, out int indexOffset, out int dataOffset)
@@ -303,23 +357,25 @@ private void LoadHeader(Span<byte> buffer, out int indexOffset, out int dataOffs
             [MemberNotNull(nameof(_ids))]
             [MemberNotNull(nameof(_byteOffsets))]
             [MemberNotNull(nameof(_lengths))]
-            private void ReadIndex(Stream fs, int indexOffset, int dataOffset)
+            [MemberNotNull(nameof(_isBackwards))]
+            private void ReadIndex(string tzFileDir, Stream fs, int indexOffset, int dataOffset)
             {
                 int indexSize = dataOffset - indexOffset;
                 const int entrySize = 52; // Data entry size
                 int entryCount = indexSize / entrySize;
-
                 _byteOffsets = new int[entryCount];
                 _ids = new string[entryCount];
                 _lengths = new int[entryCount];
-
+                _isBackwards = new bool[entryCount];
+                FilterBackwardIDs(tzFileDir, out HashSet<string> tzLookupIDs);
                 for (int i = 0; i < entryCount; ++i)
                 {
                     LoadEntryAt(fs, indexOffset + (entrySize*i), out string id, out int byteOffset, out int length);
 
                     _byteOffsets[i] = byteOffset + dataOffset;
                     _ids[i] = id;
                     _lengths[i] = length;
+                    _isBackwards[i] = !tzLookupIDs.Contains(id);
 
                     if (length < 24) // Header Size
                     {
@@ -372,7 +428,25 @@ private void LoadEntryAt(Stream fs, long position, out string id, out int byteOf
 
             public string[] GetTimeZoneIds()
             {
-                return _ids;
+                int numTimeZoneIDs = 0;
+                for (int i = 0; i < _ids.Length; i++)
+                {
+                    if (!_isBackwards[i])
+                    {
+                        numTimeZoneIDs++;
+                    }
+                }
+                string[] nonBackwardsTZIDs = new string[numTimeZoneIDs];
+                var index = 0;
+                for (int i = 0; i < _ids.Length; i++)
+                {
+                    if (!_isBackwards[i])
+                    {
+                        nonBackwardsTZIDs[index] = _ids[i];
+                        index++;
+                    }
+                }
+                return nonBackwardsTZIDs;
             }
 
             public string GetTimeZoneDirectory()

src/libraries/System.Runtime/tests/System/TimeZoneInfoTests.cs

@@ -2873,6 +2873,20 @@ public static void AdjustmentRuleBaseUtcOffsetDeltaTest()
             Assert.Equal(new TimeSpan(2, 0, 0), customTimeZone.GetUtcOffset(new DateTime(2021, 3, 10, 2, 0, 0)));
         }
 
+        [Fact]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/64111", TestPlatforms.Linux)]
+        public static void NoBackwardTimeZones()
+        {
+            ReadOnlyCollection<TimeZoneInfo> tzCollection = TimeZoneInfo.GetSystemTimeZones();
+            HashSet<String> tzDisplayNames = new HashSet<String>();
+
+            foreach (TimeZoneInfo timezone in tzCollection)
+            {
+                tzDisplayNames.Add(timezone.DisplayName);
+            }
+            Assert.Equal(tzCollection.Count, tzDisplayNames.Count);
+        }
+
         private static bool IsEnglishUILanguage => CultureInfo.CurrentUICulture.Name.Length == 0 || CultureInfo.CurrentUICulture.TwoLetterISOLanguageName == "en";
 
         private static bool IsEnglishUILanguageAndRemoteExecutorSupported => IsEnglishUILanguage && RemoteExecutor.IsSupported;