[Android][libs] Introduce GetLocalUtcOffset temporary fast result

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -254,6 +254,8 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\DateTime.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\DateTimeKind.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\DateTimeOffset.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\DateTimeOffset.NonAndroid.cs" Condition="'$(TargetsAndroid)' != 'true'" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\DateTimeOffset.Android.cs" Condition="'$(TargetsAndroid)' == 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\DayOfWeek.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\DBNull.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Decimal.cs" />

src/libraries/System.Private.CoreLib/src/System/DateTimeOffset.Android.cs

@@ -0,0 +1,76 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Threading;
+
+namespace System
+{
+    public readonly partial struct DateTimeOffset
+    {
+        private static bool s_androidTZDataLoaded;
+        private static readonly object s_localUtcOffsetLock = new();
+        private static Thread? s_loadAndroidTZData;
+
+        // Now on Android does the following
+        // 1) quickly returning a fast path result when first called if the right AppContext data element is set
+        // 2) starting a background thread to load TimeZoneInfo local cache
+        //
+        // On Android, loading AndroidTZData is expensive for startup performance.
+        // The fast result relies on `System.TimeZoneInfo.LocalDateTimeOffset` being set
+        // in the App Context's properties as the appropriate local date time offset from UTC.
+        // monovm_initialize(_preparsed) can be leveraged to do so.
+        // However, to handle timezone changes during the app lifetime, AndroidTZData needs to be loaded.
+        // So, on first call, we return the fast path and start a background thread to load
+        // the TimeZoneInfo Local cache implementation which loads AndroidTZData.
+        public static DateTimeOffset Now
+        {
+            get
+            {
+                DateTime utcDateTime = DateTime.UtcNow;
+
+                if (s_androidTZDataLoaded) // The background thread finished, the cache is loaded.
+                    return ToLocalTime(utcDateTime, true);
+
+                if (!s_androidTZDataLoaded && s_loadAndroidTZData == null) // The cache isn't loaded and no background thread has been created
+                {
+                    lock (s_localUtcOffsetLock)
+                    {
+                        // Now may be called multiple times before a cache is loaded and a background thread is running,
+                        // once the lock is available, check for a cache and background thread.
+                        if (s_androidTZDataLoaded)
+                            return ToLocalTime(utcDateTime, true);
+
+                        if (s_loadAndroidTZData == null)
+                        {
+                            s_loadAndroidTZData = new Thread(() => {
+                                // Delay the background thread to avoid impacting startup, if it still coincides after 1s, startup is already perceived as slow
+                                Thread.Sleep(1000);
+                                _ = TimeZoneInfo.Local; // Load AndroidTZData
+                                lock (s_localUtcOffsetLock)
+                                {
+                                    s_androidTZDataLoaded = true;
+                                    s_loadAndroidTZData = null; // Ensure thread is cleared when cache is loaded
+                                }
+                            });
+                            s_loadAndroidTZData.IsBackground = true;
+                            s_loadAndroidTZData.Start();
+                        }
+                    }
+                }
+
+                object? localDateTimeOffset = AppContext.GetData("System.TimeZoneInfo.LocalDateTimeOffset");
+                if (localDateTimeOffset == null) // If no offset property provided through monovm app context, default
+                    return ToLocalTime(utcDateTime, true);
+
+                // Fast path obtained offset incorporated into ToLocalTime(DateTime.UtcNow, true) logic
+                int localDateTimeOffsetSeconds = Convert.ToInt32(localDateTimeOffset);
+                TimeSpan offset = TimeSpan.FromSeconds(localDateTimeOffsetSeconds);
+                long localTicks = utcDateTime.Ticks + offset.Ticks;
+                if (localTicks < DateTime.MinTicks || localTicks > DateTime.MaxTicks)
+                    throw new ArgumentException(SR.Arg_ArgumentOutOfRangeException);
+
+                return new DateTimeOffset(localTicks, offset);
+            }
+        }
+    }
+}

src/libraries/System.Private.CoreLib/src/System/DateTimeOffset.NonAndroid.cs

@@ -0,0 +1,12 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System
+{
+    public readonly partial struct DateTimeOffset
+    {
+        // Returns a DateTimeOffset representing the current date and time. The
+        // resolution of the returned value depends on the system timer.
+        public static DateTimeOffset Now => ToLocalTime(DateTime.UtcNow, true);
+    }
+}

src/libraries/System.Private.CoreLib/src/System/DateTimeOffset.cs

@@ -34,7 +34,7 @@ namespace System
     [StructLayout(LayoutKind.Auto)]
     [Serializable]
     [TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")]
-    public readonly struct DateTimeOffset
+    public readonly partial struct DateTimeOffset
         : IComparable,
           ISpanFormattable,
           IComparable<DateTimeOffset>,
@@ -321,10 +321,6 @@ public DateTimeOffset(int year, int month, int day, int hour, int minute, int se
             _dateTime = _dateTime.AddMicroseconds(microsecond);
         }
 
-        // Returns a DateTimeOffset representing the current date and time. The
-        // resolution of the returned value depends on the system timer.
-        public static DateTimeOffset Now => ToLocalTime(DateTime.UtcNow, true);
-
         public static DateTimeOffset UtcNow
         {
             get

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

@@ -107,7 +107,7 @@ private static int ParseGMTNumericZone(string name)
             {
                 return new TimeZoneInfo(id, TimeSpan.FromSeconds(0), id, name, name, null, disableDaylightSavingTime:true);
             }
-            if (name.StartsWith("GMT", StringComparison.Ordinal))
+            if (name.Length >= 3 && name[0] == 'G' && name[1] == 'M' && name[2] == 'T')
             {
                 return new TimeZoneInfo(id, TimeSpan.FromSeconds(ParseGMTNumericZone(name)), id, name, name, null, disableDaylightSavingTime:true);
             }

src/tasks/AndroidAppBuilder/Templates/MonoRunner.java

@@ -26,6 +26,8 @@
 import java.util.ArrayList;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipInputStream;
+import java.time.OffsetDateTime;
+import java.time.ZoneOffset;
 
 public class MonoRunner extends Instrumentation
 {
@@ -88,7 +90,8 @@ public static int initialize(String entryPointLibName, String[] args, Context co
         unzipAssets(context, filesDir, "assets.zip");
 
         Log.i("DOTNET", "MonoRunner initialize,, entryPointLibName=" + entryPointLibName);
-        return initRuntime(filesDir, cacheDir, testResultsDir, entryPointLibName, args);
+        int localDateTimeOffset = OffsetDateTime.now().getOffset().getTotalSeconds();
+        return initRuntime(filesDir, cacheDir, testResultsDir, entryPointLibName, args, localDateTimeOffset);
     }
 
     @Override
@@ -149,7 +152,7 @@ static void unzipAssets(Context context, String toPath, String zipName) {
         }
     }
 
-    static native int initRuntime(String libsDir, String cacheDir, String testResultsDir, String entryPointLibName, String[] args);
+    static native int initRuntime(String libsDir, String cacheDir, String testResultsDir, String entryPointLibName, String[] args, int local_date_time_offset);
 
     static native int setEnv(String key, String value);
 }

src/tasks/AndroidAppBuilder/Templates/monodroid.c

@@ -203,7 +203,7 @@ cleanup_runtime_config (MonovmRuntimeConfigArguments *args, void *user_data)
 }
 
 int
-mono_droid_runtime_init (const char* executable, int managed_argc, char* managed_argv[])
+mono_droid_runtime_init (const char* executable, int managed_argc, char* managed_argv[], int local_date_time_offset)
 {
     // NOTE: these options can be set via command line args for adb or xharness, see AndroidSampleApp.csproj
 
@@ -225,13 +225,17 @@ mono_droid_runtime_init (const char* executable, int managed_argc, char* managed
 
     // TODO: set TRUSTED_PLATFORM_ASSEMBLIES, APP_PATHS and NATIVE_DLL_SEARCH_DIRECTORIES
 
-    const char* appctx_keys[2];
+    const char* appctx_keys[3];
     appctx_keys[0] = "RUNTIME_IDENTIFIER";
     appctx_keys[1] = "APP_CONTEXT_BASE_DIRECTORY";
+    appctx_keys[2] = "System.TimeZoneInfo.LocalDateTimeOffset";
 
-    const char* appctx_values[2];
+    const char* appctx_values[3];
     appctx_values[0] = ANDROID_RUNTIME_IDENTIFIER;
     appctx_values[1] = bundle_path;
+    char local_date_time_offset_buffer[32];
+    snprintf (local_date_time_offset_buffer, sizeof(local_date_time_offset_buffer), "%d", local_date_time_offset);
+    appctx_values[2] = strdup (local_date_time_offset_buffer);
 
     char *file_name = RUNTIMECONFIG_BIN_FILE;
     int str_len = strlen (bundle_path) + strlen (file_name) + 1; // +1 is for the "/"
@@ -251,7 +255,7 @@ mono_droid_runtime_init (const char* executable, int managed_argc, char* managed
         free (file_path);
     }
 
-    monovm_initialize(2, appctx_keys, appctx_values);
+    monovm_initialize(3, appctx_keys, appctx_values);
 
     mono_debug_init (MONO_DEBUG_FORMAT_MONO);
     mono_install_assembly_preload_hook (mono_droid_assembly_preload_hook, NULL);
@@ -318,7 +322,7 @@ Java_net_dot_MonoRunner_setEnv (JNIEnv* env, jobject thiz, jstring j_key, jstrin
 }
 
 int
-Java_net_dot_MonoRunner_initRuntime (JNIEnv* env, jobject thiz, jstring j_files_dir, jstring j_cache_dir, jstring j_testresults_dir, jstring j_entryPointLibName, jobjectArray j_args)
+Java_net_dot_MonoRunner_initRuntime (JNIEnv* env, jobject thiz, jstring j_files_dir, jstring j_cache_dir, jstring j_testresults_dir, jstring j_entryPointLibName, jobjectArray j_args, long current_local_time)
 {
     char file_dir[2048];
     char cache_dir[2048];
@@ -347,7 +351,7 @@ Java_net_dot_MonoRunner_initRuntime (JNIEnv* env, jobject thiz, jstring j_files_
         managed_argv[i + 1] = (*env)->GetStringUTFChars(env, j_arg, NULL);
     }
 
-    int res = mono_droid_runtime_init (executable, managed_argc, managed_argv);
+    int res = mono_droid_runtime_init (executable, managed_argc, managed_argv, current_local_time);
 
     for (int i = 0; i < args_len; ++i)
     {