[Android][libs] Introduce GetLocalUtcOffset temporary fast result

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[0] == 'G' && name[1] == 'M' && name[2] == 'T')
             {
                 return new TimeZoneInfo(id, TimeSpan.FromSeconds(ParseGMTNumericZone(name)), id, name, name, null, disableDaylightSavingTime:true);
             }
@@ -142,6 +142,56 @@ private static TimeZoneInfo GetLocalTimeZoneCore()
             return Utc;
         }
 
+        private static TimeSpan GetCacheLocalUtcOffset(DateTime dateTime, TimeZoneInfoOptions flags)
+        {
+                CachedData cachedData = s_cachedData;
+                return cachedData.Local.GetUtcOffset(dateTime, flags, cachedData);
+        }
+
+        private static TimeSpan? _localUtcOffset;
+        private static object _localUtcOffsetLock = new();
+        private static Thread? _loadAndroidTZData;
+        // Shortcut for TimeZoneInfo.Local.GetUtcOffset
+        // On Android, loading AndroidTZData while obtaining cachedData.Local is expensive for startup.
+        // We introduce a fast result for GetLocalUtcOffset that relies on the date time offset being
+        // passed into monovm_initialize(_preparsed) from Java in seconds as LOCAL_DATE_TIME_OFFSET.
+        // However, to handle timezone changes during the app lifetime, AndroidTZData needs to be loaded.
+        // The fast path is initially used, and we start a background thread to get cachedData.Local
+        internal static TimeSpan GetLocalUtcOffset(DateTime dateTime, TimeZoneInfoOptions flags)
+        {
+            if (_localUtcOffset != null) // The background thread finished, the cache is loaded.
+                return GetCacheLocalUtcOffset(dateTime, flags);
+
+            if (_localUtcOffset == null && _loadAndroidTZData == null) // The cache isn't loaded and no background thread has been created
+            {
+                lock (_localUtcOffsetLock)
+                {
+                    // GetLocalUtcOffset 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 (_localUtcOffset != null)
+                        return GetCacheLocalUtcOffset(dateTime, flags);
+
+                    if (_loadAndroidTZData == null)
+                    {
+                        _loadAndroidTZData = new Thread(() => {
+                            Thread.Sleep(1000);
+                            _localUtcOffset = GetCacheLocalUtcOffset(dateTime, flags);
+                        });
+                        _loadAndroidTZData.IsBackground = true;
+                        _loadAndroidTZData.Start();
+                    }
+                }
+            }
+
+            object? localDateTimeOffset = AppContext.GetData("System.TimeZoneInfo.LocalDateTimeOffset");
+            if (localDateTimeOffset == null)
+                return GetCacheLocalUtcOffset(dateTime, flags); // If no offset property provided through monovm app context, default
+
+            long localDateTimeOffsetSeconds = Convert.ToInt32(localDateTimeOffset);
+            TimeSpan offset = TimeSpan.FromSeconds(localDateTimeOffsetSeconds);
+            return offset;
+        }
+
         private static TimeZoneInfoResult TryGetTimeZoneFromLocalMachineCore(string id, out TimeZoneInfo? value, out Exception? e)
         {
 

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

@@ -24,6 +24,13 @@ private static TimeZoneInfo GetLocalTimeZoneCore()
             return GetLocalTimeZoneFromTzFile();
         }
 
+        // Shortcut for TimeZoneInfo.Local.GetUtcOffset
+        internal static TimeSpan GetLocalUtcOffset(DateTime dateTime, TimeZoneInfoOptions flags)
+        {
+            CachedData cachedData = s_cachedData;
+            return cachedData.Local.GetUtcOffset(dateTime, flags, cachedData);
+        }
+
         private static TimeZoneInfoResult TryGetTimeZoneFromLocalMachineCore(string id, out TimeZoneInfo? value, out Exception? e)
         {
             value = null;

src/libraries/System.Private.CoreLib/src/System/TimeZoneInfo.Win32.cs

@@ -76,6 +76,13 @@ public OffsetAndRule(int year, TimeSpan offset, AdjustmentRule? rule)
             }
         }
 
+        // Shortcut for TimeZoneInfo.Local.GetUtcOffset
+        internal static TimeSpan GetLocalUtcOffset(DateTime dateTime, TimeZoneInfoOptions flags)
+        {
+            CachedData cachedData = s_cachedData;
+            return cachedData.Local.GetUtcOffset(dateTime, flags, cachedData);
+        }
+
         /// <summary>
         /// Returns a cloned array of AdjustmentRule objects
         /// </summary>

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

@@ -312,13 +312,6 @@ public TimeSpan GetUtcOffset(DateTimeOffset dateTimeOffset) =>
         public TimeSpan GetUtcOffset(DateTime dateTime) =>
             GetUtcOffset(dateTime, TimeZoneInfoOptions.NoThrowOnInvalidTime, s_cachedData);
 
-        // Shortcut for TimeZoneInfo.Local.GetUtcOffset
-        internal static TimeSpan GetLocalUtcOffset(DateTime dateTime, TimeZoneInfoOptions flags)
-        {
-            CachedData cachedData = s_cachedData;
-            return cachedData.Local.GetUtcOffset(dateTime, flags, cachedData);
-        }
-
         /// <summary>
         /// Returns the Universal Coordinated Time (UTC) Offset for the current TimeZoneInfo instance.
         /// </summary>

src/mono/mono/mini/monovm.c

@@ -18,6 +18,7 @@ static MonoCoreTrustedPlatformAssemblies *trusted_platform_assemblies;
 static MonoCoreLookupPaths *native_lib_paths;
 static MonoCoreLookupPaths *app_paths;
 static MonoCoreLookupPaths *platform_resource_roots;
+static MonoCoreLocalTime *local_time;
 
 static void
 mono_core_trusted_platform_assemblies_free (MonoCoreTrustedPlatformAssemblies *a)
@@ -220,6 +221,7 @@ monovm_initialize_preparsed (MonoCoreRuntimeProperties *parsed_properties, int p
 	trusted_platform_assemblies = parsed_properties->trusted_platform_assemblies;
 	app_paths = parsed_properties->app_paths;
 	native_lib_paths = parsed_properties->native_dll_search_directories;
+	local_time = parsed_properties->local_time;
 	mono_loader_install_pinvoke_override (parsed_properties->pinvoke_override);
 
 	finish_initialization ();

src/native/public/mono/jit/details/mono-private-unstable-types.h

@@ -45,10 +45,15 @@ typedef struct {
 	char **dirs;
 } MonoCoreLookupPaths;
 
+typedef struct {
+	uint64_t current_local_time;
+} MonoCoreLocalTime;
+
 typedef struct {
 	MonoCoreTrustedPlatformAssemblies *trusted_platform_assemblies;
 	MonoCoreLookupPaths *app_paths;
 	MonoCoreLookupPaths *native_dll_search_directories;
+	MonoCoreLocalTime *local_time;
 	PInvokeOverrideFn pinvoke_override;
 } MonoCoreRuntimeProperties;
 

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 +225,24 @@ 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);
+
+    MonoCoreLocalTime mclt = {
+        local_date_time_offset,
+    };
+    MonoCoreRuntimeProperties mcrp = {
+        &mclt,
+    };
 
     char *file_name = RUNTIMECONFIG_BIN_FILE;
     int str_len = strlen (bundle_path) + strlen (file_name) + 1; // +1 is for the "/"
@@ -251,7 +262,7 @@ mono_droid_runtime_init (const char* executable, int managed_argc, char* managed
         free (file_path);
     }
 
-    monovm_initialize(2, appctx_keys, appctx_values);
+    monovm_initialize_preparsed(&mcrp, 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 +329,7 @@ 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 +358,7 @@ 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)
     {