From c9fbbad50f95ea89ae859e515247fe2d815d35f3 Mon Sep 17 00:00:00 2001
From: Steve Pfister <steve.pfister@microsoft.com>
Date: Mon, 12 Sep 2022 11:47:17 -0400
Subject: [PATCH] [Android] Update DateTimeOffset.Now fast path routine

https://github.com/dotnet/runtime/pull/74965 contained improvements for the routine. This change brings them back to main.
---
 .../src/System/DateTimeOffset.Android.cs      | 66 +++++++------------
 1 file changed, 25 insertions(+), 41 deletions(-)

diff --git a/src/libraries/System.Private.CoreLib/src/System/DateTimeOffset.Android.cs b/src/libraries/System.Private.CoreLib/src/System/DateTimeOffset.Android.cs
index c78dadcdfc3bd..c40e4708e47aa 100644
--- a/src/libraries/System.Private.CoreLib/src/System/DateTimeOffset.Android.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/DateTimeOffset.Android.cs
@@ -7,10 +7,8 @@ namespace System
 {
     public readonly partial struct DateTimeOffset
     {
-        private static bool s_androidTZDataLoaded;
-        private static readonly object s_localUtcOffsetLock = new();
-        private static Thread? s_loadAndroidTZData;
-        private static bool s_startNewBackgroundThread = true;
+        // 0 == in process of being loaded, 1 == loaded
+        private static volatile int s_androidTZDataLoaded = -1;
 
         // Now on Android does the following
         // 1) quickly returning a fast path result when first called if the right AppContext data element is set
@@ -29,52 +27,38 @@ public static DateTimeOffset Now
             {
                 DateTime utcDateTime = DateTime.UtcNow;
 
-                if (s_androidTZDataLoaded) // The background thread finished, the cache is loaded.
+                if (s_androidTZDataLoaded == 1) // The background thread finished, the cache is loaded.
+                {
                     return ToLocalTime(utcDateTime, true);
+                }
 
-                if (s_startNewBackgroundThread) // The cache isn't loaded and no background thread has been created
+                object? localDateTimeOffset = AppContext.GetData("System.TimeZoneInfo.LocalDateTimeOffset");
+                if (localDateTimeOffset == null) // If no offset property provided through monovm app context, default
                 {
-                    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);
+                    // no need to create the thread, load tzdata now
+                    s_androidTZDataLoaded = 1;
+                    return ToLocalTime(utcDateTime, true);
+                }
 
-                        if (s_loadAndroidTZData == null)
+                // The cache isn't loaded yet.
+                if (Interlocked.CompareExchange(ref s_androidTZDataLoaded, 0, -1) == -1)
+                {
+                    new Thread(() =>
+                    {
+                        try
                         {
-                            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
-                                s_androidTZDataLoaded = true;
+                            // Delay the background thread to avoid impacting startup, if it still coincides after 1s, startup is already perceived as slow
+                            Thread.Sleep(1000);
 
-                                lock (s_localUtcOffsetLock)
-                                {
-                                    s_loadAndroidTZData = null; // Ensure thread is cleared when cache is loaded
-                                }
-                            });
-                            s_loadAndroidTZData.IsBackground = true;
+                            _ = TimeZoneInfo.Local; // Load AndroidTZData
                         }
-                    }
-
-                    if (s_startNewBackgroundThread)
-                    {
-                        // Because Start does not block the calling thread,
-                        // setting the boolean flag to false immediately after should
-                        // prevent two calls to DateTimeOffset.Now in quick succession
-                        // from both reaching here.
-                        s_loadAndroidTZData.Start();
-                        s_startNewBackgroundThread = false;
-                    }
+                        finally
+                        {
+                            s_androidTZDataLoaded = 1;
+                        }
+                    }) { IsBackground = true }.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);