fix: do not recreate Random each time (#945)

* fix: do not recreate Random each time

On .NET Framework it is incorrect to recreate Random instances each time, these should instead be one per instance. On .NET Core and later this is not a limitation, but the libraries share a codebase.

Fixes #944

* Use a thread safe Random instance #944

* Forgot to commit csproj change #944

* Address PR comment #944

Box.V2/Box.V2.csproj

@@ -276,6 +276,7 @@
     <Compile Include="Utility\ContentTypeMapper.cs" />
     <Compile Include="Utility\Retry.cs" />
     <Compile Include="Utility\SharedLinkUtils.cs" />
+    <Compile Include="Utility\ThreadSafeRandom.cs" />
     <Compile Include="Wrappers\BoxError.cs" />
     <Compile Include="Wrappers\BoxErrorContextInfo.cs" />
     <Compile Include="Wrappers\BoxBinaryRequest.cs" />
@@ -343,4 +344,4 @@
     <None Include="packages.config" />
   </ItemGroup>
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
-</Project>
+</Project>

Box.V2/Utility/ExponentialBackoff.cs

@@ -10,9 +10,10 @@ public TimeSpan GetRetryTimeout(int numRetries)
             const double RETRY_RANDOMIZATION_FACTOR = 0.5;
             var minRandomization = 1 - RETRY_RANDOMIZATION_FACTOR;
             var maxRandomization = 1 + RETRY_RANDOMIZATION_FACTOR;
-            var random = new Random();
 
-            var randomization = random.NextDouble() * (maxRandomization - minRandomization) + minRandomization;
+            var randomization = ThreadSafeRandom.Instance.NextDouble()
+                * (maxRandomization - minRandomization) + minRandomization;
+
             var exponential = Math.Pow(2, numRetries - 1);
             var result = Math.Ceiling(exponential * baseInterval.TotalSeconds * randomization);
             return TimeSpan.FromSeconds(result);

Box.V2/Utility/ThreadSafeRandom.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Runtime.CompilerServices;
+
+namespace Box.V2.Utility
+{
+    /// <summary>
+    /// A thread safe implementation of <see cref="Random"/>, following best practices
+    /// for .NET Framework and .NET Standard.
+    /// </summary>
+    /// <seealso href="https://learn.microsoft.com/en-us/dotnet/fundamentals/runtime-libraries/system-random"/>
+    internal static class ThreadSafeRandom
+    {
+        //
+        // Adapted from https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/Random.cs
+        //
+        // NOTE: when/if this library updates to .NET 6+, this code can be replaced with `Random.Shared`
+        //
+
+        [ThreadStatic]
+        private static Random _random;
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private static Random CreateRandom() => _random = new Random();
+
+        /// <summary>
+        /// An instance of <see cref="Random"/> specific to the calling thread.
+        /// Do not pass this instance to other threads or contexts.
+        /// </summary>
+        public static Random Instance => _random ?? CreateRandom();
+    }
+}