Update locking and caching for `OpenIdConnectCachingSecurityTokenProv…

…ider`

src/Microsoft.Identity.Web.OWIN/OpenIdConnectCachingSecurityTokenProvider.cs

@@ -1,30 +1,28 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
 using Microsoft.IdentityModel.Protocols;
 using Microsoft.IdentityModel.Protocols.OpenIdConnect;
 using Microsoft.IdentityModel.Tokens;
 using Microsoft.Owin.Security.Jwt;
-using System.Collections.Generic;
-using System.Threading;
-using System.Threading.Tasks;
 
 namespace Microsoft.Identity.Web
 {
     // This class is necessary because the OAuthBearer Middleware does not leverage
     // the OpenID Connect metadata endpoint exposed by the STS by default.
     internal class OpenIdConnectCachingSecurityTokenProvider : IIssuerSecurityKeyProvider
     {
+        private int _lock = 0;
+
         public ConfigurationManager<OpenIdConnectConfiguration> _configManager;
         private string? _issuer;
         private IEnumerable<SecurityKey>? _keys;
-        private readonly string _metadataEndpoint;
-
-        private readonly ReaderWriterLockSlim _synclock = new ReaderWriterLockSlim();
 
         public OpenIdConnectCachingSecurityTokenProvider(string metadataEndpoint)
         {
-            _metadataEndpoint = metadataEndpoint;
             _configManager = new ConfigurationManager<OpenIdConnectConfiguration>(metadataEndpoint, new OpenIdConnectConfigurationRetriever());
 
             RetrieveMetadata();
@@ -41,15 +39,7 @@ public string? Issuer
             get
             {
                 RetrieveMetadata();
-                _synclock.EnterReadLock();
-                try
-                {
-                    return _issuer;
-                }
-                finally
-                {
-                    _synclock.ExitReadLock();
-                }
+                return _issuer;
             }
         }
 
@@ -64,32 +54,29 @@ public IEnumerable<SecurityKey>? SecurityKeys
             get
             {
                 RetrieveMetadata();
-                _synclock.EnterReadLock();
-                try
-                {
-                    return _keys;
-                }
-                finally
-                {
-                    _synclock.ExitReadLock();
-                }
+                return _keys;
             }
         }
 
         private void RetrieveMetadata()
         {
-            _synclock.EnterWriteLock();
-            try
+            // Try to acquire the lock
+            //
+            // Interlocked.Exchange returns the original value of _lock before it was swapped.
+            // If it's 0, it means it went from 0 to 1, so we did acquire the lock.
+            // If it's 1, then the lock was already acquired by another thread.
+            //
+            // See the example in the Exchange(Int32, Int32) overload: https://learn.microsoft.com/en-us/dotnet/api/system.threading.interlocked.exchange?view=netframework-4.7.2
+            if (Interlocked.Exchange(ref _lock, 1) == 0)
             {
 #pragma warning disable VSTHRD002 // Avoid problematic synchronous waits
                 OpenIdConnectConfiguration config = Task.Run(_configManager.GetConfigurationAsync).Result;
 #pragma warning restore VSTHRD002 // Avoid problematic synchronous waits
                 _issuer = config.Issuer;
                 _keys = config.SigningKeys;
-            }
-            finally
-            {
-                _synclock.ExitWriteLock();
+
+                // Release the lock
+                Interlocked.Exchange(ref _lock, 0);
             }
         }
     }