Perform background refresh of credentials during preempt expiry period (#3541)

* Perform background refresh of credentials during preempt expiry time period
* Remove unnecessary warning suppressions
* Remove ToUniversalTime usage

Author: kevinstapleton

sdk/src/Core/Amazon.Runtime/Credentials/RefreshingAWSCredentials.cs

@@ -36,11 +36,7 @@ public abstract class RefreshingAWSCredentials : AWSCredentials, IDisposable
         /// </summary>
         public class CredentialsRefreshState
         {
-            public ImmutableCredentials Credentials
-            {
-                get; 
-                set;
-            }
+            public ImmutableCredentials Credentials { get; set; }
             public DateTime Expiration { get; set; }
 
             public CredentialsRefreshState()
@@ -56,8 +52,16 @@ public CredentialsRefreshState(ImmutableCredentials credentials, DateTime expira
             internal bool IsExpiredWithin(TimeSpan preemptExpiryTime)
             {
                 var now = AWSSDKUtils.CorrectedUtcNow;
-                var exp = Expiration.ToUniversalTime();
-                return (now > exp - preemptExpiryTime);
+                var exp = Expiration;
+                return now > exp - preemptExpiryTime;
+            }
+
+            internal TimeSpan GetTimeToLive(TimeSpan preemptExpiryTime)
+            {
+                var now = AWSSDKUtils.CorrectedUtcNow;
+                var exp = Expiration;
+
+                return exp - now + preemptExpiryTime;
             }
         }
 
@@ -110,46 +114,106 @@ public TimeSpan PreemptExpiryTime
         /// <returns></returns>
         public override ImmutableCredentials GetCredentials()
         {
-            _updateGeneratedCredentialsSemaphore.Wait();
-            try
+            // We save the currentState as it might be modified or cleared.
+            var tempState = currentState;
+
+            var ttl = tempState?.GetTimeToLive(PreemptExpiryTime);
+
+            if (ttl > TimeSpan.Zero)
             {
-                // We save the currentState as it might be modified or cleared.
-                var tempState = currentState;
-                // If credentials are expired or we don't have any state yet, update
-                if (ShouldUpdateState(tempState, PreemptExpiryTime))
+                if (ttl < PreemptExpiryTime)
                 {
-                    tempState = GenerateNewCredentials();
-                    UpdateToGeneratedCredentials(tempState, PreemptExpiryTime);
-                    currentState = tempState;
+                    // background refresh (fire & forget)
+                    if (_updateGeneratedCredentialsSemaphore.Wait(0))
+                    {
+                        _ = System.Threading.Tasks.Task.Run(GenerateCredentialsAndUpdateState);
+                    }
                 }
-                return tempState.Credentials.Copy();
             }
-            finally
+            else
             {
-                _updateGeneratedCredentialsSemaphore.Release();
+                // If credentials are expired, update
+                _updateGeneratedCredentialsSemaphore.Wait();
+                tempState = GenerateCredentialsAndUpdateState();
+            }
+
+            return tempState.Credentials.Copy();
+
+            CredentialsRefreshState GenerateCredentialsAndUpdateState()
+            {
+                System.Diagnostics.Debug.Assert(_updateGeneratedCredentialsSemaphore.CurrentCount == 0);
+
+                try
+                {
+                    var tempState = currentState;
+                    // double-check that the credentials still need updating
+                    // as it's possible that multiple requests were queued acquiring the semaphore
+                    if (ShouldUpdateState(tempState, PreemptExpiryTime))
+                    {
+                        tempState = GenerateNewCredentials();
+                        UpdateToGeneratedCredentials(tempState, PreemptExpiryTime);
+                        currentState = tempState;
+                    }
+
+                    return tempState;
+                }
+                finally
+                {
+                    _updateGeneratedCredentialsSemaphore.Release();
+                }
             }
         }
 
 #if AWS_ASYNC_API
         public override async System.Threading.Tasks.Task<ImmutableCredentials> GetCredentialsAsync()
         {
-            await _updateGeneratedCredentialsSemaphore.WaitAsync().ConfigureAwait(false);
-            try
+            // We save the currentState as it might be modified or cleared.
+            var tempState = currentState;
+
+            var ttl = tempState?.GetTimeToLive(PreemptExpiryTime);
+
+            if (ttl > TimeSpan.Zero)
             {
-                // We save the currentState as it might be modified or cleared.
-                var tempState = currentState;
-                // If credentials are expired, update
-                if (ShouldUpdateState(tempState, PreemptExpiryTime))
+                if (ttl < PreemptExpiryTime)
                 {
-                    tempState = await GenerateNewCredentialsAsync().ConfigureAwait(false);
-                    UpdateToGeneratedCredentials(tempState, PreemptExpiryTime);
-                    currentState = tempState;
+                    // background refresh (fire & forget)
+                    if (_updateGeneratedCredentialsSemaphore.Wait(0))
+                    {
+                        _ = GenerateCredentialsAndUpdateStateAsync();
+                    }
                 }
-                return tempState.Credentials.Copy();
             }
-            finally
+            else
+            {
+                // If credentials are expired, update
+                await _updateGeneratedCredentialsSemaphore.WaitAsync().ConfigureAwait(false);
+                tempState = await GenerateCredentialsAndUpdateStateAsync().ConfigureAwait(false);
+            }
+
+            return tempState.Credentials.Copy();
+
+            async System.Threading.Tasks.Task<CredentialsRefreshState> GenerateCredentialsAndUpdateStateAsync()
             {
-                _updateGeneratedCredentialsSemaphore.Release();
+                System.Diagnostics.Debug.Assert(_updateGeneratedCredentialsSemaphore.CurrentCount == 0);
+
+                try
+                {
+                    var tempState = currentState;
+                    // double-check that the credentials still need updating
+                    // as it's possible that multiple requests were queued acquiring the semaphore
+                    if (ShouldUpdateState(tempState, PreemptExpiryTime))
+                    {
+                        tempState = await GenerateNewCredentialsAsync().ConfigureAwait(false);
+                        UpdateToGeneratedCredentials(tempState, PreemptExpiryTime);
+                        currentState = tempState;
+                    }
+
+                    return tempState;
+                }
+                finally
+                {
+                    _updateGeneratedCredentialsSemaphore.Release();
+                }
             }
         }
 #endif
@@ -246,7 +310,7 @@ protected virtual CredentialsRefreshState GenerateNewCredentials()
             throw new NotImplementedException();
         }
 #if AWS_ASYNC_API
-        /// <summary>
+        /// <summary> 
         /// When overridden in a derived class, generates new credentials and new expiration date.
         /// 
         /// Called on first credentials request and when expiration date is in the past.

sdk/test/NetStandard/UnitTests/AWSSDK.UnitTests.Custom.NetStandard.csproj

@@ -59,7 +59,7 @@ This project file should not be used as part of a release pipeline.
 		<PackageReference Include="System.Text.RegularExpressions" Version="4.3.1" />
 		<PackageReference Include="xunit" Version="2.4.2" />
 		<PackageReference Include="Moq" Version="4.18.4" />
-		<PackageReference Include="xunit.runner.visualstudio" Version="2.4.5" >
+		<PackageReference Include="xunit.runner.visualstudio" Version="2.4.5">
 		    <PrivateAssets>all</PrivateAssets>
 		    <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
 		</PackageReference>