Fall back to local cache if broker fails to return result for silent token call (#4395)

* Update SilentRequest.cs

* Fix failing unit tests

Author: ashok672

src/client/Microsoft.Identity.Client/Internal/Requests/Silent/SilentRequest.cs

@@ -66,20 +66,12 @@ protected override async Task<AuthenticationResult> ExecuteAsync(CancellationTok
                     _logger.Info("Broker is configured and enabled, attempting to use broker instead.");
                     var brokerResult = await _brokerStrategyLazy.Value.ExecuteAsync(cancellationToken).ConfigureAwait(false);
 
+                    // fallback to local cache if broker fails
                     if (brokerResult != null)
                     {
                         _logger.Verbose(() => "Broker responded to silent request.");
                         return brokerResult;
                     }
-                    else
-                    {
-                        _logger.Verbose(() => "Broker could not satisfy the silent request.");
-                        throw new MsalUiRequiredException(
-                           MsalError.FailedToAcquireTokenSilentlyFromBroker,
-                           "Broker could not satisfy the silent request.",
-                           null,
-                           UiRequiredExceptionClassification.AcquireTokenSilentFailed);
-                    }
                 }
 
                 _logger.Verbose(() => "Attempting to acquire token using local cache.");

tests/Microsoft.Identity.Test.Unit/BrokerTests/BrokerRequestTests.cs

@@ -526,7 +526,7 @@ public async Task NullBroker_AcquireSilentInteractive_Async()
                 }
                 catch (MsalUiRequiredException e)
                 {
-                    Assert.AreEqual("failed_to_acquire_token_silently_from_broker", e.ErrorCode);
+                    Assert.AreEqual("no_tokens_found", e.ErrorCode);
 
                     harness.HttpManager.AddInstanceDiscoveryMockHandler();
                     harness.HttpManager.AddSuccessTokenResponseMockHandlerForPost();

tests/Microsoft.Identity.Test.Unit/RequestsTests/SilentRequestTests.cs

@@ -130,25 +130,15 @@ public async Task BrokerSilentRequestLocalCacheTestAsync(bool brokerConfiguredBy
 
                 var request = new SilentRequest(harness.ServiceBundle, parameters, silentParameters);
 
-                if (brokerConfiguredByUser) //When broker is enabled, local cache should not be used
-                {
-                    var exception = await AssertException.TaskThrowsAsync<MsalUiRequiredException>(() =>
-                        request.RunAsync(default)
-                        ,false
-                        ).ConfigureAwait(false);
-
-                    Assert.IsNotNull(exception);
-                    Assert.AreEqual(MsalError.FailedToAcquireTokenSilentlyFromBroker, exception.ErrorCode);
-                    Assert.AreEqual("Broker could not satisfy the silent request.", exception.Message);
-                }
-                else
-                {
-                    var result = await request.RunAsync(default).ConfigureAwait(false);
+                var result = await request.RunAsync(default).ConfigureAwait(false);
 
-                    Assert.IsNotNull(result);
-                    Assert.IsNotNull(result.AccessToken);
-                    Assert.AreEqual(TestConstants.s_scope.AsSingleString(), result.Scopes.AsSingleString());
-                    Assert.AreEqual(brokerID, result.Account.Username);
+                // even if broker is configured by user but not installed, the result will be from the local cache
+                Assert.IsNotNull(result);
+                Assert.IsNotNull(result.AccessToken);
+                Assert.AreEqual(TestConstants.s_scope.AsSingleString(), result.Scopes.AsSingleString());
+                Assert.AreEqual(brokerID, result.Account.Username);
+                if (!brokerConfiguredByUser)
+                {
                     await mockBroker.DidNotReceiveWithAnyArgs().AcquireTokenSilentAsync(null, null).ConfigureAwait(false);
                 }
             }