Add msi source detect logic

src/client/Microsoft.Identity.Client/ManagedIdentity/ManagedIdentityClient.cs

@@ -47,5 +47,41 @@ private static AbstractManagedIdentity SelectManagedIdentitySource(RequestContex
                 AzureArcManagedIdentitySource.TryCreate(requestContext) ??
                 new ImdsManagedIdentitySource(requestContext);
         }
+
+        internal static ManagedIdentitySource GetManagedIdentitySource()
+        {
+            string identityEndpoint = EnvironmentVariables.IdentityEndpoint;
+            string identityHeader = EnvironmentVariables.IdentityHeader;
+            string identityServerThumbprint = EnvironmentVariables.IdentityServerThumbprint;
+            string msiSecret = EnvironmentVariables.IdentityHeader;
+            string msiEndpoint = EnvironmentVariables.MsiEndpoint;
+            string imdsEndpoint = EnvironmentVariables.ImdsEndpoint;
+            string podIdentityEndpoint = EnvironmentVariables.PodIdentityEndpoint;
+
+            if (!string.IsNullOrEmpty(identityEndpoint) && !string.IsNullOrEmpty(identityHeader) && !string.IsNullOrEmpty(identityServerThumbprint))
+            {
+                return ManagedIdentitySource.ServiceFabric;
+            }
+            else if (!string.IsNullOrEmpty(identityEndpoint) && !string.IsNullOrEmpty(identityHeader))
+            {
+                return ManagedIdentitySource.AppService;
+            }
+            else if (!string.IsNullOrEmpty(msiEndpoint))
+            {
+                return ManagedIdentitySource.CloudShell;
+            }
+            else if (!string.IsNullOrEmpty(identityEndpoint) && !string.IsNullOrEmpty(imdsEndpoint))
+            {
+                return ManagedIdentitySource.AzureArc;
+            }
+            else if (!string.IsNullOrEmpty(podIdentityEndpoint))
+            {
+                return ManagedIdentitySource.Imds;
+            }
+            else
+            {
+                return ManagedIdentitySource.DefaultToImds;
+            }
+        }
     }
 }

src/client/Microsoft.Identity.Client/ManagedIdentity/ManagedIdentitySource.cs

@@ -42,6 +42,12 @@ public enum ManagedIdentitySource
         /// <summary>
         /// The source to acquire token for managed identity is Service Fabric.
         /// </summary>
-        ServiceFabric
+        ServiceFabric,
+
+        /// <summary>
+        /// The source to acquire token for managed identity is defaulted to IMDS as no environment variables are found. 
+        /// This is specifically used when the source is not known.
+        /// </summary>
+        DefaultToImds
     }
 }

src/client/Microsoft.Identity.Client/ManagedIdentityApplication.cs

@@ -54,5 +54,14 @@ public AcquireTokenForManagedIdentityParameterBuilder AcquireTokenForManagedIden
                 ClientExecutorFactory.CreateManagedIdentityExecutor(this),
                 resource);
         }
+
+        /// <summary>
+        /// Detects and returns the managed identity source available on the environment.
+        /// </summary>
+        /// <returns>Managed identity source detected on the environment if any.</returns>
+        public static ManagedIdentitySource GetManagedIdentitySource()
+        {
+            return ManagedIdentityClient.GetManagedIdentitySource();
+        }
     }
 }

tests/Microsoft.Identity.Test.Unit/ManagedIdentityTests/ManagedIdentityTests.cs

@@ -37,6 +37,27 @@ public class ManagedIdentityTests : TestBase
         internal const string ExpectedErrorCode = "ErrorCode";
         internal const string ExpectedCorrelationId = "Some GUID";
 
+        [DataTestMethod]
+        [DataRow("http://127.0.0.1:41564/msi/token/", ManagedIdentitySource.AppService, ManagedIdentitySource.AppService)]
+        [DataRow(AppServiceEndpoint, ManagedIdentitySource.AppService, ManagedIdentitySource.AppService)]
+        [DataRow(ImdsEndpoint, ManagedIdentitySource.Imds, ManagedIdentitySource.Imds)]
+        [DataRow(null, ManagedIdentitySource.Imds, ManagedIdentitySource.DefaultToImds)]
+        [DataRow(AzureArcEndpoint, ManagedIdentitySource.AzureArc, ManagedIdentitySource.AzureArc)]
+        [DataRow(CloudShellEndpoint, ManagedIdentitySource.CloudShell, ManagedIdentitySource.CloudShell)]
+        [DataRow(ServiceFabricEndpoint, ManagedIdentitySource.ServiceFabric, ManagedIdentitySource.ServiceFabric)]
+        public void GetManagedIdentityTests(
+            string endpoint,
+            ManagedIdentitySource managedIdentitySource, 
+            ManagedIdentitySource expectedManagedIdentitySource)
+        {
+            using (new EnvVariableContext())
+            {
+                SetEnvironmentVariables(managedIdentitySource, endpoint);
+
+                Assert.AreEqual(expectedManagedIdentitySource, ManagedIdentityApplication.GetManagedIdentitySource());
+            }
+        }
+
         [DataTestMethod]
         [DataRow("http://127.0.0.1:41564/msi/token/", Resource, ManagedIdentitySource.AppService)]
         [DataRow(AppServiceEndpoint, Resource, ManagedIdentitySource.AppService)]