msauth: add MSAL app token cache support for CCAs

Add app token cache support for confidential client applications
(service principals). This is a different cache than the one for user
tokens that is used by public client applications (for normal users).

We do not know of any other app token cache that we can share with
currently, so we just use our own in the GCM data directory.

src/shared/Core/Authentication/MicrosoftAuthentication.cs

@@ -270,7 +270,7 @@ public async Task<IMicrosoftAuthenticationResult> GetTokenForUserAsync(
 
         public async Task<IMicrosoftAuthenticationResult> GetTokenForServicePrincipalAsync(ServicePrincipalIdentity sp, string[] scopes)
         {
-            IConfidentialClientApplication app = CreateConfidentialClientApplication(sp);
+            IConfidentialClientApplication app = await CreateConfidentialClientApplicationAsync(sp);
 
             try
             {
@@ -528,7 +528,7 @@ private async Task<IPublicClientApplication> CreatePublicClientApplicationAsync(
             return app;
         }
 
-        private IConfidentialClientApplication CreateConfidentialClientApplication(ServicePrincipalIdentity sp)
+        private async Task<IConfidentialClientApplication> CreateConfidentialClientApplicationAsync(ServicePrincipalIdentity sp)
         {
             var httpFactoryAdaptor = new MsalHttpClientFactoryAdaptor(Context.HttpClientFactory);
 
@@ -554,6 +554,8 @@ private IConfidentialClientApplication CreateConfidentialClientApplication(Servi
 
             IConfidentialClientApplication app = appBuilder.Build();
 
+            await RegisterTokenCacheAsync(app.AppTokenCache, CreateAppTokenCacheProps, Context.Trace2);
+
             return app;
         }
 
@@ -713,6 +715,38 @@ internal static ManagedIdentityId GetManagedIdentity(string str)
             throw new ArgumentException("Invalid managed identity value.", nameof(str));
         }
 
+        /// <summary>
+        /// Create the properties for the application token cache. This is used by confidential client applications only
+        /// and is not shared between applications other than GCM.
+        /// </summary>
+        internal StorageCreationProperties CreateAppTokenCacheProps(bool useLinuxFallback)
+        {
+            const string cacheFileName = "app.cache";
+
+            // The confidential client MSAL cache is located at "%UserProfile%\.gcm\msal\app.cache" on Windows
+            // and at "~/.gcm/msal/app.cache" on UNIX.
+            string cacheDirectory = Path.Combine(Context.FileSystem.UserDataDirectoryPath, "msal");
+
+            // The keychain is used on macOS with the following service & account names
+            var builder = new StorageCreationPropertiesBuilder(cacheFileName, cacheDirectory)
+                .WithMacKeyChain("GitCredentialManager.MSAL", "AppCache");
+
+            if (useLinuxFallback)
+            {
+                builder.WithLinuxUnprotectedFile();
+            }
+            else
+            {
+                // The SecretService/keyring is used on Linux with the following collection name and attributes
+                builder.WithLinuxKeyring(cacheFileName,
+                    "default", "AppCache",
+                    new KeyValuePair<string, string>("MsalClientID", "GitCredentialManager.MSAL"),
+                    new KeyValuePair<string, string>("GitCredentialManager.MSAL", "1.0.0.0"));
+            }
+
+            return builder.Build();
+        }
+
         private static EmbeddedWebViewOptions GetEmbeddedWebViewOptions()
         {
             return new EmbeddedWebViewOptions