settings: add extra settings layer from WinRegistry

Introduce the concept of another layer of settings (lowest precedence;
default values only) below the existing environment variable and Git
configuration file mechanisms.

Implement on Windows using the Registry, under key:

  HKLM\SOFTWARE\GitCredentialManager\Configuration

Author: mjcheetham

src/shared/Microsoft.Git.CredentialManager/CommandContext.cs

@@ -101,7 +101,7 @@ public CommandContext(string appPath)
                                             gitPath,
                                             FileSystem.GetCurrentDirectory()
                                         );
-                Settings          = new Settings(Environment, Git);
+                Settings          = new WindowsSettings(Environment, Git, Trace);
                 CredentialStore   = new WindowsCredentialManager(Settings.CredentialNamespace);
             }
             else if (PlatformUtils.IsMacOS())

src/shared/Microsoft.Git.CredentialManager/Constants.cs

@@ -107,6 +107,12 @@ public static class Remote
             }
         }
 
+        public static class WindowsRegistry
+        {
+            public const string HKAppBasePath = @"SOFTWARE\GitCredentialManager";
+            public const string HKConfigurationPath = HKAppBasePath + @"\Configuration";
+        }
+
         public static class HelpUrls
         {
             public const string GcmProjectUrl          = "https://aka.ms/gcmcore";

src/shared/Microsoft.Git.CredentialManager/Interop/Windows/WindowsSettings.cs

@@ -0,0 +1,55 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+namespace Microsoft.Git.CredentialManager.Interop.Windows
+{
+    /// <summary>
+    /// Reads settings from Git configuration, environment variables, and defaults from the Windows Registry.
+    /// </summary>
+    public class WindowsSettings : Settings
+    {
+        private readonly ITrace _trace;
+
+        public WindowsSettings(IEnvironment environment, IGit git, ITrace trace)
+            : base(environment, git)
+        {
+            EnsureArgument.NotNull(trace, nameof(trace));
+            _trace = trace;
+
+            PlatformUtils.EnsureWindows();
+        }
+
+        protected override bool TryGetExternalDefault(string section, string property, out string value)
+        {
+            value = null;
+
+#if NETFRAMEWORK
+            // Check for machine (HKLM) registry keys that match the Git configuration name.
+            // These can be set by system administrators via Group Policy, so make useful defaults.
+            using (Win32.RegistryKey configKey = Win32.Registry.LocalMachine.OpenSubKey(Constants.WindowsRegistry.HKConfigurationPath))
+            {
+                if (configKey is null)
+                {
+                    // No configuration key exists
+                    return false;
+                }
+
+                string name = $"{section}.{property}";
+                object registryValue = configKey.GetValue(name);
+                if (registryValue is null)
+                {
+                    // No property exists
+                    return false;
+                }
+
+                value = registryValue.ToString();
+                _trace.WriteLine($"Default setting found in registry: {name}={value}");
+
+                return true;
+            }
+#else
+            return base.TryGetExternalDefault(section, property, out value);
+#endif
+        }
+    }
+}

src/shared/Microsoft.Git.CredentialManager/Settings.cs

@@ -289,9 +289,29 @@ public IEnumerable<string> GetSettingValues(string envarName, string section, st
                 {
                     yield return value;
                 }
+
+                // Check for an externally specified default value
+                if (TryGetExternalDefault(section, property, out string defaultValue))
+                {
+                    yield return defaultValue;
+                }
             }
         }
 
+        /// <summary>
+        /// Try to get the default value for a configuration setting.
+        /// This may come from external policies or the Operating System.
+        /// </summary>
+        /// <param name="section">Configuration section name.</param>
+        /// <param name="property">Configuration property name.</param>
+        /// <param name="value">Value of the configuration setting, or null.</param>
+        /// <returns>True if a default setting has been set, false otherwise.</returns>
+        protected virtual bool TryGetExternalDefault(string section, string property, out string value)
+        {
+            value = null;
+            return false;
+        }
+
         public Uri RemoteUri { get; set; }
 
         public bool IsDebuggingEnabled => _environment.Variables.GetBooleanyOrDefault(KnownEnvars.GcmDebug, false);