macossettings: implement default settings for macOS

docs/enterprise-config.md

@@ -55,7 +55,38 @@ those of the [Git configuration][config] settings.
 The type of each registry key can be either `REG_SZ` (string) or `REG_DWORD`
 (integer).
 
-## macOS/Linux
+## macOS
+
+Default settings values come from macOS's preferences system. Configuration
+profiles can be deployed to devices using a compatible Mobile Device Management
+(MDM) solution.
+
+Configuration for Git Credential Manager must take the form of a dictionary, set
+for the domain `git-credential-manager` under the key `configuration`. For
+example:
+
+```shell
+defaults write git-credential-manager configuration -dict-add <key> <value>
+```
+
+..where `<key>` is the name of the settings from the [Git configuration][config]
+reference, and `<value>` is the desired value.
+
+All values in the `configuration` dictionary must be strings. For boolean values
+use `true` or `false`, and for integer values use the number in string form.
+
+To read the current configuration:
+
+```console
+$ defaults read git-credential-manager configuration
+{
+    <key1> = <value1>;
+    ...
+    <keyN> = <valueN>;
+}
+```
+
+## Linux
 
 Default configuration setting stores has not been implemented.
 

src/shared/Core/CommandContext.cs

@@ -131,7 +131,7 @@ public CommandContext()
                                             gitPath,
                                             FileSystem.GetCurrentDirectory()
                                         );
-                Settings          = new Settings(Environment, Git);
+                Settings          = new MacOSSettings(Environment, Git, Trace);
             }
             else if (PlatformUtils.IsLinux())
             {

src/shared/Core/Interop/MacOS/MacOSSettings.cs

@@ -0,0 +1,67 @@
+using System;
+using System.Collections.Generic;
+
+namespace GitCredentialManager.Interop.MacOS
+{
+    /// <summary>
+    /// Reads settings from Git configuration, environment variables, and defaults from the system.
+    /// </summary>
+    public class MacOSSettings : Settings
+    {
+        private readonly ITrace _trace;
+
+        public MacOSSettings(IEnvironment environment, IGit git, ITrace trace)
+            : base(environment, git)
+        {
+            EnsureArgument.NotNull(trace, nameof(trace));
+            _trace = trace;
+
+            PlatformUtils.EnsureMacOS();
+        }
+
+        protected override bool TryGetExternalDefault(string section, string scope, string property, out string value)
+        {
+            value = null;
+
+            try
+            {
+                // Check for app default preferences for our bundle ID.
+                // Defaults can be deployed system administrators via device management profiles.
+                var prefs = new MacOSPreferences("git-credential-manager"); // TODO: move to a constant
+                IDictionary<string, string> dict = prefs.GetDictionary("configuration");
+
+                if (dict is null)
+                {
+                    // No configuration key exists
+                    return false;
+                }
+
+                // Wrap the raw dictionary in one configured with the Git configuration key comparer.
+                // This means we can use the same key comparison rules as Git in our configuration plist dict,
+                // That is, sections and names are insensitive to case, but the scope is case-sensitive.
+                var config = new Dictionary<string, string>(dict, GitConfigurationKeyComparer.Instance);
+
+                string name = string.IsNullOrWhiteSpace(scope)
+                    ? $"{section}.{property}"
+                    : $"{section}.{scope}.{property}";
+
+                if (!config.TryGetValue(name, out value))
+                {
+                    // No property exists
+                    return false;
+                }
+
+                _trace.WriteLine($"Default setting found in app preferences: {name}={value}");
+                return true;
+            }
+            catch (Exception ex)
+            {
+                // Reading defaults is not critical to the operation of the application
+                // so we can ignore any errors and just log the failure.
+                _trace.WriteLine("Failed to read default setting from app preferences.");
+                _trace.WriteException(ex);
+                return false;
+            }
+        }
+    }
+}