linuxsettings: implement default settings for Linux

Author: mjcheetham

docs/enterprise-config.md

@@ -55,9 +55,42 @@ 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 configuration setting stores has not been implemented.
 
+## Linux
+
+Default settings values come from the `/etc/git-credential-manager/config.d`
+directory. Each file in this directory represents a single settings dictionary.
+
+All files in this directory are read at runtime and merged into a single
+collection of settings, in the order they are read from the file system.
+To provide a stable ordering, it is recommended to prefix each filename with a
+number, e.g. `42-my-settings`.
+
+The format of each file is a simple set of key/value pairs, separated by an
+`=` sign, and each line separated by a line-feed (`\n`, LF) character.
+Comments are identified by a `#` character at the beginning of a line.
+
+For example:
+
+```text
+# /etc/git-credential-manager/config.d/00-example1
+credential.noguiprompt=0
+```
+
+```text
+# /etc/git-credential-manager/config.d/01-example2
+credential.trace=true
+credential.traceMsAuth=true
+```
+
+All settings names and values are the same as the [Git configuration][config]
+reference.
+
+> Note: These files are read once at startup. If changes are made to these files
+they will not be reflected in an already running process.
+
 [environment]: environment.md
 [config]: configuration.md

src/shared/Core/CommandContext.cs

@@ -148,7 +148,7 @@ public CommandContext()
                                             gitPath,
                                             FileSystem.GetCurrentDirectory()
                                         );
-                Settings          = new Settings(Environment, Git);
+                Settings          = new LinuxSettings(Environment, Git, Trace, FileSystem);
             }
             else
             {

src/shared/Core/Constants.cs

@@ -15,6 +15,7 @@ public static class Constants
         public const string AuthorityIdAuto = "auto";
 
         public const string GcmDataDirectoryName = ".gcm";
+        public const string LinuxAppDefaultsDirectoryPath = "/etc/git-credential-manager/config.d";
 
         public static readonly Guid DevBoxPartnerId = new("e3171dd9-9a5f-e5be-b36c-cc7c4f3f3bcf");
 

src/shared/Core/Interop/Posix/LinuxSettings.cs

@@ -0,0 +1,120 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Text;
+
+namespace GitCredentialManager.Interop.Posix;
+
+public class LinuxSettings : Settings
+{
+    private readonly ITrace _trace;
+    private readonly IFileSystem _fs;
+
+    private IDictionary<string, string> _extConfigCache;
+
+    /// <summary>
+    /// Reads settings from Git configuration, environment variables, and defaults from the
+    /// /etc/git-credential-manager.d app configuration directory.
+    /// </summary>
+    public LinuxSettings(IEnvironment environment, IGit git, ITrace trace, IFileSystem fs)
+        : base(environment, git)
+    {
+        EnsureArgument.NotNull(trace, nameof(trace));
+        EnsureArgument.NotNull(fs, nameof(fs));
+
+        _trace = trace;
+        _fs = fs;
+
+        PlatformUtils.EnsurePosix();
+    }
+
+    protected override bool TryGetExternalDefault(string section, string scope, string property, out string value)
+    {
+        value = null;
+
+        _extConfigCache ??= ReadExternalConfiguration();
+
+        string name = string.IsNullOrWhiteSpace(scope)
+            ? $"{section}.{property}"
+            : $"{section}.{scope}.{property}";
+
+        // Check if the setting exists in the configuration
+        if (!_extConfigCache?.TryGetValue(name, out value) ?? false)
+        {
+            // No property exists (or failed to read config)
+            return false;
+        }
+
+        _trace.WriteLine($"Default setting found in app configuration directory: {name}={value}");
+        return true;
+    }
+
+    private Dictionary<string, string> ReadExternalConfiguration()
+    {
+        try
+        {
+            // Check for system-wide config files in /etc/git-credential-manager/config.d and concatenate them together
+            // in alphabetical order to form a single configuration.
+            const string configDir = Constants.LinuxAppDefaultsDirectoryPath;
+            if (!_fs.DirectoryExists(configDir))
+            {
+                // No configuration directory exists
+                return null;
+            }
+
+            // Get all the files in the configuration directory
+            IEnumerable<string> files = _fs.EnumerateFiles(configDir, "*");
+
+            // Read the contents of each file and concatenate them together
+            var combinedFile = new StringBuilder();
+            foreach (string file in files)
+            {
+                using Stream stream = _fs.OpenFileStream(file, FileMode.Open, FileAccess.Read, FileShare.Read);
+                using var reader = new StreamReader(stream);
+                string contents = reader.ReadToEnd();
+                combinedFile.Append(contents);
+                combinedFile.Append('\n');
+            }
+
+            // Parse the configuration in to a dictionary
+            // Each line is a setting in the format: name=value\n
+            var config = new Dictionary<string, string>(GitConfigurationKeyComparer.Instance);
+            string[] lines = combinedFile.ToString().Split('\n', StringSplitOptions.RemoveEmptyEntries);
+            foreach (string line in lines)
+            {
+                if (string.IsNullOrWhiteSpace(line))
+                {
+                    // Skip empty lines
+                    continue;
+                }
+
+                if (line.TrimStart().StartsWith('#'))
+                {
+                    // Skip comments
+                    continue;
+                }
+
+                string[] parts = line.Split('=', count: 2);
+                if (parts.Length != 2)
+                {
+                    // Invalid line format
+                    continue;
+                }
+
+                string k = parts[0];
+                string v = parts[1];
+                config[k] = v;
+            }
+
+            return config;
+        }
+        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 configuration directory.");
+            _trace.WriteException(ex);
+            return null;
+        }
+    }
+}