Add support for Linux enterprise deployable default settings (git-ecosystem#1814)

Mirroring git-ecosystem#1811, add support to set default settings for Linux, via MDM
tooling. For this platform we look for files in the
`/etc/git-credential-manager/config.d` directory.

Files are a simple format of `key=value\n`, where the `key` is a
configuration value name from `docs/config.md`. This format prevents
values from containing a `\n` line feed character, but this is unlikely
to be an issue in practice.

Author: mjcheetham

docs/enterprise-config.md

@@ -88,7 +88,36 @@ $ defaults read git-credential-manager configuration
 
 ## Linux
 
-Default configuration setting stores has not been implemented.
+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.Tests/Interop/Linux/LinuxConfigParserTests.cs

@@ -0,0 +1,57 @@
+using System.Collections.Generic;
+using GitCredentialManager.Interop.Linux;
+using GitCredentialManager.Tests.Objects;
+using Xunit;
+
+namespace GitCredentialManager.Tests.Interop.Linux;
+
+public class LinuxConfigParserTests
+{
+    [Fact]
+    public void LinuxConfigParser_Parse()
+    {
+        const string contents =
+            """
+            #
+            # This is a config file complete with comments
+            # and empty..
+
+            # lines, as well as lines with..
+            #                              
+            # only whitespace (like above ^), and..
+            invalid lines like this one, not a comment
+            # Here's the first real properties:
+            core.overrideMe=This is the first config value
+            baz.specialChars=I contain special chars like = in my value # this is a comment
+            # and let's have with a comment that also contains a = in side
+            #
+            core.overrideMe=This is the second config value
+            bar.scope.foo=123456
+            core.overrideMe=This is the correct value
+                  ###### comments that start ## with whitespace and extra ## inside
+            strings.one="here we have a dq string"
+            strings.two='here we have a sq string'
+            strings.three=    'here we have another sq string'   # have another sq string
+            strings.four="this has 'nested quotes' inside"
+            strings.five='mixed "quotes" the other way around'
+            strings.six='this has an \'escaped\' set of quotes'
+            """;
+
+        var expected = new Dictionary<string, string>
+        {
+            ["core.overrideMe"] = "This is the correct value",
+            ["bar.scope.foo"] = "123456",
+            ["baz.specialChars"] = "I contain special chars like = in my value",
+            ["strings.one"] = "here we have a dq string",
+            ["strings.two"] = "here we have a sq string",
+            ["strings.three"] = "here we have another sq string",
+            ["strings.four"] = "this has 'nested quotes' inside",
+            ["strings.five"] = "mixed \"quotes\" the other way around",
+            ["strings.six"] = "this has an \\'escaped\\' set of quotes",
+        };
+
+        var parser = new LinuxConfigParser(new NullTrace());
+
+        Assert.Equal(expected, parser.Parse(contents));
+    }
+}

src/shared/Core.Tests/Interop/Linux/LinuxSettingsTests.cs

@@ -0,0 +1,47 @@
+using System.Collections.Generic;
+using GitCredentialManager.Interop.Linux;
+using GitCredentialManager.Tests.Objects;
+using Xunit;
+
+namespace GitCredentialManager.Tests.Interop.Linux;
+
+public class LinuxSettingsTests
+{
+    [LinuxFact]
+    public void LinuxSettings_TryGetExternalDefault_CombinesFiles()
+    {
+        var env = new TestEnvironment();
+        var git = new TestGit();
+        var trace = new NullTrace();
+        var fs = new TestFileSystem();
+
+        var utf8 = EncodingEx.UTF8NoBom;
+
+        fs.Directories = new HashSet<string>
+        {
+            "/",
+            "/etc",
+            "/etc/git-credential-manager",
+            "/etc/git-credential-manager/config.d"
+        };
+
+        const string config1 = "core.overrideMe=value1";
+        const string config2 = "core.overrideMe=value2";
+        const string config3 = "core.overrideMe=value3";
+
+        fs.Files = new Dictionary<string, byte[]>
+        {
+            ["/etc/git-credential-manager/config.d/01-first"] = utf8.GetBytes(config1),
+            ["/etc/git-credential-manager/config.d/02-second"] = utf8.GetBytes(config2),
+            ["/etc/git-credential-manager/config.d/03-third"] = utf8.GetBytes(config3),
+        };
+
+        var settings = new LinuxSettings(env, git, trace, fs);
+
+        bool result = settings.TryGetExternalDefault(
+            "core", null, "overrideMe", out string value);
+
+        Assert.True(result);
+        Assert.Equal("value3", value);
+    }
+}

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 const string MacOSBundleId = "git-credential-manager";
         public static readonly Guid DevBoxPartnerId = new("e3171dd9-9a5f-e5be-b36c-cc7c4f3f3bcf");

src/shared/Core/Interop/Linux/LinuxConfigParser.cs

@@ -0,0 +1,62 @@
+using System;
+using System.Collections.Generic;
+using System.Text.RegularExpressions;
+
+namespace GitCredentialManager.Interop.Linux;
+
+public class LinuxConfigParser
+{
+#if NETFRAMEWORK
+    private const string SQ = "'";
+    private const string DQ = "\"";
+    private const string Hash = "#";
+#else
+    private const char SQ = '\'';
+    private const char DQ = '"';
+    private const char Hash = '#';
+#endif
+
+    private static readonly Regex LineRegex = new(@"^\s*(?<key>[a-zA-Z0-9\.-]+)\s*=\s*(?<value>.+?)\s*(?:#.*)?$");
+
+    private readonly ITrace _trace;
+
+    public LinuxConfigParser(ITrace trace)
+    {
+        EnsureArgument.NotNull(trace, nameof(trace));
+
+        _trace = trace;
+    }
+
+    public IDictionary<string, string> Parse(string content)
+    {
+        var result = new Dictionary<string, string>(GitConfigurationKeyComparer.Instance);
+
+        IEnumerable<string> lines = content.Split(['\n'], StringSplitOptions.RemoveEmptyEntries);
+
+        foreach (string line in lines)
+        {
+            // Ignore empty lines or full-line comments
+            var trimmedLine = line.Trim();
+            if (string.IsNullOrEmpty(trimmedLine) || trimmedLine.StartsWith(Hash))
+                continue;
+
+            var match = LineRegex.Match(trimmedLine);
+            if (!match.Success)
+            {
+                _trace.WriteLine($"Invalid config line format: {line}");
+                continue;
+            }
+
+            string key = match.Groups["key"].Value;
+            string value = match.Groups["value"].Value;
+
+            // Remove enclosing quotes from the value, if any
+            if ((value.StartsWith(DQ) && value.EndsWith(DQ)) || (value.StartsWith(SQ) && value.EndsWith(SQ)))
+                value = value.Substring(1, value.Length - 2);
+
+            result[key] = value;
+        }
+
+        return result;
+    }
+}

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

@@ -0,0 +1,93 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Text;
+using Avalonia.Markup.Xaml.MarkupExtensions;
+
+namespace GitCredentialManager.Interop.Linux;
+
+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.EnsureLinux();
+    }
+
+    protected internal 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 IDictionary<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');
+            }
+
+            var parser = new LinuxConfigParser(_trace);
+
+            return parser.Parse(combinedFile.ToString());
+        }
+        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;
+        }
+    }
+}

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

@@ -19,7 +19,7 @@ public MacOSSettings(IEnvironment environment, IGit git, ITrace trace)
             PlatformUtils.EnsureMacOS();
         }
 
-        protected override bool TryGetExternalDefault(string section, string scope, string property, out string value)
+        protected internal override bool TryGetExternalDefault(string section, string scope, string property, out string value)
         {
             value = null;
 

src/shared/Core/Interop/Windows/WindowsSettings.cs

@@ -17,7 +17,7 @@ public WindowsSettings(IEnvironment environment, IGit git, ITrace trace)
             PlatformUtils.EnsureWindows();
         }
 
-        protected override bool TryGetExternalDefault(string section, string scope, string property, out string value)
+        protected internal override bool TryGetExternalDefault(string section, string scope, string property, out string value)
         {
             value = null;
 

src/shared/Core/Settings.cs

@@ -489,7 +489,7 @@ public IEnumerable<string> GetSettingValues(string envarName, string section, st
         /// <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 scope, string property, out string value)
+        protected internal virtual bool TryGetExternalDefault(string section, string scope, string property, out string value)
         {
             value = null;
             return false;