Write settings, presets and favourites whole or not at all

A failed write leaves the old file in place. Settings that could not be read
are not saved over. UI exceptions reach the log. Started from a shortcut set
up elsewhere, the starter moves to its own folder when only that one holds a
game.

StarterNG.Tests/ConfigFileTests.cs

@@ -0,0 +1,161 @@
+using StarterNG.Classes;
+
+namespace StarterNG.Tests;
+
+public class ConfigFileTests
+{
+    private static string[] Lines(ConfigFile config) =>
+        config.ToText().Split(Environment.NewLine);
+
+    [Fact]
+    public void A_file_is_written_with_the_hosts_line_endings_whichever_it_came_with()
+    {
+        // parser.cpp breaks words on \r as on \n, so the simulator takes either.
+        string nl = Environment.NewLine;
+
+        Assert.Equal($"a 1{nl}b 2{nl}", ConfigFile.Parse("a 1\nb 2\n").ToText());
+        Assert.Equal($"a 1{nl}b 2{nl}", ConfigFile.Parse("a 1\r\nb 2\r\n").ToText());
+    }
+
+    [Fact]
+    public void Removing_a_key_comments_every_line_that_sets_it()
+    {
+        var config = ConfigFile.Parse("width 800\nheight 600\nwidth 1024\nWIDTH 1920\n");
+
+        config.Remove("width");
+
+        Assert.False(config.Has("width"));
+        Assert.Equal(new[] { "// width 800", "height 600", "// width 1024", "// WIDTH 1920", "" }, Lines(config));
+        // Parsed again, nothing of it comes back.
+        Assert.False(ConfigFile.Parse(config.ToText()).Has("width"));
+    }
+
+    [Fact]
+    public void A_removed_key_keeps_its_comment_and_indent()
+    {
+        var config = ConfigFile.Parse("  width 800\t// mine\n");
+
+        config.Remove("width");
+
+        Assert.Equal("  // width 800\t// mine", Lines(config)[0]);
+    }
+
+    [Fact]
+    public void Removing_a_key_leaves_what_only_mentions_it()
+    {
+        var config = ConfigFile.Parse("// width 800\nwidth.max 900\nwidth 1024\n");
+
+        config.Remove("width");
+
+        Assert.Equal(new[] { "// width 800", "width.max 900", "// width 1024", "" }, Lines(config));
+    }
+
+    [Fact]
+    public void A_new_key_goes_before_endconfig()
+    {
+        // Globals.cpp stops reading at endconfig.
+        var config = ConfigFile.Parse("width 800\nendconfig\n// notes\n");
+
+        config.Set("height", "600");
+        config.Set("fullscreen", "no");
+
+        Assert.Equal(new[] { "width 800", "height 600", "fullscreen no", "endconfig", "// notes", "" }, Lines(config));
+    }
+
+    [Theory]
+    [InlineData("endconfig")]
+    [InlineData("ENDCONFIG")]
+    [InlineData("  EndConfig  // the end")]
+    public void Endconfig_is_known_as_the_simulator_knows_it(string line)
+    {
+        // Its parser lower-cases every word.
+        var config = ConfigFile.Parse(line + "\n");
+
+        config.Set("width", "800");
+
+        Assert.Equal("width 800", Lines(config)[0]);
+    }
+
+    [Fact]
+    public void Endconfig_in_a_comment_ends_nothing()
+    {
+        var config = ConfigFile.Parse("// endconfig\n");
+
+        config.Set("width", "800");
+
+        Assert.Equal(new[] { "// endconfig", "", "width 800" }, Lines(config));
+    }
+
+    [Fact]
+    public void Keys_found_after_an_insertion_are_still_the_right_lines()
+    {
+        var config = ConfigFile.Parse("width 800\nendconfig\nlate 1\n");
+
+        config.Set("height", "600");
+        config.Set("width", "1024");
+        config.Set("height", "720");
+
+        Assert.Equal(new[] { "width 1024", "height 720", "endconfig", "late 1", "" }, Lines(config));
+        Assert.Equal("1024", config.GetRaw("width"));
+        Assert.Equal("720", config.GetRaw("height"));
+        Assert.Equal("1", config.GetRaw("late"));
+    }
+
+    [Fact]
+    public void A_key_past_endconfig_is_read_but_known_to_be_past_it()
+    {
+        var config = ConfigFile.Parse("width 800\nendconfig\nheight 600\n");
+
+        Assert.Equal("600", config.GetRaw("height"));
+        Assert.True(config.IsPastEnd("height"));
+        Assert.False(config.IsPastEnd("width"));
+        Assert.False(config.IsPastEnd("endconfig"));
+        Assert.False(config.IsPastEnd("absent"));
+    }
+
+    [Fact]
+    public void A_key_set_past_endconfig_moves_before_it_with_its_comment()
+    {
+        var config = ConfigFile.Parse("endconfig\n  height 600\t// mine\nlate 1\n");
+
+        config.Set("height", "720");
+
+        Assert.Equal(new[] { "  height 720\t// mine", "endconfig", "late 1", "" }, Lines(config));
+        Assert.False(config.IsPastEnd("height"));
+        Assert.True(config.IsPastEnd("late"));
+    }
+
+    [Fact]
+    public void Of_a_key_on_both_sides_of_endconfig_the_line_the_simulator_reads_counts()
+    {
+        var config = ConfigFile.Parse("width 800\nendconfig\nwidth 1920\n");
+
+        Assert.Equal("800", config.GetRaw("width"));
+        Assert.False(config.IsPastEnd("width"));
+
+        config.Set("width", "1024");
+
+        Assert.Equal(new[] { "width 1024", "endconfig", "width 1920", "" }, Lines(config));
+    }
+
+    [Fact]
+    public void A_key_past_endconfig_is_removed_like_any_other()
+    {
+        var config = ConfigFile.Parse("width 800\nendconfig\nwidth 1920\nheight 600\n");
+
+        config.Remove("width");
+        config.Remove("height");
+
+        Assert.Equal(new[] { "// width 800", "endconfig", "// width 1920", "// height 600", "" }, Lines(config));
+    }
+
+    [Fact]
+    public void Without_endconfig_a_new_key_goes_at_the_end_as_it_did()
+    {
+        var config = ConfigFile.Parse("width 800");
+
+        config.Set("height", "600");
+
+        Assert.Equal(new[] { "width 800", "height 600" }, Lines(config));
+    }
+}

StarterNG.Tests/Fakes/InMemoryFileSystem.cs

@@ -9,16 +9,61 @@ namespace StarterNG.Tests.Fakes;
 /// </summary>
 /// <remarks>
 /// Paths are normalised to forward slashes and compared case-insensitively,
-/// matching how the parsers treat installation paths on both platforms.
+/// matching how the parsers treat installation paths on both platforms - or,
+/// asked for, case-sensitively, as a Linux disk holds them.
 /// </remarks>
 public sealed class InMemoryFileSystem : IFileSystem
 {
-    private readonly Dictionary<string, byte[]> _files = new(StringComparer.OrdinalIgnoreCase);
-    private readonly HashSet<string> _directories = new(StringComparer.OrdinalIgnoreCase);
-    private readonly HashSet<string> _executable = new(StringComparer.OrdinalIgnoreCase);
+    private readonly StringComparison _comparison;
+    private readonly Dictionary<string, byte[]> _files;
+    private readonly HashSet<string> _directories;
+    private readonly HashSet<string> _executable;
+    private readonly Dictionary<string, string> _links;
+    private readonly HashSet<string> _unreadable;
+    private readonly HashSet<string> _unlistable;
+    private readonly Dictionary<string, DateTime> _written;
+
+    public InMemoryFileSystem(bool caseSensitive = false)
+    {
+        _comparison = caseSensitive ? StringComparison.Ordinal : StringComparison.OrdinalIgnoreCase;
+        var comparer = StringComparer.FromComparison(_comparison);
+        _files = new Dictionary<string, byte[]>(comparer);
+        _directories = new HashSet<string>(comparer);
+        _executable = new HashSet<string>(comparer);
+        _links = new Dictionary<string, string>(comparer);
+        _unreadable = new HashSet<string>(comparer);
+        _unlistable = new HashSet<string>(comparer);
+        _written = new Dictionary<string, DateTime>(comparer);
+    }
 
     public DateTime LastWriteTimeUtc { get; set; } = new(2026, 1, 1, 0, 0, 0, DateTimeKind.Utc);
 
+    /// <summary>Making a link throws, as on a disk that cannot hold one.</summary>
+    public bool LinksFail { get; set; }
+
+    /// <summary>A folder moved to a destination this says yes to, its path with forward slashes, throws.</summary>
+    public Func<string, bool>? MoveFails { get; set; }
+
+    /// <summary>The file stays, but reading or copying it throws, as a locked one does; writing it still goes through.</summary>
+    public InMemoryFileSystem FailReading(string path)
+    {
+        _unreadable.Add(Normalize(path));
+        return this;
+    }
+
+    public InMemoryFileSystem AllowReading(string path)
+    {
+        _unreadable.Remove(Normalize(path));
+        return this;
+    }
+
+    /// <summary>The folder is there, but listing it throws, as one without the permission does.</summary>
+    public InMemoryFileSystem DenyListing(string path)
+    {
+        _unlistable.Add(Normalize(path));
+        return this;
+    }
+
     static InMemoryFileSystem()
     {
         // The game's own files are code page 1250; a test that writes Polish text
@@ -54,7 +99,10 @@ public sealed class InMemoryFileSystem : IFileSystem
 
     public bool FileExists(string path) => _files.ContainsKey(Normalize(path));
 
-    public bool DirectoryExists(string path) => _directories.Contains(Normalize(path));
+    /// <summary>A link counts as the directory it points at, as the disk has it.</summary>
+    public bool DirectoryExists(string path) =>
+        _directories.Contains(Normalize(path)) ||
+        (_links.TryGetValue(Normalize(path), out string? target) && _directories.Contains(Normalize(target)));
 
     public string ReadAllText(string path) => Encoding.UTF8.GetString(Bytes(path));
 
@@ -76,6 +124,10 @@ public sealed class InMemoryFileSystem : IFileSystem
         AddParents(key);
     }
 
+    public void WriteAllTextAtomic(string path, string contents) => WithFile(path, contents);
+
+    public void WriteAllBytesAtomic(string path, byte[] bytes) => WriteAllBytes(path, bytes);
+
     public void WriteAllLines(string path, IEnumerable<string> lines) =>
         WithFile(path, string.Join("\n", lines));
 
@@ -103,7 +155,7 @@ public sealed class InMemoryFileSystem : IFileSystem
         string prefix = Normalize(path) + "/";
         var pattern = ToRegex(searchPattern);
         return _files.Keys
-            .Where(key => key.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
+            .Where(key => key.StartsWith(prefix, _comparison))
             .Where(key => !key[prefix.Length..].Contains('/'))
             .Where(key => pattern.IsMatch(key[prefix.Length..]))
             .OrderBy(key => key, StringComparer.OrdinalIgnoreCase)
@@ -115,7 +167,8 @@ public sealed class InMemoryFileSystem : IFileSystem
         string prefix = Normalize(path) + "/";
         var pattern = ToRegex(searchPattern);
         return _files.Keys
-            .Where(key => key.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
+            .Where(key => key.StartsWith(prefix, _comparison))
+            .Where(key => !_unlistable.Any(folder => key.StartsWith(folder + "/", _comparison)))
             .Where(key => pattern.IsMatch(key[(key.LastIndexOf('/') + 1)..]))
             .OrderBy(key => key, StringComparer.OrdinalIgnoreCase)
             .ToList();
@@ -123,15 +176,26 @@ public sealed class InMemoryFileSystem : IFileSystem
 
     public IReadOnlyList<string> GetDirectories(string path)
     {
+        if (_unlistable.Contains(Normalize(path)))
+            throw new UnauthorizedAccessException($"Access to the path '{path}' is denied.");
+
         string prefix = Normalize(path) + "/";
         return _directories
-            .Where(key => key.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
+            .Where(key => key.StartsWith(prefix, _comparison))
             .Where(key => !key[prefix.Length..].Contains('/'))
             .OrderBy(key => key, StringComparer.OrdinalIgnoreCase)
             .ToList();
     }
 
-    public DateTime GetLastWriteTimeUtc(string path) => LastWriteTimeUtc;
+    public DateTime GetLastWriteTimeUtc(string path) =>
+        _written.TryGetValue(Normalize(path), out var written) ? written : LastWriteTimeUtc;
+
+    /// <summary>One file's own time, in place of <see cref="LastWriteTimeUtc"/>.</summary>
+    public InMemoryFileSystem WrittenAt(string path, DateTime utc)
+    {
+        _written[Normalize(path)] = utc;
+        return this;
+    }
 
     /// <summary>Files added through <see cref="WithExecutable"/> carry the execute bit.</summary>
     public long GetFileSize(string path) => Bytes(path).LongLength;
@@ -145,10 +209,56 @@ public sealed class InMemoryFileSystem : IFileSystem
         return this;
     }
 
-    private byte[] Bytes(string path) =>
-        _files.TryGetValue(Normalize(path), out var bytes)
+    public string? LinkTarget(string path) => _links.GetValueOrDefault(Normalize(path));
+
+    public void CreateDirectoryLink(string path, string target)
+    {
+        if (LinksFail)
+            throw new IOException($"cannot link {path}");
+
+        string key = Normalize(path);
+        if (_links.ContainsKey(key) || _directories.Contains(key) || _files.ContainsKey(key))
+            throw new IOException($"{path} already exists");
+        _links[key] = target;
+        AddParents(key);
+    }
+
+    public void MoveDirectory(string sourcePath, string destinationPath)
+    {
+        if (MoveFails?.Invoke(Normalize(destinationPath)) == true)
+            throw new IOException($"cannot move to {destinationPath}");
+
+        string from = Normalize(sourcePath), to = Normalize(destinationPath);
+        if (!_directories.Contains(from) && !_links.ContainsKey(from))
+            throw new DirectoryNotFoundException(sourcePath);
+        if (_directories.Contains(to) || _links.ContainsKey(to) || _files.ContainsKey(to))
+            throw new IOException($"{destinationPath} already exists");
+
+        string? Moved(string key) =>
+            string.Equals(key, from, _comparison) ? to
+            : key.StartsWith(from + "/", _comparison) ? to + key[from.Length..]
+            : null;
+
+        foreach (var (key, bytes) in _files.ToList())
+            if (Moved(key) is { } moved) { _files.Remove(key); _files[moved] = bytes; }
+        foreach (string key in _executable.ToList())
+            if (Moved(key) is { } moved) { _executable.Remove(key); _executable.Add(moved); }
+        foreach (var (key, target) in _links.ToList())
+            if (Moved(key) is { } moved) { _links.Remove(key); _links[moved] = target; }
+        foreach (string key in _directories.ToList())
+            if (Moved(key) is { } moved) { _directories.Remove(key); _directories.Add(moved); }
+        AddParents(to);
+    }
+
+    private byte[] Bytes(string path)
+    {
+        if (_unreadable.Contains(Normalize(path)))
+            throw new IOException($"The process cannot access the file '{path}'.");
+
+        return _files.TryGetValue(Normalize(path), out var bytes)
             ? bytes
             : throw new FileNotFoundException(path);
+    }
 
     private void AddParents(string key)
     {

StarterNG.Tests/Fakes/TestLog.cs

@@ -0,0 +1,12 @@
+using StarterNG.Application.Abstractions;
+using StarterNG.Infrastructure.Adapters;
+
+namespace StarterNG.Tests.Fakes;
+
+/// <summary>The diagnostics log for a test that only wants one to write to or read back.</summary>
+public static class TestLog
+{
+    /// <param name="files">Where the log file goes; a disk of its own when left out.</param>
+    public static FileDiagnosticsLog Over(IFileSystem? files = null) =>
+        new(files ?? new InMemoryFileSystem(), new FixedClock(new DateTime(2026, 10, 13)), new GamePaths("game"));
+}

StarterNG.Tests/FavoriteSceneriesTests.cs

@@ -1,5 +1,4 @@
 using StarterNG.Application;
-using StarterNG.Domain.Sceneries;
 using StarterNG.Tests.Fakes;
 
 namespace StarterNG.Tests;
@@ -35,4 +34,43 @@ public class FavoriteSceneriesTests
 
         Assert.False(favorites.Contains(scenery));
     }
+
+    [Fact]
+    public void A_file_that_could_not_be_read_goes_aside_before_a_star_replaces_it()
+    {
+        var installation = new TestInstallation(new InMemoryFileSystem()
+            .WithLegacyFile(TestInstallation.At("scenery", "td.scn"), "//$n Testowa")
+            .WithFile(FavoritesPath, "one.scn\ntwo.scn")
+            .FailReading(FavoritesPath));
+        var scenery = Assert.Single(installation.Sceneries.LoadAll());
+        var favorites = new FavoriteSceneries(installation.Files, installation.Log, FavoritesPath);
+
+        installation.Files.AllowReading(FavoritesPath);
+        favorites.Toggle(scenery);
+
+        Assert.Equal("one.scn\ntwo.scn", installation.Files.ReadAllText(FavoritesPath + ".unreadable"));
+        Assert.Equal("td.scn", installation.Files.ReadAllText(FavoritesPath));
+        Assert.Contains(FavoritesPath + ".unreadable", installation.Log.Text);
+
+        // Set aside once: the file is the starter's own again.
+        favorites.Toggle(scenery);
+        Assert.False(installation.Files.FileExists(FavoritesPath + ".unreadable-2"));
+    }
+
+    [Fact]
+    public void A_locked_file_is_left_alone_when_no_copy_of_it_can_be_made()
+    {
+        var installation = new TestInstallation(new InMemoryFileSystem()
+            .WithLegacyFile(TestInstallation.At("scenery", "td.scn"), "//$n Testowa")
+            .WithFile(FavoritesPath, "one.scn\ntwo.scn")
+            .FailReading(FavoritesPath));
+        var scenery = Assert.Single(installation.Sceneries.LoadAll());
+        var favorites = new FavoriteSceneries(installation.Files, installation.Log, FavoritesPath);
+
+        favorites.Toggle(scenery);
+
+        installation.Files.AllowReading(FavoritesPath);
+        Assert.Equal("one.scn\ntwo.scn", installation.Files.ReadAllText(FavoritesPath));
+        Assert.Contains("writing", installation.Log.Text);
+    }
 }

StarterNG.Tests/FileDiagnosticsLogTests.cs

@@ -42,4 +42,128 @@ public class FileDiagnosticsLogTests
 
         Assert.Equal(string.Empty, log.Text);
     }
+
+    private static string Written(string path)
+    {
+        // The writer holds it open for as long as the session lasts.
+        using var stream = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
+        using var reader = new StreamReader(stream);
+        return reader.ReadToEnd();
+    }
+
+    [Fact]
+    public void The_first_entry_of_a_session_keeps_the_log_of_the_one_before()
+    {
+        string root = Directory.CreateTempSubdirectory("starterng-log-").FullName;
+        try
+        {
+            var paths = new GamePaths(root);
+            Directory.CreateDirectory(paths.StarterConfig);
+            File.WriteAllText(paths.DiagnosticsLog, "the crash\n");
+            var log = new FileDiagnosticsLog(new PhysicalFileSystem(), new FixedClock(new DateTime(2026, 3, 7)), paths);
+
+            log.Log("after the restart");
+            log.Log("and on");
+
+            Assert.Equal("the crash\n", Written(paths.DiagnosticsLog + ".1"));
+            string now = Written(paths.DiagnosticsLog);
+            Assert.DoesNotContain("the crash", now);
+            Assert.Contains("after the restart", now);
+            Assert.Contains("and on", now);
+            log.Clear();
+        }
+        finally
+        {
+            Directory.Delete(root, recursive: true);
+        }
+    }
+
+    [Fact]
+    public void Only_the_session_before_is_kept()
+    {
+        string root = Directory.CreateTempSubdirectory("starterng-log-").FullName;
+        try
+        {
+            var paths = new GamePaths(root);
+            Directory.CreateDirectory(paths.StarterConfig);
+            File.WriteAllText(paths.DiagnosticsLog, "the last one\n");
+            File.WriteAllText(paths.DiagnosticsLog + ".1", "the one before it\n");
+            var log = new FileDiagnosticsLog(new PhysicalFileSystem(), new FixedClock(new DateTime(2026, 3, 7)), paths);
+
+            log.Log("now");
+
+            Assert.Equal("the last one\n", Written(paths.DiagnosticsLog + ".1"));
+            log.Clear();
+        }
+        finally
+        {
+            Directory.Delete(root, recursive: true);
+        }
+    }
+
+    [Fact]
+    public void Clearing_the_log_does_not_touch_the_session_kept()
+    {
+        var log = Build(out var files);
+        string path = new GamePaths("game").DiagnosticsLog;
+        files.WithFile(path, "the crash\n");
+
+        log.Log("after the restart");
+        log.Clear();
+        log.Log("cleared");
+
+        Assert.Equal("the crash\n", files.ReadAllText(path + ".1"));
+    }
+
+    [Fact]
+    public void A_log_that_cannot_be_copied_is_added_to_and_not_wiped()
+    {
+        string root = Directory.CreateTempSubdirectory("starterng-log-").FullName;
+        try
+        {
+            var paths = new GamePaths(root);
+            Directory.CreateDirectory(paths.StarterConfig);
+            File.WriteAllText(paths.DiagnosticsLog, "the crash\n");
+            // A folder where the copy would go: nothing can be written in its place.
+            Directory.CreateDirectory(paths.DiagnosticsLog + ".1");
+            var log = new FileDiagnosticsLog(new PhysicalFileSystem(), new FixedClock(new DateTime(2026, 3, 7)), paths);
+
+            log.Log("after the restart");
+
+            string now = Written(paths.DiagnosticsLog);
+            Assert.Contains("the crash", now);
+            Assert.Contains("after the restart", now);
+            log.Clear();
+        }
+        finally
+        {
+            Directory.Delete(root, recursive: true);
+        }
+    }
+
+    [Fact]
+    public void A_folder_it_cannot_write_in_does_not_stop_the_program()
+    {
+        string root = Directory.CreateTempSubdirectory("starterng-log-").FullName;
+        try
+        {
+            var paths = new GamePaths(root);
+            // A file where the log's folder should be, as unwritable as read-only gets
+            // on every platform and for every user.
+            File.WriteAllText(paths.StarterConfig, "");
+            var log = new FileDiagnosticsLog(new PhysicalFileSystem(), new FixedClock(new DateTime(2026, 3, 7)), paths);
+
+            log.Log("first");
+            log.Log("second");
+            log.Flush();
+            log.Clear();
+            log.Log("third");
+
+            Assert.Contains("third", log.Text);
+        }
+        finally
+        {
+            Directory.Delete(root, recursive: true);
+        }
+    }
 }

StarterNG.Tests/PhysicalFileSystemTests.cs

@@ -0,0 +1,36 @@
+using StarterNG.Infrastructure.Adapters;
+
+namespace StarterNG.Tests;
+
+public sealed class PhysicalFileSystemTests : IDisposable
+{
+    private readonly string _root = Directory.CreateTempSubdirectory("starterng-disk-").FullName;
+    private readonly PhysicalFileSystem _files = new();
+
+    public void Dispose() => Directory.Delete(_root, recursive: true);
+
+    [Fact]
+    public void An_atomic_write_makes_or_replaces_the_file_and_leaves_nothing_beside_it()
+    {
+        string path = Path.Combine(_root, "file.json");
+
+        _files.WriteAllTextAtomic(path, "first");
+        _files.WriteAllTextAtomic(path, "zażółć");
+
+        Assert.Equal("zażółć", File.ReadAllText(path));
+        Assert.Equal(new[] { path }, Directory.GetFiles(_root));
+    }
+
+    [Fact]
+    public void An_atomic_write_that_cannot_finish_leaves_what_stood_there()
+    {
+        string path = Path.Combine(_root, "file.json");
+        File.WriteAllText(path, "what stood there");
+        // Nowhere to put the new text on its way in.
+        Directory.CreateDirectory(path + PhysicalFileSystem.TempSuffix);
+
+        Assert.ThrowsAny<Exception>(() => _files.WriteAllTextAtomic(path, "half"));
+
+        Assert.Equal("what stood there", File.ReadAllText(path));
+    }
+}

StarterNG.Tests/PresetFileTests.cs

@@ -0,0 +1,160 @@
+using StarterNG.Classes;
+using StarterNG.Infrastructure.Adapters;
+using StarterNG.Tests.Fakes;
+
+namespace StarterNG.Tests;
+
+// Over the fake disk throughout: PresetStore itself is the player's real file.
+public class PresetFileTests
+{
+    private const string PresetsPath = "/home/user/.config/MaSzyna/starter/userpresets.json";
+
+    private const string ThreePresets = """
+        { "Presets": [
+          { "Name": "a", "Entry": "trainset a" },
+          { "Name": "b", "Entry": "trainset b" },
+          { "Name": "c", "Entry": "trainset c" }
+        ] }
+        """;
+
+    private static (PresetFile Presets, InMemoryFileSystem Files, FileDiagnosticsLog Log) Rig(string? held = null)
+    {
+        var files = new InMemoryFileSystem();
+        if (held is not null)
+            files.WithFile(PresetsPath, held);
+        var log = TestLog.Over(files);
+        return (new PresetFile(files, log, PresetsPath), files, log);
+    }
+
+    [Fact]
+    public void A_saved_preset_joins_the_ones_in_the_file()
+    {
+        var (presets, files, _) = Rig(ThreePresets);
+
+        presets.Save("d", "trainset d");
+
+        Assert.Equal(new[] { "a", "b", "c", "d" }, presets.Presets().Select(p => p.Name));
+        Assert.False(files.FileExists(PresetsPath + ".unreadable"));
+    }
+
+    [Fact]
+    public void Saving_under_a_name_there_replaces_that_one()
+    {
+        var (presets, _, _) = Rig(ThreePresets);
+
+        presets.Save("B", "trainset other");
+
+        Assert.Equal(new[] { "a", "B", "c" }, presets.Presets().Select(p => p.Name));
+    }
+
+    [Fact]
+    public void A_first_preset_needs_no_file_to_be_there()
+    {
+        var (presets, files, log) = Rig();
+
+        presets.Save("a", "trainset a");
+
+        Assert.Equal("a", Assert.Single(presets.Presets()).Name);
+        Assert.False(files.FileExists(PresetsPath + ".unreadable"));
+        Assert.Equal(string.Empty, log.Text);
+    }
+
+    [Theory]
+    [InlineData("""{ "Presets": [ { "Name": "a", "Entry": "trainset a" }, { "Na""")]
+    [InlineData("")]
+    [InlineData("null")]
+    [InlineData("""{ "Presets": null }""")]
+    public void A_file_cut_short_goes_aside_before_a_save_replaces_it(string held)
+    {
+        var (presets, files, log) = Rig(held);
+
+        presets.Save("new", "trainset new");
+
+        Assert.Equal(held, files.ReadAllText(PresetsPath + ".unreadable"));
+        Assert.Equal("new", Assert.Single(presets.Presets()).Name);
+        Assert.Contains(PresetsPath + ".unreadable", log.Text);
+    }
+
+    [Fact]
+    public void An_earlier_copy_set_aside_is_not_overwritten()
+    {
+        var (presets, files, _) = Rig("{ cut");
+        files.WithFile(PresetsPath + ".unreadable", "older");
+
+        presets.Save("new", "trainset new");
+
+        Assert.Equal("older", files.ReadAllText(PresetsPath + ".unreadable"));
+        Assert.Equal("{ cut", files.ReadAllText(PresetsPath + ".unreadable-2"));
+    }
+
+    [Fact]
+    public void A_locked_file_is_left_alone_when_no_copy_of_it_can_be_made()
+    {
+        var (presets, files, log) = Rig(ThreePresets);
+        files.FailReading(PresetsPath);
+
+        presets.Save("new", "trainset new");
+
+        files.AllowReading(PresetsPath);
+        Assert.Equal(ThreePresets, files.ReadAllText(PresetsPath));
+        Assert.False(files.FileExists(PresetsPath + ".unreadable"));
+        Assert.Contains("saving", log.Text);
+    }
+
+    [Fact]
+    public void A_file_unlocked_between_the_read_and_the_write_is_still_kept()
+    {
+        // The list in hand is the empty one from the read that failed: the file has
+        // to be kept whatever it looks like by the time it would be replaced.
+        var (presets, files, _) = Rig(ThreePresets);
+        var unlocking = new UnlockingLog(files, PresetsPath);
+        var racing = new PresetFile(files, unlocking, PresetsPath);
+        files.FailReading(PresetsPath);
+
+        racing.Save("new", "trainset new");
+
+        Assert.Equal(ThreePresets, files.ReadAllText(PresetsPath + ".unreadable"));
+        Assert.Equal("new", Assert.Single(presets.Presets()).Name);
+    }
+
+    [Fact]
+    public void Deleting_from_an_unreadable_file_leaves_it_as_it_is()
+    {
+        var (presets, files, _) = Rig("{ cut");
+
+        presets.Delete("a");
+
+        Assert.Equal("{ cut", files.ReadAllText(PresetsPath));
+        Assert.False(files.FileExists(PresetsPath + ".unreadable"));
+    }
+
+    [Fact]
+    public void An_unreadable_file_shows_as_no_presets_and_is_logged()
+    {
+        var (presets, _, log) = Rig("{ cut");
+
+        Assert.Empty(presets.Presets());
+        Assert.Contains("reading", log.Text);
+    }
+
+    /// <summary>Lets the file go the moment its read has failed and been logged.</summary>
+    private sealed class UnlockingLog : StarterNG.Application.Abstractions.IDiagnosticsLog
+    {
+        private readonly InMemoryFileSystem _files;
+        private readonly string _path;
+
+        public UnlockingLog(InMemoryFileSystem files, string path)
+        {
+            _files = files;
+            _path = path;
+        }
+
+        public string LogPath => "";
+        public string Text => "";
+        public void Log(string message) => _files.AllowReading(_path);
+        public void Log(string context, Exception exception) => _files.AllowReading(_path);
+        public void Log(IEnumerable<string> messages) => _files.AllowReading(_path);
+        public void Clear() { }
+        public void Flush() { }
+    }
+}

StarterNG/Application/Abstractions/IFileSystem.cs

@@ -25,6 +25,16 @@ public interface IFileSystem
 
     void WriteAllText(string path, string contents, Encoding encoding);
 
+    /// <summary>
+    /// Writes the file whole or not at all: a crash or a full disk half way leaves
+    /// what stood there, not the start of what was on its way. A link stays a link,
+    /// its file gets the contents; a file keeps its mode.
+    /// </summary>
+    void WriteAllBytesAtomic(string path, byte[] bytes);
+
+    /// <summary>As UTF-8 without a mark, by <see cref="WriteAllBytesAtomic"/>.</summary>
+    void WriteAllTextAtomic(string path, string contents);
+
     void WriteAllLines(string path, IEnumerable<string> lines);
 
     void WriteAllBytes(string path, byte[] bytes);
@@ -46,4 +56,32 @@ public interface IFileSystem
     long GetFileSize(string path);
 
     bool IsExecutable(string path);
+
+    /// <summary>Where the symbolic link at <paramref name="path"/> points, or null for anything else.</summary>
+    string? LinkTarget(string path);
+
+    /// <summary>Makes <paramref name="path"/> a symbolic link to the directory <paramref name="target"/>.</summary>
+    void CreateDirectoryLink(string path, string target);
+
+    void MoveDirectory(string sourcePath, string destinationPath);
+}
+
+public static class FileSystemExtensions
+{
+    /// <summary>
+    /// Copies a file that could not be read beside itself, before something replaces
+    /// it: it may hold what was not read. Throws when it cannot; then it must stay.
+    /// </summary>
+    public static void KeepAside(this IFileSystem files, string path, IDiagnosticsLog log)
+    {
+        if (!files.FileExists(path))
+            return;
+
+        string aside = path + ".unreadable";
+        for (int n = 2; files.FileExists(aside); n++)
+            aside = $"{path}.unreadable-{n}";
+
+        files.CopyFile(path, aside, overwrite: false);
+        log.Log($"{path} could not be read; a copy is kept as {aside}");
+    }
 }

StarterNG/Application/FavoriteSceneries.cs

@@ -18,6 +18,9 @@ public sealed class FavoriteSceneries
     private readonly string _path;
     private readonly HashSet<string> _names = new(StringComparer.OrdinalIgnoreCase);
 
+    // The file holds more than _names does: it goes aside before anything replaces it.
+    private bool _unread;
+
     public FavoriteSceneries(IFileSystem files, IDiagnosticsLog log, string path)
     {
         _files = files;
@@ -52,6 +55,7 @@ public sealed class FavoriteSceneries
         catch (Exception ex)
         {
             _log.Log($"reading {_path}", ex);
+            _unread = true;
         }
     }
 
@@ -62,7 +66,12 @@ public sealed class FavoriteSceneries
             if (Path.GetDirectoryName(_path) is { Length: > 0 } directory)
                 _files.CreateDirectory(directory);
 
-            _files.WriteAllText(_path, string.Join('\n', _names.Order(StringComparer.OrdinalIgnoreCase)));
+            if (_unread)
+            {
+                _files.KeepAside(_path, _log);
+                _unread = false;
+            }
+            _files.WriteAllTextAtomic(_path, string.Join('\n', _names.Order(StringComparer.OrdinalIgnoreCase)));
         }
         catch (Exception ex)
         {

StarterNG/Classes/ConfigFile.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Globalization;
+using System.Linq;
 using System.Text;
 
 namespace StarterNG.Classes;
@@ -12,6 +13,12 @@ public sealed class ConfigFile
     private readonly Dictionary<string, int> _index =
         new(StringComparer.OrdinalIgnoreCase);
 
+    // The simulator stops reading at this word (Globals.cpp), whatever its case.
+    private const string EndKey = "endconfig";
+
+    // Its first line, or -1 when the file runs to its end.
+    private int _end = -1;
+
     public static ConfigFile Parse(string text)
     {
         var cfg = new ConfigFile();
@@ -24,14 +31,44 @@ public sealed class ConfigFile
 
     private void AddLine(string raw)
     {
-        int idx = _lines.Count;
         _lines.Add(raw);
-        if (TryParseKey(raw, out string key))
-            _index[key] = idx;
+        IndexLine(_lines.Count - 1);
+    }
+
+    private void IndexLine(int line)
+    {
+        if (!TryParseKey(_lines[line], out string key))
+            return;
+
+        if (_end < 0 && key.Equals(EndKey, StringComparison.OrdinalIgnoreCase))
+            _end = line;
+
+        // Of a key on both sides of the end, the line the simulator reads counts.
+        if (_end < 0 || line == _end || !_index.TryGetValue(key, out int seen) || seen > _end)
+            _index[key] = line;
+    }
+
+    private void Reindex()
+    {
+        _index.Clear();
+        _end = -1;
+        for (int line = 0; line < _lines.Count; line++)
+            IndexLine(line);
     }
 
     public bool Has(string key) => _index.ContainsKey(key);
 
+    /// <summary>Whether the line setting the key is one the simulator never gets to.</summary>
+    public bool IsPastEnd(string key) =>
+        _end >= 0 && _index.TryGetValue(key, out int line) && line > _end;
+
+    /// <summary>Every key the file sets, in the order of its lines, with its value.</summary>
+    public IEnumerable<KeyValuePair<string, string>> Entries()
+    {
+        foreach (var (key, _) in _index.OrderBy(pair => pair.Value))
+            yield return new KeyValuePair<string, string>(key, GetRaw(key) ?? string.Empty);
+    }
+
     public string? GetRaw(string key)
     {
         if (!_index.TryGetValue(key, out int i))
@@ -83,35 +120,61 @@ public sealed class ConfigFile
         if (_index.TryGetValue(key, out int i))
         {
             SplitLine(_lines[i], out string indent, out _, out _, out string comment);
-            var sb = new StringBuilder();
-            sb.Append(indent).Append(key);
-            if (value.Length > 0)
-                sb.Append(' ').Append(value);
-            if (comment.Length > 0)
-                sb.Append('\t').Append(comment);
-            _lines[i] = sb.ToString();
+            string line = Line(indent, key, value, comment);
+
+            if (i > _end && _end >= 0)
+            {
+                _lines.RemoveAt(i);
+                InsertBeforeEnd(line);
+            }
+            else
+            {
+                _lines[i] = line;
+            }
+        }
+        else if (_end >= 0)
+        {
+            InsertBeforeEnd(Line("", key, value, ""));
         }
         else
         {
             int idx = _lines.Count;
-            _lines.Add(value.Length > 0 ? $"{key} {value}" : key);
+            _lines.Add(Line("", key, value, ""));
             _index[key] = idx;
         }
     }
 
-    public void Remove(string key)
+    private static string Line(string indent, string key, string value, string comment)
     {
-        if (!_index.TryGetValue(key, out int i))
-            return;
-
-        SplitLine(_lines[i], out string indent, out _, out string value, out string comment);
         var sb = new StringBuilder();
-        sb.Append(indent).Append("// ").Append(key);
+        sb.Append(indent).Append(key);
         if (value.Length > 0)
             sb.Append(' ').Append(value);
         if (comment.Length > 0)
             sb.Append('\t').Append(comment);
-        _lines[i] = sb.ToString();
+        return sb.ToString();
+    }
+
+    private void InsertBeforeEnd(string line)
+    {
+        _lines.Insert(_end, line);
+        Reindex();
+    }
+
+    /// <summary>Comments out every line setting the key: the last one alone would bring the one before it back.</summary>
+    public void Remove(string key)
+    {
+        if (!_index.ContainsKey(key))
+            return;
+
+        for (int i = 0; i < _lines.Count; i++)
+        {
+            SplitLine(_lines[i], out string indent, out string found, out string value, out string comment);
+            if (!found.Equals(key, StringComparison.OrdinalIgnoreCase))
+                continue;
+
+            _lines[i] = Line(indent + "// ", found, value, comment);
+        }
 
         _index.Remove(key);
     }
@@ -133,7 +196,7 @@ public sealed class ConfigFile
     private static void SplitLine(string raw, out string indent, out string key,
         out string value, out string comment)
     {
-        indent = key = value = comment = string.Empty;
+        key = value = comment = string.Empty;
 
         int start = 0;
         while (start < raw.Length && char.IsWhiteSpace(raw[start]))

StarterNG/Classes/PresetStore.cs

@@ -1,13 +1,13 @@
 using System;
 using System.Collections.Generic;
-using System.Globalization;
 using System.IO;
-using System.Linq;
 using System.Text;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 using System.Text.Json.Serialization.Metadata;
 using System.Text.RegularExpressions;
+using StarterNG.Application;
+using StarterNG.Application.Abstractions;
 using StarterNG.Infrastructure.Adapters;
 
 namespace StarterNG.Classes;
@@ -54,18 +54,14 @@ public static class PresetStore
 
     public static PresetSort SortMode { get; set; } = PresetSort.Name;
 
-    private static readonly JsonSerializerOptions Options = new()
-    {
-        WriteIndented = true,
-        TypeInfoResolver = PresetJsonContext.Default,
-    };
+    private static PresetFile? _disk;
 
-    private static JsonTypeInfo<PresetCollection> TypeInfo =>
-        (JsonTypeInfo<PresetCollection>)Options.GetTypeInfo(typeof(PresetCollection));
+    private static PresetFile Disk =>
+        _disk ??= new PresetFile(AppServices.Current.Files, AppServices.Current.Log, FilePath);
 
     public static IReadOnlyList<TrainsetPreset> All()
     {
-        var list = Load().Presets.ToList();
+        var list = Disk.Presets();
         if (SortMode == PresetSort.Track)
             list.Sort((a, b) => string.Compare(TrackOf(a), TrackOf(b), StringComparison.OrdinalIgnoreCase));
         else
@@ -73,25 +69,9 @@ public static class PresetStore
         return list;
     }
 
-    public static void Save(string? name, string? entry)
-    {
-        name = name?.Trim() ?? "";
-        if (name.Length == 0 || string.IsNullOrWhiteSpace(entry))
-            return;
+    public static void Save(string? name, string? entry) => Disk.Save(name, entry);
 
-        var col = Load();
-        col.Presets.RemoveAll(p => string.Equals(p.Name, name, StringComparison.OrdinalIgnoreCase));
-        col.Presets.Add(new TrainsetPreset { Name = name, Entry = entry! });
-        col.Presets.Sort((a, b) => string.Compare(a.Name, b.Name, StringComparison.OrdinalIgnoreCase));
-        Persist(col);
-    }
-
-    public static void Delete(string name)
-    {
-        var col = Load();
-        if (col.Presets.RemoveAll(p => string.Equals(p.Name, name, StringComparison.OrdinalIgnoreCase)) > 0)
-            Persist(col);
-    }
+    public static void Delete(string name) => Disk.Delete(name);
 
     public static int ImportMagazyn(string? path = null)
     {
@@ -162,35 +142,95 @@ public static class PresetStore
 
     private static string Sanitize(string name) =>
         string.IsNullOrWhiteSpace(name) ? "preset" : name.Replace(' ', '_');
+}
 
-    private static PresetCollection Load()
+/// <summary>The presets as one file holds them; <see cref="PresetStore"/> is the player's own.</summary>
+public sealed class PresetFile
+{
+    private static readonly JsonSerializerOptions Options = new()
     {
-        try
-        {
-            if (File.Exists(FilePath))
-            {
-                var col = JsonSerializer.Deserialize(File.ReadAllText(FilePath), TypeInfo);
-                if (col?.Presets != null)
-                    return col;
-            }
-        }
-        catch (Exception ex)
-        {
-            StarterNG.Infrastructure.Diagnostics.Log($"reading {FilePath}", ex);
-        }
-        return new PresetCollection();
+        WriteIndented = true,
+        TypeInfoResolver = PresetJsonContext.Default,
+    };
+
+    private static JsonTypeInfo<PresetCollection> TypeInfo =>
+        (JsonTypeInfo<PresetCollection>)Options.GetTypeInfo(typeof(PresetCollection));
+
+    private readonly IFileSystem _files;
+    private readonly IDiagnosticsLog _log;
+    private readonly string _path;
+
+    public PresetFile(IFileSystem files, IDiagnosticsLog log, string path)
+    {
+        _files = files;
+        _log = log;
+        _path = path;
     }
 
-    private static void Persist(PresetCollection col)
+    public List<TrainsetPreset> Presets()
     {
+        TryRead(out var col);
+        return col.Presets;
+    }
+
+    public void Save(string? name, string? entry)
+    {
+        name = name?.Trim() ?? "";
+        if (name.Length == 0 || string.IsNullOrWhiteSpace(entry))
+            return;
+
+        bool read = TryRead(out var col);
+        col.Presets.RemoveAll(p => string.Equals(p.Name, name, StringComparison.OrdinalIgnoreCase));
+        col.Presets.Add(new TrainsetPreset { Name = name, Entry = entry! });
+        col.Presets.Sort((a, b) => string.Compare(a.Name, b.Name, StringComparison.OrdinalIgnoreCase));
+        Persist(col, read);
+    }
+
+    public void Delete(string name)
+    {
+        bool read = TryRead(out var col);
+        if (col.Presets.RemoveAll(p => string.Equals(p.Name, name, StringComparison.OrdinalIgnoreCase)) > 0)
+            Persist(col, read);
+    }
+
+    /// <summary>The file's presets; false, with none, when there is a file but they cannot be had from it.</summary>
+    private bool TryRead(out PresetCollection col)
+    {
+        col = new PresetCollection();
         try
         {
-            Directory.CreateDirectory(Path.GetDirectoryName(FilePath)!);
-            File.WriteAllText(FilePath, JsonSerializer.Serialize(col, TypeInfo));
+            if (!_files.FileExists(_path))
+                return true;
+
+            var read = JsonSerializer.Deserialize(_files.ReadAllText(_path), TypeInfo);
+            if (read?.Presets != null)
+            {
+                col = read;
+                return true;
+            }
+            _log.Log($"reading {_path}: no presets in it");
         }
         catch (Exception ex)
         {
-            StarterNG.Infrastructure.Diagnostics.Log($"saving {FilePath}", ex);
+            _log.Log($"reading {_path}", ex);
+        }
+        return false;
+    }
+
+    // What was not read is not in the list: written over, those presets are gone, so
+    // a copy goes first and without one the file stays - the new preset waits.
+    private void Persist(PresetCollection col, bool read)
+    {
+        try
+        {
+            _files.CreateDirectory(Path.GetDirectoryName(_path)!);
+            if (!read)
+                _files.KeepAside(_path, _log);
+            _files.WriteAllTextAtomic(_path, JsonSerializer.Serialize(col, TypeInfo));
+        }
+        catch (Exception ex)
+        {
+            _log.Log($"saving {_path}", ex);
         }
     }
 }

StarterNG/Infrastructure/Adapters/FileDiagnosticsLog.cs

@@ -17,6 +17,7 @@ public sealed class FileDiagnosticsLog : IDiagnosticsLog
 
     private StreamWriter? _writer;
     private bool _writerFailed;
+    private bool _previousKept;
 
     public FileDiagnosticsLog(IFileSystem files, IClock clock, IGamePaths paths)
     {
@@ -91,7 +92,8 @@ public sealed class FileDiagnosticsLog : IDiagnosticsLog
             try
             {
                 _files.CreateDirectory(Path.GetDirectoryName(_logPath)!);
-                _files.WriteAllLines(_logPath, _lines);
+                if (KeepPreviousSession())
+                    _files.WriteAllLines(_logPath, _lines);
             }
             catch
             {
@@ -99,6 +101,26 @@ public sealed class FileDiagnosticsLog : IDiagnosticsLog
         }
     }
 
+    // What the last session left, a crash's traces above all, is what this log is
+    // read for: one copy of it goes aside before the first entry of this one. False
+    // when it could not, and the log is then added to instead of started over.
+    private bool KeepPreviousSession()
+    {
+        if (_previousKept)
+            return true;
+
+        try
+        {
+            if (_files.FileExists(_logPath))
+                _files.CopyFile(_logPath, _logPath + ".1", overwrite: true);
+            return _previousKept = true;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
     private void Append(string line)
     {
         if (_writerFailed)
@@ -109,7 +131,7 @@ public sealed class FileDiagnosticsLog : IDiagnosticsLog
             if (_writer is null)
             {
                 _files.CreateDirectory(Path.GetDirectoryName(_logPath)!);
-                _writer = new StreamWriter(_logPath, append: false) { AutoFlush = true };
+                _writer = new StreamWriter(_logPath, append: !KeepPreviousSession()) { AutoFlush = true };
             }
             _writer.WriteLine(line);
         }

StarterNG/Infrastructure/Adapters/GamePaths.cs

@@ -10,7 +10,24 @@ public sealed class GamePaths : IGamePaths
         Root = root;
     }
 
-    public static GamePaths ForCurrentDirectory() => new(Directory.GetCurrentDirectory());
+    /// <summary>
+    /// The working directory, which is how the game's own shortcuts point at it - or
+    /// the starter's folder where only that one holds a game, as when started from a
+    /// shortcut set up elsewhere. Moved there too: more than these paths go by it.
+    /// </summary>
+    public static GamePaths ForCurrentDirectory()
+    {
+        string current = Directory.GetCurrentDirectory();
+        string own = System.AppContext.BaseDirectory;
+        if (HoldsGame(current) || !HoldsGame(own))
+            return new GamePaths(current);
+
+        Directory.SetCurrentDirectory(own);
+        return new GamePaths(Directory.GetCurrentDirectory());
+    }
+
+    private static bool HoldsGame(string folder) =>
+        Directory.Exists(Path.Combine(folder, "scenery")) && Directory.Exists(Path.Combine(folder, "dynamic"));
 
     public string Root { get; }
 

StarterNG/Infrastructure/Adapters/LegacyText.cs

@@ -32,14 +32,26 @@ public static class LegacyText
     /// </summary>
     public static string Decode(byte[] bytes)
     {
-        ReadOnlySpan<byte> text = bytes;
-        if (text.StartsWith(Utf8Bom))
-            text = text[Utf8Bom.Length..];
-
-        return Utf8.IsValid(text) ? Encoding.UTF8.GetString(text) : CodePage1250.GetString(text);
+        var (encoding, bom) = Detect(bytes);
+        return encoding.GetString(bytes, bom.Length, bytes.Length - bom.Length);
     }
 
-    private static ReadOnlySpan<byte> Utf8Bom => [0xEF, 0xBB, 0xBF];
+    /// <summary>
+    /// What a game file is written in, so it is written back the way it was read:
+    /// UTF-8 only for a file that is UTF-8 and says so with its mark or a letter
+    /// outside ASCII, code page 1250 - the game's own - else. That one gives every
+    /// byte back as it was, also behind a mark that lied; UTF-8 would turn what is
+    /// not UTF-8 into replacement characters.
+    /// </summary>
+    public static (Encoding Encoding, byte[] Bom) Detect(byte[] bytes)
+    {
+        byte[] bom = bytes.AsSpan().StartsWith(Utf8Bom) ? Utf8Bom : Array.Empty<byte>();
+        var body = bytes.AsSpan(bom.Length);
+        bool utf8 = Utf8.IsValid(body) && (bom.Length > 0 || body.ContainsAnyInRange((byte)0x80, (byte)0xFF));
+        return (utf8 ? new UTF8Encoding(false) : CodePage1250, bom);
+    }
+
+    private static readonly byte[] Utf8Bom = { 0xEF, 0xBB, 0xBF };
 
     private static Encoding Resolve()
     {

StarterNG/Infrastructure/Adapters/PhysicalFileSystem.cs

@@ -8,6 +8,8 @@ namespace StarterNG.Infrastructure.Adapters;
 
 public sealed class PhysicalFileSystem : IFileSystem
 {
+    public const string TempSuffix = ".starter.tmp";
+
     public bool FileExists(string path) => File.Exists(path);
 
     public bool DirectoryExists(string path) => Directory.Exists(path);
@@ -20,13 +22,61 @@ public sealed class PhysicalFileSystem : IFileSystem
 
     public byte[] ReadAllBytes(string path) => File.ReadAllBytes(path);
 
-    public Stream OpenRead(string path) => File.OpenRead(path);
+    // Shared for writing too, so a file the simulator still holds open - a physics log of
+    // a running trip - can be read.
+    public Stream OpenRead(string path) =>
+        new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete);
 
     public void WriteAllText(string path, string contents) => File.WriteAllText(path, contents);
 
     public void WriteAllText(string path, string contents, Encoding encoding) =>
         File.WriteAllText(path, contents, encoding);
 
+    public void WriteAllTextAtomic(string path, string contents) =>
+        WriteAllBytesAtomic(path, new UTF8Encoding(encoderShouldEmitUTF8Identifier: false).GetBytes(contents));
+
+    /// <summary>
+    /// Written beside the file and moved over it. A crash at any moment leaves the old
+    /// file or the new one whole - at worst with the temporary one beside it, which
+    /// the next save writes over, under a name nothing in the game reads.
+    /// </summary>
+    public void WriteAllBytesAtomic(string path, byte[] bytes)
+    {
+        // Through a link, to its file: moved over the link, the text would take its place.
+        if (File.Exists(path) && File.ResolveLinkTarget(path, returnFinalTarget: true) is { } linked)
+            path = linked.FullName;
+
+        string temporary = path + TempSuffix;
+        try
+        {
+            using (var stream = new FileStream(temporary, FileMode.Create, FileAccess.Write, FileShare.None))
+            {
+                stream.Write(bytes);
+                // On the disk itself first, or a power cut could leave the new name with no text under it.
+                stream.Flush(flushToDisk: true);
+            }
+
+            // A new file gets the default mode; a write in place kept the old one.
+            if (!OperatingSystem.IsWindows() && File.Exists(path) &&
+                File.GetUnixFileMode(path) is var mode && mode != File.GetUnixFileMode(temporary))
+                File.SetUnixFileMode(temporary, mode);
+
+            File.Move(temporary, path, overwrite: true);
+        }
+        catch
+        {
+            try
+            {
+                File.Delete(temporary);
+            }
+            catch (Exception ex) when (ex is IOException or UnauthorizedAccessException)
+            {
+                // The failure to tell is the one that brought us here.
+            }
+            throw;
+        }
+    }
+
     public void WriteAllLines(string path, IEnumerable<string> lines) => File.WriteAllLines(path, lines);
 
     public void WriteAllBytes(string path, byte[] bytes) => File.WriteAllBytes(path, bytes);
@@ -41,9 +91,16 @@ public sealed class PhysicalFileSystem : IFileSystem
     public IReadOnlyList<string> GetFiles(string path, string searchPattern = "*") =>
         Directory.Exists(path) ? Directory.GetFiles(path, searchPattern) : Array.Empty<string>();
 
+    // A folder it may not enter costs what is in it, not the whole listing.
     public IReadOnlyList<string> GetFilesRecursive(string path, string searchPattern) =>
         Directory.Exists(path)
-            ? Directory.GetFiles(path, searchPattern, SearchOption.AllDirectories)
+            ? Directory.GetFiles(path, searchPattern, new EnumerationOptions
+            {
+                RecurseSubdirectories = true,
+                IgnoreInaccessible = true,
+                AttributesToSkip = 0,
+                MatchType = MatchType.Win32
+            })
             : Array.Empty<string>();
 
     public IReadOnlyList<string> GetDirectories(string path) =>
@@ -62,4 +119,11 @@ public sealed class PhysicalFileSystem : IFileSystem
             UnixFileMode.UserExecute | UnixFileMode.GroupExecute | UnixFileMode.OtherExecute;
         return (File.GetUnixFileMode(path) & anyExecute) != 0;
     }
+
+    public string? LinkTarget(string path) => new FileInfo(path).LinkTarget;
+
+    public void CreateDirectoryLink(string path, string target) => Directory.CreateSymbolicLink(path, target);
+
+    public void MoveDirectory(string sourcePath, string destinationPath) =>
+        Directory.Move(sourcePath, destinationPath);
 }

StarterNG/Infrastructure/Diagnostics.cs

@@ -1,5 +1,7 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
 using System.Threading.Tasks;
 using Avalonia.Controls;
 using Avalonia.Controls.ApplicationLifetimes;
@@ -53,6 +55,25 @@ public static class Diagnostics
             Dispatcher.UIThread.Post(() => _ = ReportAsync(message, title));
     }
 
+    /// <summary>Opens a file or folder the way the system does; a dialog says if it is not there or will not open.</summary>
+    public static void OpenOrReport(string path)
+    {
+        if (!File.Exists(path) && !Directory.Exists(path))
+        {
+            ReportOnUiThread(string.Format(App.Loc["FaultFileNotFound"], path));
+            return;
+        }
+
+        try
+        {
+            Process.Start(new ProcessStartInfo(path) { UseShellExecute = true });
+        }
+        catch (Exception ex)
+        {
+            _ = ReportAsync(path, ex);
+        }
+    }
+
     public static InstallationCheck InstallationCheck() =>
         new(AppServices.Current.Paths,
             AppServices.Current.Files,