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/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)
     {