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/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() { }
+    }
+}