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/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);
+        }
+    }
 }