feat(bake): publish validated pak artifacts atomically

src/AcDream.Bake/BakeOutputTransaction.cs

@@ -0,0 +1,61 @@
+using AcDream.Content.Pak;
+
+namespace AcDream.Bake;
+
+/// <summary>
+/// Publishes a bake as one same-volume filesystem transaction. Work and
+/// validation happen against a unique adjacent temporary file; failure or
+/// cancellation leaves an existing destination untouched.
+/// </summary>
+public static class BakeOutputTransaction
+{
+    public static TResult WriteValidateAndPublish<TResult>(
+        string destinationPath,
+        Func<string, TResult> writeTemporary,
+        Action<string, TResult> validateTemporary,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(destinationPath);
+        ArgumentNullException.ThrowIfNull(writeTemporary);
+        ArgumentNullException.ThrowIfNull(validateTemporary);
+
+        string fullDestination = Path.GetFullPath(destinationPath);
+        string? directory = Path.GetDirectoryName(fullDestination);
+        if (string.IsNullOrEmpty(directory))
+            throw new InvalidOperationException("destination has no parent directory");
+        Directory.CreateDirectory(directory);
+
+        string temporaryPath = Path.Combine(
+            directory,
+            $".{Path.GetFileName(fullDestination)}.{Guid.NewGuid():N}.tmp");
+
+        try
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            TResult result = writeTemporary(temporaryPath);
+            cancellationToken.ThrowIfCancellationRequested();
+            validateTemporary(temporaryPath, result);
+            cancellationToken.ThrowIfCancellationRequested();
+
+            if (File.Exists(fullDestination))
+                File.Replace(temporaryPath, fullDestination, null, ignoreMetadataErrors: true);
+            else
+                File.Move(temporaryPath, fullDestination);
+
+            return result;
+        }
+        finally
+        {
+            try
+            {
+                if (File.Exists(temporaryPath))
+                    File.Delete(temporaryPath);
+            }
+            catch
+            {
+                // Preserve the original exception. An adjacent .tmp is
+                // recognizable and never mistaken for a published pak.
+            }
+        }
+    }
+}