Streamable HTTP resumability + redelivery + SSE polling via server-side disconnect (#1077)

Co-authored-by: Stephen Halter <halter73@gmail.com>

src/ModelContextProtocol.Core/Server/McpSseEventWriterExtensions.cs

@@ -0,0 +1,70 @@
+using ModelContextProtocol.Protocol;
+using System.Buffers;
+using System.Net.ServerSentEvents;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Server;
+
+/// <summary>
+/// Provides MCP extension methods for <see cref="SseEventWriter"/>.
+/// </summary>
+internal static class McpSseEventWriterExtensions
+{
+    [ThreadStatic]
+    private static Utf8JsonWriter? _jsonWriter;
+
+    /// <summary>
+    /// Writes an SSE item containing a <see cref="JsonRpcMessage"/>.
+    /// </summary>
+    /// <param name="writer">The <see cref="SseEventWriter"/>.</param>
+    /// <param name="item">The SSE item containing the <see cref="JsonRpcMessage"/>.</param>
+    /// <param name="cancellationToken">The token to monitor for cancellation requests.</param>
+    /// <returns>A task representing the asynchronous write operation.</returns>
+    public static ValueTask WriteAsync(this SseEventWriter writer, SseItem<JsonRpcMessage?> item, CancellationToken cancellationToken = default)
+        => writer.WriteAsync(item, FormatJsonRpcMessage, cancellationToken);
+
+    /// <summary>
+    /// Writes an SSE item containing a <see cref="string"/>.
+    /// </summary>
+    /// <param name="writer">The <see cref="SseEventWriter"/>.</param>
+    /// <param name="item">The SSE item containing the string.</param>
+    /// <param name="cancellationToken">The token to monitor for cancellation requests.</param>
+    /// <returns>A task representing the asynchronous write operation.</returns>
+    public static ValueTask WriteAsync(this SseEventWriter writer, SseItem<string> item, CancellationToken cancellationToken = default)
+        => writer.WriteAsync(item, FormatString, cancellationToken);
+
+    /// <summary>
+    /// Formats a <see cref="JsonRpcMessage"/> message by writing it as JSON to the buffer writer.
+    /// </summary>
+    private static void FormatJsonRpcMessage(SseItem<JsonRpcMessage?> item, IBufferWriter<byte> writer)
+    {
+        if (item.Data is null)
+        {
+            return;
+        }
+
+        if (_jsonWriter is null)
+        {
+            _jsonWriter = new Utf8JsonWriter(writer);
+        }
+        else
+        {
+            _jsonWriter.Reset(writer);
+        }
+
+        JsonSerializer.Serialize(_jsonWriter, item.Data, McpJsonUtilities.JsonContext.Default.JsonRpcMessage!);
+    }
+
+    /// <summary>
+    /// Formats a string by writing it as UTF-8 to the buffer writer.
+    /// </summary>
+    private static void FormatString(SseItem<string> item, IBufferWriter<byte> writer)
+    {
+        if (item.Data is null)
+        {
+            return;
+        }
+
+        writer.WriteUtf8String(item.Data);
+    }
+}