Add ModelContextProtocol.Core package (#428)

src/ModelContextProtocol.Core/Server/StreamableHttpPostTransport.cs

@@ -0,0 +1,93 @@
+using ModelContextProtocol.Protocol;
+using System.IO.Pipelines;
+using System.Net.ServerSentEvents;
+using System.Runtime.CompilerServices;
+using System.Text.Json;
+using System.Threading.Channels;
+
+namespace ModelContextProtocol.Server;
+
+/// <summary>
+/// Handles processing the request/response body pairs for the Streamable HTTP transport.
+/// This is typically used via <see cref="JsonRpcMessage.RelatedTransport"/>.
+/// </summary>
+internal sealed class StreamableHttpPostTransport(StreamableHttpServerTransport parentTransport, IDuplexPipe httpBodies) : ITransport
+{
+    private readonly SseWriter _sseWriter = new();
+    private RequestId _pendingRequest;
+
+    public ChannelReader<JsonRpcMessage> MessageReader => throw new NotSupportedException("JsonRpcMessage.RelatedTransport should only be used for sending messages.");
+
+    /// <returns>
+    /// True, if data was written to the respond body.
+    /// False, if nothing was written because the request body did not contain any <see cref="JsonRpcRequest"/> messages to respond to.
+    /// The HTTP application should typically respond with an empty "202 Accepted" response in this scenario.
+    /// </returns>
+    public async ValueTask<bool> RunAsync(CancellationToken cancellationToken)
+    {
+        var message = await JsonSerializer.DeserializeAsync(httpBodies.Input.AsStream(),
+            McpJsonUtilities.JsonContext.Default.JsonRpcMessage, cancellationToken).ConfigureAwait(false);
+        await OnMessageReceivedAsync(message, cancellationToken).ConfigureAwait(false);
+
+        if (_pendingRequest.Id is null)
+        {
+            return false;
+        }
+
+        _sseWriter.MessageFilter = StopOnFinalResponseFilter;
+        await _sseWriter.WriteAllAsync(httpBodies.Output.AsStream(), cancellationToken).ConfigureAwait(false);
+        return true;
+    }
+
+    public async Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
+    {
+        if (parentTransport.Stateless && message is JsonRpcRequest)
+        {
+            throw new InvalidOperationException("Server to client requests are not supported in stateless mode.");
+        }
+
+        await _sseWriter.SendMessageAsync(message, cancellationToken).ConfigureAwait(false);
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        await _sseWriter.DisposeAsync().ConfigureAwait(false);
+    }
+
+    private async IAsyncEnumerable<SseItem<JsonRpcMessage?>> StopOnFinalResponseFilter(IAsyncEnumerable<SseItem<JsonRpcMessage?>> messages, [EnumeratorCancellation] CancellationToken cancellationToken)
+    {
+        await foreach (var message in messages.WithCancellation(cancellationToken))
+        {
+            yield return message;
+
+            if (message.Data is JsonRpcMessageWithId response && response.Id == _pendingRequest)
+            {
+                // Complete the SSE response stream now that all pending requests have been processed.
+                break;
+            }
+        }
+    }
+
+    private async ValueTask OnMessageReceivedAsync(JsonRpcMessage? message, CancellationToken cancellationToken)
+    {
+        if (message is null)
+        {
+            throw new InvalidOperationException("Received invalid null message.");
+        }
+
+        if (message is JsonRpcRequest request)
+        {
+            _pendingRequest = request.Id;
+
+            // Store client capabilities so they can be serialized by "stateless" callers for use in later requests.
+            if (parentTransport.Stateless && request.Method == RequestMethods.Initialize)
+            {
+                parentTransport.InitializeRequest = JsonSerializer.Deserialize(request.Params, McpJsonUtilities.JsonContext.Default.InitializeRequestParams);
+            }
+        }
+
+        message.RelatedTransport = this;
+
+        await parentTransport.MessageWriter.WriteAsync(message, cancellationToken).ConfigureAwait(false);
+    }
+}