[lldb] Adopt JSONTransport in the MCP Server (Reland) (#155322)

This PR adopts JSONTransport in the MCP server implementation. It
required a slight change in design in the relationship between the two
server classes. Previously, these two had an "is-a" connection, while
now they have a "has-a" connection.

The "generic" protocol server in Protocol/MCP now operates using a
single connection (Transport). This matches the design in DAP where each
DAP instance has its own connection. The protocol server in Plugins
still supports multiple clients and creates a new server instance for
each connection.

I believe the new design makes sense in the long term (as proved by DAP)
and allows us to make the server stateful if we choose to do so. There's
no reason that multiple client support can't live in the generic
protocol library, but for now I kept it in ProtocolServerMCP to avoid
creating unnecessary abstractions.

This is a reland of #155034 but with significant changes to the tests.
The unit tests now test the generic server implementation, which matches
the original intent. This also means the test are now single threaded
and therefore fully deterministic using the MainLoop.

Author: JDevlieghere

lldb/include/lldb/Protocol/MCP/Server.h

@@ -9,6 +9,8 @@
 #ifndef LLDB_PROTOCOL_MCP_SERVER_H
 #define LLDB_PROTOCOL_MCP_SERVER_H
 
+#include "lldb/Host/JSONTransport.h"
+#include "lldb/Host/MainLoop.h"
 #include "lldb/Protocol/MCP/Protocol.h"
 #include "lldb/Protocol/MCP/Resource.h"
 #include "lldb/Protocol/MCP/Tool.h"
@@ -18,31 +20,57 @@
 
 namespace lldb_protocol::mcp {
 
-class Server {
+class MCPTransport
+    : public lldb_private::JSONRPCTransport<Request, Response, Notification> {
 public:
-  Server(std::string name, std::string version);
-  virtual ~Server() = default;
+  using LogCallback = std::function<void(llvm::StringRef message)>;
+
+  MCPTransport(lldb::IOObjectSP in, lldb::IOObjectSP out,
+               std::string client_name, LogCallback log_callback = {})
+      : JSONRPCTransport(in, out), m_client_name(std::move(client_name)),
+        m_log_callback(log_callback) {}
+  virtual ~MCPTransport() = default;
+
+  void Log(llvm::StringRef message) override {
+    if (m_log_callback)
+      m_log_callback(llvm::formatv("{0}: {1}", m_client_name, message).str());
+  }
+
+private:
+  std::string m_client_name;
+  LogCallback m_log_callback;
+};
+
+class Server : public MCPTransport::MessageHandler {
+public:
+  Server(std::string name, std::string version,
+         std::unique_ptr<MCPTransport> transport_up,
+         lldb_private::MainLoop &loop);
+  ~Server() = default;
+
+  using NotificationHandler = std::function<void(const Notification &)>;
 
   void AddTool(std::unique_ptr<Tool> tool);
   void AddResourceProvider(std::unique_ptr<ResourceProvider> resource_provider);
+  void AddNotificationHandler(llvm::StringRef method,
+                              NotificationHandler handler);
+
+  llvm::Error Run();
 
 protected:
-  virtual Capabilities GetCapabilities() = 0;
+  Capabilities GetCapabilities();
 
   using RequestHandler =
       std::function<llvm::Expected<Response>(const Request &)>;
-  using NotificationHandler = std::function<void(const Notification &)>;
 
   void AddRequestHandlers();
 
   void AddRequestHandler(llvm::StringRef method, RequestHandler handler);
-  void AddNotificationHandler(llvm::StringRef method,
-                              NotificationHandler handler);
 
   llvm::Expected<std::optional<Message>> HandleData(llvm::StringRef data);
 
-  llvm::Expected<Response> Handle(Request request);
-  void Handle(Notification notification);
+  llvm::Expected<Response> Handle(const Request &request);
+  void Handle(const Notification &notification);
 
   llvm::Expected<Response> InitializeHandler(const Request &);
 
@@ -52,12 +80,21 @@ class Server {
   llvm::Expected<Response> ResourcesListHandler(const Request &);
   llvm::Expected<Response> ResourcesReadHandler(const Request &);
 
-  std::mutex m_mutex;
+  void Received(const Request &) override;
+  void Received(const Response &) override;
+  void Received(const Notification &) override;
+  void OnError(llvm::Error) override;
+  void OnClosed() override;
+
+  void TerminateLoop();
 
 private:
   const std::string m_name;
   const std::string m_version;
 
+  std::unique_ptr<MCPTransport> m_transport_up;
+  lldb_private::MainLoop &m_loop;
+
   llvm::StringMap<std::unique_ptr<Tool>> m_tools;
   std::vector<std::unique_ptr<ResourceProvider>> m_resource_providers;
 

lldb/source/Plugins/Protocol/MCP/ProtocolServerMCP.cpp

@@ -26,24 +26,10 @@ using namespace llvm;
 
 LLDB_PLUGIN_DEFINE(ProtocolServerMCP)
 
-static constexpr size_t kChunkSize = 1024;
 static constexpr llvm::StringLiteral kName = "lldb-mcp";
 static constexpr llvm::StringLiteral kVersion = "0.1.0";
 
-ProtocolServerMCP::ProtocolServerMCP()
-    : ProtocolServer(),
-      lldb_protocol::mcp::Server(std::string(kName), std::string(kVersion)) {
-  AddNotificationHandler("notifications/initialized",
-                         [](const lldb_protocol::mcp::Notification &) {
-                           LLDB_LOG(GetLog(LLDBLog::Host),
-                                    "MCP initialization complete");
-                         });
-
-  AddTool(
-      std::make_unique<CommandTool>("lldb_command", "Run an lldb command."));
-
-  AddResourceProvider(std::make_unique<DebuggerResourceProvider>());
-}
+ProtocolServerMCP::ProtocolServerMCP() : ProtocolServer() {}
 
 ProtocolServerMCP::~ProtocolServerMCP() { llvm::consumeError(Stop()); }
 
@@ -64,57 +50,37 @@ llvm::StringRef ProtocolServerMCP::GetPluginDescriptionStatic() {
   return "MCP Server.";
 }
 
+void ProtocolServerMCP::Extend(lldb_protocol::mcp::Server &server) const {
+  server.AddNotificationHandler("notifications/initialized",
+                                [](const lldb_protocol::mcp::Notification &) {
+                                  LLDB_LOG(GetLog(LLDBLog::Host),
+                                           "MCP initialization complete");
+                                });
+  server.AddTool(
+      std::make_unique<CommandTool>("lldb_command", "Run an lldb command."));
+  server.AddResourceProvider(std::make_unique<DebuggerResourceProvider>());
+}
+
 void ProtocolServerMCP::AcceptCallback(std::unique_ptr<Socket> socket) {
-  LLDB_LOG(GetLog(LLDBLog::Host), "New MCP client ({0}) connected",
-           m_clients.size() + 1);
+  Log *log = GetLog(LLDBLog::Host);
+  std::string client_name = llvm::formatv("client_{0}", m_instances.size() + 1);
+  LLDB_LOG(log, "New MCP client connected: {0}", client_name);
 
   lldb::IOObjectSP io_sp = std::move(socket);
-  auto client_up = std::make_unique<Client>();
-  client_up->io_sp = io_sp;
-  Client *client = client_up.get();
-
-  Status status;
-  auto read_handle_up = m_loop.RegisterReadObject(
-      io_sp,
-      [this, client](MainLoopBase &loop) {
-        if (llvm::Error error = ReadCallback(*client)) {
-          LLDB_LOG_ERROR(GetLog(LLDBLog::Host), std::move(error), "{0}");
-          client->read_handle_up.reset();
-        }
-      },
-      status);
-  if (status.Fail())
+  auto transport_up = std::make_unique<lldb_protocol::mcp::MCPTransport>(
+      io_sp, io_sp, std::move(client_name), [&](llvm::StringRef message) {
+        LLDB_LOG(GetLog(LLDBLog::Host), "{0}", message);
+      });
+  auto instance_up = std::make_unique<lldb_protocol::mcp::Server>(
+      std::string(kName), std::string(kVersion), std::move(transport_up),
+      m_loop);
+  Extend(*instance_up);
+  llvm::Error error = instance_up->Run();
+  if (error) {
+    LLDB_LOG_ERROR(log, std::move(error), "Failed to run MCP server: {0}");
     return;
-
-  client_up->read_handle_up = std::move(read_handle_up);
-  m_clients.emplace_back(std::move(client_up));
-}
-
-llvm::Error ProtocolServerMCP::ReadCallback(Client &client) {
-  char chunk[kChunkSize];
-  size_t bytes_read = sizeof(chunk);
-  if (Status status = client.io_sp->Read(chunk, bytes_read); status.Fail())
-    return status.takeError();
-  client.buffer.append(chunk, bytes_read);
-
-  for (std::string::size_type pos;
-       (pos = client.buffer.find('\n')) != std::string::npos;) {
-    llvm::Expected<std::optional<lldb_protocol::mcp::Message>> message =
-        HandleData(StringRef(client.buffer.data(), pos));
-    client.buffer = client.buffer.erase(0, pos + 1);
-    if (!message)
-      return message.takeError();
-
-    if (*message) {
-      std::string Output;
-      llvm::raw_string_ostream OS(Output);
-      OS << llvm::formatv("{0}", toJSON(**message)) << '\n';
-      size_t num_bytes = Output.size();
-      return client.io_sp->Write(Output.data(), num_bytes).takeError();
-    }
   }
-
-  return llvm::Error::success();
+  m_instances.push_back(std::move(instance_up));
 }
 
 llvm::Error ProtocolServerMCP::Start(ProtocolServer::Connection connection) {
@@ -158,27 +124,11 @@ llvm::Error ProtocolServerMCP::Stop() {
 
   // Stop the main loop.
   m_loop.AddPendingCallback(
-      [](MainLoopBase &loop) { loop.RequestTermination(); });
+      [](lldb_private::MainLoopBase &loop) { loop.RequestTermination(); });
 
   // Wait for the main loop to exit.
   if (m_loop_thread.joinable())
     m_loop_thread.join();
 
-  {
-    std::lock_guard<std::mutex> guard(m_mutex);
-    m_listener.reset();
-    m_listen_handlers.clear();
-    m_clients.clear();
-  }
-
   return llvm::Error::success();
 }
-
-lldb_protocol::mcp::Capabilities ProtocolServerMCP::GetCapabilities() {
-  lldb_protocol::mcp::Capabilities capabilities;
-  capabilities.tools.listChanged = true;
-  // FIXME: Support sending notifications when a debugger/target are
-  // added/removed.
-  capabilities.resources.listChanged = false;
-  return capabilities;
-}

lldb/source/Plugins/Protocol/MCP/ProtocolServerMCP.h

@@ -18,8 +18,7 @@
 
 namespace lldb_private::mcp {
 
-class ProtocolServerMCP : public ProtocolServer,
-                          public lldb_protocol::mcp::Server {
+class ProtocolServerMCP : public ProtocolServer {
 public:
   ProtocolServerMCP();
   virtual ~ProtocolServerMCP() override;
@@ -39,26 +38,24 @@ class ProtocolServerMCP : public ProtocolServer,
 
   Socket *GetSocket() const override { return m_listener.get(); }
 
+protected:
+  // This adds tools and resource providers that
+  // are specific to this server. Overridable by the unit tests.
+  virtual void Extend(lldb_protocol::mcp::Server &server) const;
+
 private:
   void AcceptCallback(std::unique_ptr<Socket> socket);
 
-  lldb_protocol::mcp::Capabilities GetCapabilities() override;
-
   bool m_running = false;
 
-  MainLoop m_loop;
+  lldb_private::MainLoop m_loop;
   std::thread m_loop_thread;
+  std::mutex m_mutex;
 
   std::unique_ptr<Socket> m_listener;
-  std::vector<MainLoopBase::ReadHandleUP> m_listen_handlers;
 
-  struct Client {
-    lldb::IOObjectSP io_sp;
-    MainLoopBase::ReadHandleUP read_handle_up;
-    std::string buffer;
-  };
-  llvm::Error ReadCallback(Client &client);
-  std::vector<std::unique_ptr<Client>> m_clients;
+  std::vector<MainLoopBase::ReadHandleUP> m_listen_handlers;
+  std::vector<std::unique_ptr<lldb_protocol::mcp::Server>> m_instances;
 };
 } // namespace lldb_private::mcp