[lldb] Add Model Context Protocol (MCP) support to LLDB (#143628)

This PR adds an MCP (Model Context Protocol ) server to LLDB. For
motivation and background, please refer to the corresponding RFC:
https://discourse.llvm.org/t/rfc-adding-mcp-support-to-lldb/86798

I implemented this as a new kind of plugin. The idea is that we could
support multiple protocol servers (e.g. if we want to support DAP from
within LLDB). This also introduces a corresponding top-level command
(`protocol-server`) with two subcommands to `start` and `stop` the
server.

```
(lldb) protocol-server start MCP tcp://localhost:1234
MCP server started with connection listeners: connection://[::1]:1234, connection://[127.0.0.1]:1234
```

The MCP sever supports one tool (`lldb_command`) which executes a
command, but can easily be extended with more commands.

Author: JDevlieghere

lldb/cmake/modules/LLDBConfig.cmake

@@ -67,6 +67,7 @@ add_optional_dependency(LLDB_ENABLE_FBSDVMCORE "Enable libfbsdvmcore support in
 
 option(LLDB_USE_ENTITLEMENTS "When codesigning, use entitlements if available" ON)
 option(LLDB_BUILD_FRAMEWORK "Build LLDB.framework (Darwin only)" OFF)
+option(LLDB_ENABLE_PROTOCOL_SERVERS "Enable protocol servers (e.g. MCP) in LLDB" ON)
 option(LLDB_NO_INSTALL_DEFAULT_RPATH "Disable default RPATH settings in binaries" OFF)
 option(LLDB_USE_SYSTEM_DEBUGSERVER "Use the system's debugserver for testing (Darwin only)." OFF)
 option(LLDB_SKIP_STRIP "Whether to skip stripping of binaries when installing lldb." OFF)

lldb/include/lldb/Core/Debugger.h

@@ -602,6 +602,10 @@ class Debugger : public std::enable_shared_from_this<Debugger>,
   void FlushProcessOutput(Process &process, bool flush_stdout,
                           bool flush_stderr);
 
+  void AddProtocolServer(lldb::ProtocolServerSP protocol_server_sp);
+  void RemoveProtocolServer(lldb::ProtocolServerSP protocol_server_sp);
+  lldb::ProtocolServerSP GetProtocolServer(llvm::StringRef protocol) const;
+
   SourceManager::SourceFileCache &GetSourceFileCache() {
     return m_source_file_cache;
   }
@@ -772,6 +776,8 @@ class Debugger : public std::enable_shared_from_this<Debugger>,
   mutable std::mutex m_progress_reports_mutex;
   /// @}
 
+  llvm::SmallVector<lldb::ProtocolServerSP> m_protocol_servers;
+
   std::mutex m_destroy_callback_mutex;
   lldb::callback_token_t m_destroy_callback_next_token = 0;
   struct DestroyCallbackInfo {

lldb/include/lldb/Core/PluginManager.h

@@ -321,6 +321,17 @@ class PluginManager {
   static void AutoCompleteProcessName(llvm::StringRef partial_name,
                                       CompletionRequest &request);
 
+  // Protocol
+  static bool RegisterPlugin(llvm::StringRef name, llvm::StringRef description,
+                             ProtocolServerCreateInstance create_callback);
+
+  static bool UnregisterPlugin(ProtocolServerCreateInstance create_callback);
+
+  static llvm::StringRef GetProtocolServerPluginNameAtIndex(uint32_t idx);
+
+  static ProtocolServerCreateInstance
+  GetProtocolCreateCallbackForPluginName(llvm::StringRef name);
+
   // Register Type Provider
   static bool RegisterPlugin(llvm::StringRef name, llvm::StringRef description,
                              RegisterTypeBuilderCreateInstance create_callback);

lldb/include/lldb/Core/ProtocolServer.h

@@ -0,0 +1,39 @@
+//===-- ProtocolServer.h --------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_CORE_PROTOCOLSERVER_H
+#define LLDB_CORE_PROTOCOLSERVER_H
+
+#include "lldb/Core/PluginInterface.h"
+#include "lldb/Host/Socket.h"
+#include "lldb/lldb-private-interfaces.h"
+
+namespace lldb_private {
+
+class ProtocolServer : public PluginInterface {
+public:
+  ProtocolServer() = default;
+  virtual ~ProtocolServer() = default;
+
+  static lldb::ProtocolServerSP Create(llvm::StringRef name,
+                                       Debugger &debugger);
+
+  struct Connection {
+    Socket::SocketProtocol protocol;
+    std::string name;
+  };
+
+  virtual llvm::Error Start(Connection connection) = 0;
+  virtual llvm::Error Stop() = 0;
+
+  virtual Socket *GetSocket() const = 0;
+};
+
+} // namespace lldb_private
+
+#endif

lldb/include/lldb/Interpreter/CommandOptionArgumentTable.h

@@ -315,6 +315,7 @@ static constexpr CommandObject::ArgumentTableEntry g_argument_table[] = {
     { lldb::eArgTypeCPUName, "cpu-name", lldb::CompletionType::eNoCompletion, {}, { nullptr, false }, "The name of a CPU." },
     { lldb::eArgTypeCPUFeatures, "cpu-features", lldb::CompletionType::eNoCompletion, {}, { nullptr, false }, "The CPU feature string." },
     { lldb::eArgTypeManagedPlugin, "managed-plugin", lldb::CompletionType::eNoCompletion, {}, { nullptr, false }, "Plugins managed by the PluginManager" },
+    { lldb::eArgTypeProtocol, "protocol", lldb::CompletionType::eNoCompletion, {}, { nullptr, false }, "The name of the protocol." },
     // clang-format on
 };
 

lldb/include/lldb/lldb-enumerations.h

@@ -664,6 +664,7 @@ enum CommandArgumentType {
   eArgTypeCPUName,
   eArgTypeCPUFeatures,
   eArgTypeManagedPlugin,
+  eArgTypeProtocol,
   eArgTypeLastArg // Always keep this entry as the last entry in this
                   // enumeration!!
 };

lldb/include/lldb/lldb-forward.h

@@ -164,13 +164,13 @@ class PersistentExpressionState;
 class Platform;
 class Process;
 class ProcessAttachInfo;
-class ProcessLaunchInfo;
 class ProcessInfo;
 class ProcessInstanceInfo;
 class ProcessInstanceInfoMatch;
 class ProcessLaunchInfo;
 class ProcessModID;
 class Property;
+class ProtocolServer;
 class Queue;
 class QueueImpl;
 class QueueItem;
@@ -391,6 +391,7 @@ typedef std::shared_ptr<lldb_private::Platform> PlatformSP;
 typedef std::shared_ptr<lldb_private::Process> ProcessSP;
 typedef std::shared_ptr<lldb_private::ProcessAttachInfo> ProcessAttachInfoSP;
 typedef std::shared_ptr<lldb_private::ProcessLaunchInfo> ProcessLaunchInfoSP;
+typedef std::shared_ptr<lldb_private::ProtocolServer> ProtocolServerSP;
 typedef std::weak_ptr<lldb_private::Process> ProcessWP;
 typedef std::shared_ptr<lldb_private::RegisterCheckpoint> RegisterCheckpointSP;
 typedef std::shared_ptr<lldb_private::RegisterContext> RegisterContextSP;

lldb/include/lldb/lldb-private-interfaces.h

@@ -81,6 +81,8 @@ typedef lldb::PlatformSP (*PlatformCreateInstance)(bool force,
 typedef lldb::ProcessSP (*ProcessCreateInstance)(
     lldb::TargetSP target_sp, lldb::ListenerSP listener_sp,
     const FileSpec *crash_file_path, bool can_connect);
+typedef lldb::ProtocolServerSP (*ProtocolServerCreateInstance)(
+    Debugger &debugger);
 typedef lldb::RegisterTypeBuilderSP (*RegisterTypeBuilderCreateInstance)(
     Target &target);
 typedef lldb::ScriptInterpreterSP (*ScriptInterpreterCreateInstance)(

lldb/source/Commands/CMakeLists.txt

@@ -23,6 +23,7 @@ add_lldb_library(lldbCommands NO_PLUGIN_DEPENDENCIES
   CommandObjectPlatform.cpp
   CommandObjectPlugin.cpp
   CommandObjectProcess.cpp
+  CommandObjectProtocolServer.cpp
   CommandObjectQuit.cpp
   CommandObjectRegexCommand.cpp
   CommandObjectRegister.cpp

lldb/source/Commands/CommandObjectProtocolServer.cpp

@@ -0,0 +1,176 @@
+//===-- CommandObjectProtocolServer.cpp
+//----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "CommandObjectProtocolServer.h"
+#include "lldb/Core/PluginManager.h"
+#include "lldb/Core/ProtocolServer.h"
+#include "lldb/Host/Socket.h"
+#include "lldb/Interpreter/CommandInterpreter.h"
+#include "lldb/Interpreter/CommandReturnObject.h"
+#include "lldb/Utility/UriParser.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/FormatAdapters.h"
+
+using namespace llvm;
+using namespace lldb;
+using namespace lldb_private;
+
+#define LLDB_OPTIONS_mcp
+#include "CommandOptions.inc"
+
+static std::vector<llvm::StringRef> GetSupportedProtocols() {
+  std::vector<llvm::StringRef> supported_protocols;
+  size_t i = 0;
+
+  for (llvm::StringRef protocol_name =
+           PluginManager::GetProtocolServerPluginNameAtIndex(i++);
+       !protocol_name.empty();
+       protocol_name = PluginManager::GetProtocolServerPluginNameAtIndex(i++)) {
+    supported_protocols.push_back(protocol_name);
+  }
+
+  return supported_protocols;
+}
+
+class CommandObjectProtocolServerStart : public CommandObjectParsed {
+public:
+  CommandObjectProtocolServerStart(CommandInterpreter &interpreter)
+      : CommandObjectParsed(interpreter, "protocol-server start",
+                            "start protocol server",
+                            "protocol-server start <protocol> <connection>") {
+    AddSimpleArgumentList(lldb::eArgTypeProtocol, eArgRepeatPlain);
+    AddSimpleArgumentList(lldb::eArgTypeConnectURL, eArgRepeatPlain);
+  }
+
+  ~CommandObjectProtocolServerStart() override = default;
+
+protected:
+  void DoExecute(Args &args, CommandReturnObject &result) override {
+    if (args.GetArgumentCount() < 1) {
+      result.AppendError("no protocol specified");
+      return;
+    }
+
+    llvm::StringRef protocol = args.GetArgumentAtIndex(0);
+    std::vector<llvm::StringRef> supported_protocols = GetSupportedProtocols();
+    if (llvm::find(supported_protocols, protocol) ==
+        supported_protocols.end()) {
+      result.AppendErrorWithFormatv(
+          "unsupported protocol: {0}. Supported protocols are: {1}", protocol,
+          llvm::join(GetSupportedProtocols(), ", "));
+      return;
+    }
+
+    if (args.GetArgumentCount() < 2) {
+      result.AppendError("no connection specified");
+      return;
+    }
+    llvm::StringRef connection_uri = args.GetArgumentAtIndex(1);
+
+    ProtocolServerSP server_sp = GetDebugger().GetProtocolServer(protocol);
+    if (!server_sp)
+      server_sp = ProtocolServer::Create(protocol, GetDebugger());
+
+    const char *connection_error =
+        "unsupported connection specifier, expected 'accept:///path' or "
+        "'listen://[host]:port', got '{0}'.";
+    auto uri = lldb_private::URI::Parse(connection_uri);
+    if (!uri) {
+      result.AppendErrorWithFormatv(connection_error, connection_uri);
+      return;
+    }
+
+    std::optional<Socket::ProtocolModePair> protocol_and_mode =
+        Socket::GetProtocolAndMode(uri->scheme);
+    if (!protocol_and_mode || protocol_and_mode->second != Socket::ModeAccept) {
+      result.AppendErrorWithFormatv(connection_error, connection_uri);
+      return;
+    }
+
+    ProtocolServer::Connection connection;
+    connection.protocol = protocol_and_mode->first;
+    connection.name =
+        formatv("[{0}]:{1}", uri->hostname.empty() ? "0.0.0.0" : uri->hostname,
+                uri->port.value_or(0));
+
+    if (llvm::Error error = server_sp->Start(connection)) {
+      result.AppendErrorWithFormatv("{0}", llvm::fmt_consume(std::move(error)));
+      return;
+    }
+
+    GetDebugger().AddProtocolServer(server_sp);
+
+    if (Socket *socket = server_sp->GetSocket()) {
+      std::string address =
+          llvm::join(socket->GetListeningConnectionURI(), ", ");
+      result.AppendMessageWithFormatv(
+          "{0} server started with connection listeners: {1}", protocol,
+          address);
+    }
+  }
+};
+
+class CommandObjectProtocolServerStop : public CommandObjectParsed {
+public:
+  CommandObjectProtocolServerStop(CommandInterpreter &interpreter)
+      : CommandObjectParsed(interpreter, "protocol-server stop",
+                            "stop protocol server",
+                            "protocol-server stop <protocol>") {
+    AddSimpleArgumentList(lldb::eArgTypeProtocol, eArgRepeatPlain);
+  }
+
+  ~CommandObjectProtocolServerStop() override = default;
+
+protected:
+  void DoExecute(Args &args, CommandReturnObject &result) override {
+    if (args.GetArgumentCount() < 1) {
+      result.AppendError("no protocol specified");
+      return;
+    }
+
+    llvm::StringRef protocol = args.GetArgumentAtIndex(0);
+    std::vector<llvm::StringRef> supported_protocols = GetSupportedProtocols();
+    if (llvm::find(supported_protocols, protocol) ==
+        supported_protocols.end()) {
+      result.AppendErrorWithFormatv(
+          "unsupported protocol: {0}. Supported protocols are: {1}", protocol,
+          llvm::join(GetSupportedProtocols(), ", "));
+      return;
+    }
+
+    Debugger &debugger = GetDebugger();
+
+    ProtocolServerSP server_sp = debugger.GetProtocolServer(protocol);
+    if (!server_sp) {
+      result.AppendError(
+          llvm::formatv("no {0} protocol server running", protocol).str());
+      return;
+    }
+
+    if (llvm::Error error = server_sp->Stop()) {
+      result.AppendErrorWithFormatv("{0}", llvm::fmt_consume(std::move(error)));
+      return;
+    }
+
+    debugger.RemoveProtocolServer(server_sp);
+  }
+};
+
+CommandObjectProtocolServer::CommandObjectProtocolServer(
+    CommandInterpreter &interpreter)
+    : CommandObjectMultiword(interpreter, "protocol-server",
+                             "Start and stop a protocol server.",
+                             "protocol-server") {
+  LoadSubCommand("start", CommandObjectSP(new CommandObjectProtocolServerStart(
+                              interpreter)));
+  LoadSubCommand("stop", CommandObjectSP(
+                             new CommandObjectProtocolServerStop(interpreter)));
+}
+
+CommandObjectProtocolServer::~CommandObjectProtocolServer() = default;

lldb/source/Commands/CommandObjectProtocolServer.h

@@ -0,0 +1,25 @@
+//===-- CommandObjectProtocolServer.h
+//------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_SOURCE_COMMANDS_COMMANDOBJECTPROTOCOLSERVER_H
+#define LLDB_SOURCE_COMMANDS_COMMANDOBJECTPROTOCOLSERVER_H
+
+#include "lldb/Interpreter/CommandObjectMultiword.h"
+
+namespace lldb_private {
+
+class CommandObjectProtocolServer : public CommandObjectMultiword {
+public:
+  CommandObjectProtocolServer(CommandInterpreter &interpreter);
+  ~CommandObjectProtocolServer() override;
+};
+
+} // namespace lldb_private
+
+#endif // LLDB_SOURCE_COMMANDS_COMMANDOBJECTMCP_H

lldb/source/Core/CMakeLists.txt

@@ -46,6 +46,7 @@ add_lldb_library(lldbCore NO_PLUGIN_DEPENDENCIES
   Opcode.cpp
   PluginManager.cpp
   Progress.cpp
+  ProtocolServer.cpp
   Statusline.cpp
   RichManglingContext.cpp
   SearchFilter.cpp

lldb/source/Core/Debugger.cpp

@@ -16,6 +16,7 @@
 #include "lldb/Core/ModuleSpec.h"
 #include "lldb/Core/PluginManager.h"
 #include "lldb/Core/Progress.h"
+#include "lldb/Core/ProtocolServer.h"
 #include "lldb/Core/StreamAsynchronousIO.h"
 #include "lldb/Core/Telemetry.h"
 #include "lldb/DataFormatters/DataVisualization.h"
@@ -2375,3 +2376,26 @@ llvm::ThreadPoolInterface &Debugger::GetThreadPool() {
          "Debugger::GetThreadPool called before Debugger::Initialize");
   return *g_thread_pool;
 }
+
+void Debugger::AddProtocolServer(lldb::ProtocolServerSP protocol_server_sp) {
+  assert(protocol_server_sp &&
+         GetProtocolServer(protocol_server_sp->GetPluginName()) == nullptr);
+  m_protocol_servers.push_back(protocol_server_sp);
+}
+
+void Debugger::RemoveProtocolServer(lldb::ProtocolServerSP protocol_server_sp) {
+  auto it = llvm::find(m_protocol_servers, protocol_server_sp);
+  if (it != m_protocol_servers.end())
+    m_protocol_servers.erase(it);
+}
+
+lldb::ProtocolServerSP
+Debugger::GetProtocolServer(llvm::StringRef protocol) const {
+  for (ProtocolServerSP protocol_server_sp : m_protocol_servers) {
+    if (!protocol_server_sp)
+      continue;
+    if (protocol_server_sp->GetPluginName() == protocol)
+      return protocol_server_sp;
+  }
+  return nullptr;
+}