[lldb] Adjust ProtocolServer connection defaults.

This adjusts the ProtocolServer command to default to create a new connection listening on `localhost:0` and adds a new `ServerMetadata` details to `~/.lldb/mcp/lldb-<pid>.json` to record information about the current MCP server.

This can be consumed by the lldb-mcp binary to establish a connection from an LLM client.

Author: ashgti

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

@@ -14,11 +14,23 @@
 #include "lldb/Protocol/MCP/Resource.h"
 #include "lldb/Protocol/MCP/Tool.h"
 #include "lldb/Protocol/MCP/Transport.h"
+#include "lldb/lldb-types.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/Support/Error.h"
+#include "llvm/Support/JSON.h"
 
 namespace lldb_protocol::mcp {
 
+/// Metadata about this instance of lldb's MCP server for lldb-mcp to use to
+/// coordinate connecting an lldb-mcp client.
+struct ServerMetadata {
+  std::string connection_uri;
+  lldb::pid_t pid;
+};
+llvm::json::Value toJSON(const ServerMetadata &SM);
+bool fromJSON(const llvm::json::Value &V, ServerMetadata &SM,
+              llvm::json::Path P);
+
 class Server : public Transport::MessageHandler {
 public:
   Server(std::string name, std::string version,

lldb/source/Commands/CommandObjectProtocolServer.cpp

@@ -15,6 +15,7 @@
 #include "lldb/Utility/UriParser.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/FormatAdapters.h"
+#include <string>
 
 using namespace llvm;
 using namespace lldb;
@@ -28,7 +29,7 @@ class CommandObjectProtocolServerStart : public CommandObjectParsed {
   CommandObjectProtocolServerStart(CommandInterpreter &interpreter)
       : CommandObjectParsed(interpreter, "protocol-server start",
                             "start protocol server",
-                            "protocol-server start <protocol> <connection>") {
+                            "protocol-server start <protocol> [<connection>]") {
     AddSimpleArgumentList(lldb::eArgTypeProtocol, eArgRepeatPlain);
     AddSimpleArgumentList(lldb::eArgTypeConnectURL, eArgRepeatPlain);
   }
@@ -51,15 +52,13 @@ class CommandObjectProtocolServerStart : public CommandObjectParsed {
       return;
     }
 
-    if (args.GetArgumentCount() < 2) {
-      result.AppendError("no connection specified");
-      return;
-    }
-    llvm::StringRef connection_uri = args.GetArgumentAtIndex(1);
+    std::string connection_uri = "listen://[localhost]:0";
+    if (args.GetArgumentCount() > 2)
+      connection_uri = args.GetArgumentAtIndex(1);
 
     const char *connection_error =
-        "unsupported connection specifier, expected 'accept:///path' or "
-        "'listen://[host]:port', got '{0}'.";
+        "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);

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

@@ -10,14 +10,14 @@
 #include "Resource.h"
 #include "Tool.h"
 #include "lldb/Core/PluginManager.h"
-#include "lldb/Protocol/MCP/MCPError.h"
-#include "lldb/Protocol/MCP/Tool.h"
+#include "lldb/Host/FileSystem.h"
+#include "lldb/Protocol/MCP/Server.h"
 #include "lldb/Utility/LLDBLog.h"
 #include "lldb/Utility/Log.h"
 #include "llvm/ADT/StringExtras.h"
+#include "llvm/Support/Error.h"
 #include "llvm/Support/Threading.h"
 #include <thread>
-#include <variant>
 
 using namespace lldb_private;
 using namespace lldb_private::mcp;
@@ -104,6 +104,43 @@ llvm::Error ProtocolServerMCP::Start(ProtocolServer::Connection connection) {
   if (llvm::Error error = handles.takeError())
     return error;
 
+  auto listening_uris = m_listener->GetListeningConnectionURI();
+  if (listening_uris.empty())
+    return createStringError("Failed to list listening connections");
+  std::string address =
+      llvm::join(m_listener->GetListeningConnectionURI(), ", ");
+
+  llvm::SmallString<128> user_home_dir;
+  FileSystem::Instance().GetHomeDirectory(user_home_dir);
+  FileSpec mcp_registry_dir = FileSpec(user_home_dir.c_str());
+  mcp_registry_dir.AppendPathComponent(".lldb");
+  mcp_registry_dir.AppendPathComponent("mcp");
+
+  Status error(llvm::sys::fs::create_directory(mcp_registry_dir.GetPath()));
+  if (error.Fail())
+    return error.takeError();
+
+  m_mcp_registry_entry_path = mcp_registry_dir.CopyByAppendingPathComponent(
+      formatv("lldb-{0}.json", getpid()).str());
+
+  const File::OpenOptions flags = File::eOpenOptionWriteOnly |
+                                  File::eOpenOptionCanCreate |
+                                  File::eOpenOptionTruncate;
+  llvm::Expected<lldb::FileUP> file =
+      FileSystem::Instance().Open(m_mcp_registry_entry_path, flags,
+                                  lldb::eFilePermissionsFileDefault, false);
+  if (!file)
+    return file.takeError();
+
+  ServerMetadata metadata;
+  metadata.connection_uri = listening_uris[0];
+  metadata.pid = getpid();
+
+  std::string buf = formatv("{0}", toJSON(metadata)).str();
+  size_t num_bytes = buf.size();
+  if (llvm::Error error = (*file)->Write(buf.data(), num_bytes).takeError())
+    return error;
+
   m_running = true;
   m_listen_handlers = std::move(*handles);
   m_loop_thread = std::thread([=] {
@@ -122,6 +159,10 @@ llvm::Error ProtocolServerMCP::Stop() {
     m_running = false;
   }
 
+  if (!m_mcp_registry_entry_path.GetPath().empty())
+    FileSystem::Instance().RemoveFile(m_mcp_registry_entry_path);
+  m_mcp_registry_entry_path.Clear();
+
   // Stop the main loop.
   m_loop.AddPendingCallback(
       [](lldb_private::MainLoopBase &loop) { loop.RequestTermination(); });

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

@@ -48,6 +48,7 @@ class ProtocolServerMCP : public ProtocolServer {
 
   bool m_running = false;
 
+  FileSpec m_mcp_registry_entry_path;
   lldb_private::MainLoop m_loop;
   std::thread m_loop_thread;
   std::mutex m_mutex;

lldb/source/Protocol/MCP/Server.cpp

@@ -14,6 +14,18 @@
 using namespace lldb_protocol::mcp;
 using namespace llvm;
 
+llvm::json::Value toJSON(const ServerMetadata &SM) {
+  return llvm::json::Object{{"connection_uri", SM.connection_uri},
+                            {"pid", SM.pid}};
+}
+
+bool fromJSON(const llvm::json::Value &V, ServerMetadata &SM,
+              llvm::json::Path P) {
+  llvm::json::ObjectMapper O(V, P);
+  return O && O.map("connection_uri", SM.connection_uri) &&
+         O.map("pid", SM.pid);
+}
+
 Server::Server(std::string name, std::string version,
                std::unique_ptr<Transport> transport_up,
                lldb_private::MainLoop &loop)