[lldb] Adjust ProtocolServer connection defaults. (#155714)

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.

---------

Co-authored-by: Jonas Devlieghere <[email protected]>

Author: ashgti

lldb/include/lldb/Host/HostInfoBase.h

@@ -102,11 +102,19 @@ class HostInfoBase {
   /// member of the FileSpec is filled in.
   static FileSpec GetSystemPluginDir();
 
+  /// Returns the directory containing the users home (e.g. `~/`). Only the
+  /// directory member of the FileSpec is filled in.
+  static FileSpec GetUserHomeDir();
+
+  /// Returns the directory containing the users lldb home (e.g. `~/.lldb/`).
+  /// Only the directory member of the FileSpec is filled in.
+  static FileSpec GetUserLLDBDir();
+
   /// Returns the directory containing the user plugins. Only the directory
   /// member of the FileSpec is filled in.
   static FileSpec GetUserPluginDir();
 
-  /// Returns the proces temporary directory. This directory will be cleaned up
+  /// Returns the process temporary directory. This directory will be cleaned up
   /// when this process exits. Only the directory member of the FileSpec is
   /// filled in.
   static FileSpec GetProcessTempDir();
@@ -167,11 +175,13 @@ class HostInfoBase {
   static bool ComputeTempFileBaseDirectory(FileSpec &file_spec);
   static bool ComputeHeaderDirectory(FileSpec &file_spec);
   static bool ComputeSystemPluginsDirectory(FileSpec &file_spec);
+  static bool ComputeUserHomeDirectory(FileSpec &file_spec);
+  static bool ComputeUserLLDBHomeDirectory(FileSpec &file_spec);
   static bool ComputeUserPluginsDirectory(FileSpec &file_spec);
 
   static void ComputeHostArchitectureSupport(ArchSpec &arch_32,
                                              ArchSpec &arch_64);
 };
-}
+} // namespace lldb_private
 
 #endif

lldb/include/lldb/Host/macosx/HostInfoMacOSX.h

@@ -56,6 +56,7 @@ class HostInfoMacOSX : public HostInfoPosix {
   static std::string FindComponentInPath(llvm::StringRef path,
                                          llvm::StringRef component);
 };
-}
+
+} // namespace lldb_private
 
 #endif

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

@@ -41,6 +41,15 @@ class MCPTransport
   LogCallback m_log_callback;
 };
 
+/// Information about this instance of lldb's MCP server for lldb-mcp to use to
+/// coordinate connecting an lldb-mcp client.
+struct ServerInfo {
+  std::string connection_uri;
+  lldb::pid_t pid;
+};
+llvm::json::Value toJSON(const ServerInfo &);
+bool fromJSON(const llvm::json::Value &, ServerInfo &, llvm::json::Path);
+
 class Server : public MCPTransport::MessageHandler {
 public:
   Server(std::string name, std::string version,

lldb/source/API/SBHostOS.cpp

@@ -86,15 +86,7 @@ SBFileSpec SBHostOS::GetLLDBPath(lldb::PathType path_type) {
 
 SBFileSpec SBHostOS::GetUserHomeDirectory() {
   LLDB_INSTRUMENT();
-
-  FileSpec homedir;
-  FileSystem::Instance().GetHomeDirectory(homedir);
-  FileSystem::Instance().Resolve(homedir);
-
-  SBFileSpec sb_fspec;
-  sb_fspec.SetFileSpec(homedir);
-
-  return sb_fspec;
+  return HostInfo::GetUserHomeDir();
 }
 
 lldb::thread_t SBHostOS::ThreadCreate(const char *name,

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/Host/common/Editline.cpp

@@ -10,10 +10,8 @@
 #include <iomanip>
 #include <optional>
 
-#include "lldb/Host/ConnectionFileDescriptor.h"
 #include "lldb/Host/Editline.h"
-#include "lldb/Host/FileSystem.h"
-#include "lldb/Host/Host.h"
+#include "lldb/Host/HostInfo.h"
 #include "lldb/Host/StreamFile.h"
 #include "lldb/Utility/AnsiTerminal.h"
 #include "lldb/Utility/CompletionRequest.h"
@@ -219,20 +217,19 @@ class EditlineHistory {
   const char *GetHistoryFilePath() {
     // Compute the history path lazily.
     if (m_path.empty() && m_history && !m_prefix.empty()) {
-      llvm::SmallString<128> lldb_history_file;
-      FileSystem::Instance().GetHomeDirectory(lldb_history_file);
-      llvm::sys::path::append(lldb_history_file, ".lldb");
+      FileSpec lldb_dir = HostInfo::GetUserLLDBDir();
 
       // LLDB stores its history in ~/.lldb/. If for some reason this directory
       // isn't writable or cannot be created, history won't be available.
-      if (!llvm::sys::fs::create_directory(lldb_history_file)) {
+      if (!llvm::sys::fs::create_directory(lldb_dir.GetPath())) {
 #if LLDB_EDITLINE_USE_WCHAR
         std::string filename = m_prefix + "-widehistory";
 #else
         std::string filename = m_prefix + "-history";
 #endif
-        llvm::sys::path::append(lldb_history_file, filename);
-        m_path = std::string(lldb_history_file.str());
+        FileSpec lldb_history_file =
+            lldb_dir.CopyByAppendingPathComponent(filename);
+        m_path = lldb_history_file.GetPath();
       }
     }
 

lldb/source/Host/common/HostInfoBase.cpp

@@ -61,6 +61,10 @@ struct HostInfoBaseFields {
   FileSpec m_lldb_clang_resource_dir;
   llvm::once_flag m_lldb_system_plugin_dir_once;
   FileSpec m_lldb_system_plugin_dir;
+  llvm::once_flag m_lldb_user_home_dir_once;
+  FileSpec m_lldb_user_home_dir;
+  llvm::once_flag m_lldb_user_lldb_dir_once;
+  FileSpec m_lldb_user_lldb_dir;
   llvm::once_flag m_lldb_user_plugin_dir_once;
   FileSpec m_lldb_user_plugin_dir;
   llvm::once_flag m_lldb_process_tmp_dir_once;
@@ -161,6 +165,26 @@ FileSpec HostInfoBase::GetSystemPluginDir() {
   return g_fields->m_lldb_system_plugin_dir;
 }
 
+FileSpec HostInfoBase::GetUserHomeDir() {
+  llvm::call_once(g_fields->m_lldb_user_home_dir_once, []() {
+    if (!HostInfo::ComputeUserHomeDirectory(g_fields->m_lldb_user_home_dir))
+      g_fields->m_lldb_user_home_dir = FileSpec();
+    LLDB_LOG(GetLog(LLDBLog::Host), "user home dir -> `{0}`",
+             g_fields->m_lldb_user_home_dir);
+  });
+  return g_fields->m_lldb_user_home_dir;
+}
+
+FileSpec HostInfoBase::GetUserLLDBDir() {
+  llvm::call_once(g_fields->m_lldb_user_lldb_dir_once, []() {
+    if (!HostInfo::ComputeUserLLDBHomeDirectory(g_fields->m_lldb_user_lldb_dir))
+      g_fields->m_lldb_user_lldb_dir = FileSpec();
+    LLDB_LOG(GetLog(LLDBLog::Host), "user lldb home dir -> `{0}`",
+             g_fields->m_lldb_user_lldb_dir);
+  });
+  return g_fields->m_lldb_user_lldb_dir;
+}
+
 FileSpec HostInfoBase::GetUserPluginDir() {
   llvm::call_once(g_fields->m_lldb_user_plugin_dir_once, []() {
     if (!HostInfo::ComputeUserPluginsDirectory(
@@ -316,6 +340,20 @@ bool HostInfoBase::ComputeSystemPluginsDirectory(FileSpec &file_spec) {
   return false;
 }
 
+bool HostInfoBase::ComputeUserHomeDirectory(FileSpec &file_spec) {
+  FileSpec temp_file("~");
+  FileSystem::Instance().Resolve(temp_file);
+  file_spec.SetDirectory(temp_file.GetPathAsConstString());
+  return true;
+}
+
+bool HostInfoBase::ComputeUserLLDBHomeDirectory(FileSpec &file_spec) {
+  FileSpec home_dir_spec = GetUserHomeDir();
+  home_dir_spec.AppendPathComponent(".lldb");
+  file_spec.SetDirectory(home_dir_spec.GetPathAsConstString());
+  return true;
+}
+
 bool HostInfoBase::ComputeUserPluginsDirectory(FileSpec &file_spec) {
   // TODO(zturner): Figure out how to compute the user plugins directory for
   // all platforms.

lldb/source/Host/macosx/objcxx/HostInfoMacOSX.mm

@@ -39,7 +39,7 @@
 #include <Foundation/Foundation.h>
 #include <mach-o/dyld.h>
 #if defined(MAC_OS_X_VERSION_MIN_REQUIRED) && \
-  MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_VERSION_12_0
+    MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_VERSION_12_0
 #if __has_include(<mach-o/dyld_introspection.h>)
 #include <mach-o/dyld_introspection.h>
 #define SDK_HAS_NEW_DYLD_INTROSPECTION_SPIS
@@ -78,8 +78,8 @@
 static void ParseOSVersion(llvm::VersionTuple &version, NSString *Key) {
   @autoreleasepool {
     NSDictionary *version_info =
-      [NSDictionary dictionaryWithContentsOfFile:
-       @"/System/Library/CoreServices/SystemVersion.plist"];
+        [NSDictionary dictionaryWithContentsOfFile:
+                          @"/System/Library/CoreServices/SystemVersion.plist"];
     NSString *version_value = [version_info objectForKey: Key];
     const char *version_str = [version_value UTF8String];
     version.tryParse(version_str);
@@ -225,9 +225,9 @@ static bool ResolveAndVerifyCandidateSupportDir(FileSpec &path) {
 }
 
 bool HostInfoMacOSX::ComputeUserPluginsDirectory(FileSpec &file_spec) {
-  FileSpec temp_file("~/Library/Application Support/LLDB/PlugIns");
-  FileSystem::Instance().Resolve(temp_file);
-  file_spec.SetDirectory(temp_file.GetPathAsConstString());
+  FileSpec home_dir_spec = GetUserHomeDir();
+  home_dir_spec.AppendPathComponent("Library/Application Support/LLDB/PlugIns");
+  file_spec.SetDirectory(home_dir_spec.GetPathAsConstString());
   return true;
 }
 

lldb/source/Interpreter/CommandInterpreter.cpp

@@ -52,6 +52,7 @@
 #include "lldb/Core/Telemetry.h"
 #include "lldb/Host/StreamFile.h"
 #include "lldb/Utility/ErrorMessages.h"
+#include "lldb/Utility/FileSpec.h"
 #include "lldb/Utility/LLDBLog.h"
 #include "lldb/Utility/Log.h"
 #include "lldb/Utility/State.h"
@@ -2502,22 +2503,18 @@ int CommandInterpreter::GetOptionArgumentPosition(const char *in_string) {
   return position;
 }
 
-static void GetHomeInitFile(llvm::SmallVectorImpl<char> &init_file,
-                            llvm::StringRef suffix = {}) {
+static void GetHomeInitFile(FileSpec &init_file, llvm::StringRef suffix = {}) {
   std::string init_file_name = ".lldbinit";
   if (!suffix.empty()) {
     init_file_name.append("-");
     init_file_name.append(suffix.str());
   }
 
-  FileSystem::Instance().GetHomeDirectory(init_file);
-  llvm::sys::path::append(init_file, init_file_name);
-
-  FileSystem::Instance().Resolve(init_file);
+  init_file =
+      HostInfo::GetUserHomeDir().CopyByAppendingPathComponent(init_file_name);
 }
 
-static void GetHomeREPLInitFile(llvm::SmallVectorImpl<char> &init_file,
-                                LanguageType language) {
+static void GetHomeREPLInitFile(FileSpec &init_file, LanguageType language) {
   if (language == eLanguageTypeUnknown) {
     LanguageSet repl_languages = Language::GetLanguagesSupportingREPLs();
     if (auto main_repl_language = repl_languages.GetSingularLanguage())
@@ -2531,9 +2528,9 @@ static void GetHomeREPLInitFile(llvm::SmallVectorImpl<char> &init_file,
        llvm::Twine(Language::GetNameForLanguageType(language)) +
        llvm::Twine("-repl"))
           .str();
-  FileSystem::Instance().GetHomeDirectory(init_file);
-  llvm::sys::path::append(init_file, init_file_name);
-  FileSystem::Instance().Resolve(init_file);
+
+  init_file =
+      HostInfo::GetUserHomeDir().CopyByAppendingPathComponent(init_file_name);
 }
 
 static void GetCwdInitFile(llvm::SmallVectorImpl<char> &init_file) {
@@ -2588,10 +2585,10 @@ void CommandInterpreter::SourceInitFileCwd(CommandReturnObject &result) {
     SourceInitFile(FileSpec(init_file.str()), result);
     break;
   case eLoadCWDlldbinitWarn: {
-    llvm::SmallString<128> home_init_file;
+    FileSpec home_init_file;
     GetHomeInitFile(home_init_file);
     if (llvm::sys::path::parent_path(init_file) ==
-        llvm::sys::path::parent_path(home_init_file)) {
+        llvm::sys::path::parent_path(home_init_file.GetPath())) {
       result.SetStatus(eReturnStatusSuccessFinishNoResult);
     } else {
       result.AppendError(InitFileWarning);
@@ -2611,24 +2608,24 @@ void CommandInterpreter::SourceInitFileHome(CommandReturnObject &result,
     return;
   }
 
-  llvm::SmallString<128> init_file;
+  FileSpec init_file;
 
   if (is_repl)
     GetHomeREPLInitFile(init_file, GetDebugger().GetREPLLanguage());
 
-  if (init_file.empty())
+  if (init_file.GetPath().empty())
     GetHomeInitFile(init_file);
 
   if (!m_skip_app_init_files) {
     llvm::StringRef program_name =
         HostInfo::GetProgramFileSpec().GetFilename().GetStringRef();
-    llvm::SmallString<128> program_init_file;
+    FileSpec program_init_file;
     GetHomeInitFile(program_init_file, program_name);
     if (FileSystem::Instance().Exists(program_init_file))
       init_file = program_init_file;
   }
 
-  SourceInitFile(FileSpec(init_file.str()), result);
+  SourceInitFile(init_file, result);
 }
 
 void CommandInterpreter::SourceInitFileGlobal(CommandReturnObject &result) {