[lldb-dap] Add multi-session support with shared debugger instances

Summary:
This change introduces a DAPSessionManager to enable multiple concurrent
lldb-dap sessions and allow them to share debugger instances when needed.

Key changes:
- Add DAPSessionManager singleton to track and coordinate all active DAP sessions
- Support attaching to an existing target via unique target ID (targetId parameter)
- Share debugger instances across sessions when spawning new targets (e.g., GPU debugging)
- Refactor event thread management to allow sharing event threads between sessions
- Add eBroadcastBitNewTargetCreated event to notify when new targets are spawned
- Extract session names from target creation events for better identification
- Defer debugger initialization from 'initialize' request to 'launch'/'attach' requests

This enables scenarios where a native process spawns a new target (like a child process)
and the debug adapter can automatically start a new debug session for the spawned
target while sharing the parent's debugger instance.

Tests:
The refactoring maintains backward compatibility. All existing test cases pass.

Author: qxy11

lldb/include/lldb/API/SBTarget.h

@@ -44,6 +44,7 @@ class LLDB_API SBTarget {
     eBroadcastBitWatchpointChanged = (1 << 3),
     eBroadcastBitSymbolsLoaded = (1 << 4),
     eBroadcastBitSymbolsChanged = (1 << 5),
+    eBroadcastBitNewTargetCreated = (1 << 6),
   };
 
   // Constructors
@@ -69,6 +70,8 @@ class LLDB_API SBTarget {
   static lldb::SBModule GetModuleAtIndexFromEvent(const uint32_t idx,
                                                   const lldb::SBEvent &event);
 
+  static const char *GetSessionNameFromEvent(const SBEvent &event);
+
   static const char *GetBroadcasterClassName();
 
   lldb::SBProcess GetProcess();

lldb/include/lldb/Target/Target.h

@@ -537,6 +537,7 @@ class Target : public std::enable_shared_from_this<Target>,
     eBroadcastBitWatchpointChanged = (1 << 3),
     eBroadcastBitSymbolsLoaded = (1 << 4),
     eBroadcastBitSymbolsChanged = (1 << 5),
+    eBroadcastBitNewTargetCreated = (1 << 6),
   };
 
   // These two functions fill out the Broadcaster interface:
@@ -556,6 +557,11 @@ class Target : public std::enable_shared_from_this<Target>,
     TargetEventData(const lldb::TargetSP &target_sp,
                     const ModuleList &module_list);
 
+    TargetEventData(const lldb::TargetSP &target_sp, std::string session_name);
+
+    TargetEventData(const lldb::TargetSP &target_sp,
+                    const ModuleList &module_list, std::string session_name);
+
     ~TargetEventData() override;
 
     static llvm::StringRef GetFlavorString();
@@ -564,6 +570,8 @@ class Target : public std::enable_shared_from_this<Target>,
       return TargetEventData::GetFlavorString();
     }
 
+    static llvm::StringRef GetSessionNameFromEvent(const Event *event_ptr);
+
     void Dump(Stream *s) const override;
 
     static const TargetEventData *GetEventDataFromEvent(const Event *event_ptr);
@@ -579,6 +587,7 @@ class Target : public std::enable_shared_from_this<Target>,
   private:
     lldb::TargetSP m_target_sp;
     ModuleList m_module_list;
+    std::string m_session_name;
 
     TargetEventData(const TargetEventData &) = delete;
     const TargetEventData &operator=(const TargetEventData &) = delete;

lldb/packages/Python/lldbsuite/test/tools/lldb-dap/dap_server.py

@@ -785,6 +785,7 @@ def request_attach(
         *,
         program: Optional[str] = None,
         pid: Optional[int] = None,
+        targetId: Optional[int] = None,
         waitFor=False,
         initCommands: Optional[list[str]] = None,
         preRunCommands: Optional[list[str]] = None,
@@ -804,6 +805,8 @@ def request_attach(
             args_dict["pid"] = pid
         if program is not None:
             args_dict["program"] = program
+        if targetId is not None:
+            args_dict["targetId"] = targetId
         if waitFor:
             args_dict["waitFor"] = waitFor
         args_dict["initCommands"] = self.init_commands

lldb/source/API/SBTarget.cpp

@@ -145,6 +145,14 @@ SBModule SBTarget::GetModuleAtIndexFromEvent(const uint32_t idx,
   return SBModule(module_list.GetModuleAtIndex(idx));
 }
 
+const char *SBTarget::GetSessionNameFromEvent(const SBEvent &event) {
+  LLDB_INSTRUMENT_VA(event);
+
+  return ConstString(
+             Target::TargetEventData::GetSessionNameFromEvent(event.get()))
+      .AsCString();
+}
+
 const char *SBTarget::GetBroadcasterClassName() {
   LLDB_INSTRUMENT();
 

lldb/source/Target/Target.cpp

@@ -193,6 +193,7 @@ Target::Target(Debugger &debugger, const ArchSpec &target_arch,
   SetEventName(eBroadcastBitModulesUnloaded, "modules-unloaded");
   SetEventName(eBroadcastBitWatchpointChanged, "watchpoint-changed");
   SetEventName(eBroadcastBitSymbolsLoaded, "symbols-loaded");
+  SetEventName(eBroadcastBitNewTargetCreated, "new-target-created");
 
   CheckInWithManager();
 
@@ -5169,11 +5170,21 @@ void TargetProperties::SetDebugUtilityExpression(bool debug) {
 // Target::TargetEventData
 
 Target::TargetEventData::TargetEventData(const lldb::TargetSP &target_sp)
-    : EventData(), m_target_sp(target_sp), m_module_list() {}
+    : TargetEventData(target_sp, ModuleList(), "") {}
 
 Target::TargetEventData::TargetEventData(const lldb::TargetSP &target_sp,
                                          const ModuleList &module_list)
-    : EventData(), m_target_sp(target_sp), m_module_list(module_list) {}
+    : TargetEventData(target_sp, module_list, "") {}
+
+Target::TargetEventData::TargetEventData(const lldb::TargetSP &target_sp,
+                                         std::string session_name)
+    : TargetEventData(target_sp, ModuleList(), std::move(session_name)) {}
+
+Target::TargetEventData::TargetEventData(const lldb::TargetSP &target_sp,
+                                         const ModuleList &module_list,
+                                         std::string session_name)
+    : EventData(), m_target_sp(target_sp), m_module_list(module_list),
+      m_session_name(std::move(session_name)) {}
 
 Target::TargetEventData::~TargetEventData() = default;
 
@@ -5209,6 +5220,25 @@ TargetSP Target::TargetEventData::GetTargetFromEvent(const Event *event_ptr) {
   return target_sp;
 }
 
+llvm::StringRef
+Target::TargetEventData::GetSessionNameFromEvent(const Event *event_ptr) {
+  const TargetEventData *event_data = GetEventDataFromEvent(event_ptr);
+  if (!event_data)
+    return llvm::StringRef();
+
+  if (!event_data->m_session_name.empty())
+    return event_data->m_session_name;
+
+  // Generate default session name if not provided
+  if (event_data->m_target_sp) {
+    lldb::user_id_t target_id = event_data->m_target_sp->GetGloballyUniqueID();
+    std::string default_name = llvm::formatv("Session {0}", target_id).str();
+    return ConstString(default_name).GetStringRef();
+  }
+
+  return llvm::StringRef();
+}
+
 ModuleList
 Target::TargetEventData::GetModuleListFromEvent(const Event *event_ptr) {
   ModuleList module_list;

lldb/test/API/tools/lldb-dap/attach/TestDAP_attach.py

@@ -72,3 +72,16 @@ def test_by_name_waitFor(self):
         self.spawn_thread.start()
         self.attach(program=program, waitFor=True)
         self.continue_and_verify_pid()
+
+    def test_attach_with_invalid_targetId(self):
+        """
+        Test that attaching with an invalid targetId fails with the expected
+        error message.
+        """
+        self.build_and_create_debug_adapter()
+
+        resp = self.attach(targetId=99999, expectFailure=True)
+        self.assertFalse(resp["success"])
+        self.assertIn(
+            "Unable to find existing debugger", resp["body"]["error"]["format"]
+        )

lldb/tools/lldb-dap/CMakeLists.txt

@@ -12,6 +12,7 @@ add_lldb_library(lldbDAP
   DAP.cpp
   DAPError.cpp
   DAPLog.cpp
+  DAPSessionManager.cpp
   EventHelper.cpp
   ExceptionBreakpoint.cpp
   FifoFiles.cpp