[lldb-dap] Add multi-session support with shared debugger instances (#163653)

## Summary:
This change introduces a `DAPSessionManager` to enable multiple DAP
sessions to share debugger instances when needed, for things like child
process debugging and some scripting hooks that create dynamically new
targets.

Changes include:
- Add `DAPSessionManager` singleton to track and coordinate all active DAP
sessions
- Support attaching to an existing target via its globally unique target
ID (targetId parameter)
- Share debugger instances across sessions when new targets are created
dynamically
- Refactor event thread management to allow sharing event threads
between sessions and move event thread and event thread handlers to `EventHelpers`
- Add `eBroadcastBitNewTargetCreated` event to notify when new targets are
created
- Extract session names from target creation events
- Defer debugger initialization from 'initialize' request to
'launch'/'attach' requests. The only time the debugger is used currently
in between its creation in `InitializeRequestHandler` and the `Launch`
or `Attach` requests is during the `TelemetryDispatcher` destruction
call at the end of the `DAP::HandleObject` call, so this is safe.

This enables scenarios when new targets are created dynamically so that
the debug adapter can automatically start a new debug session for the
spawned target while sharing the debugger instance.

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

Also added a few basic unit tests for DAPSessionManager
```
>> ninja DAPTests
>> ./tools/lldb/unittests/DAP/DAPTests
>>./bin/llvm-lit -v ../llvm-project/lldb/test/API/tools/lldb-dap/
```

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
@@ -64,6 +65,10 @@ class LLDB_API SBTarget {
 
   static lldb::SBTarget GetTargetFromEvent(const lldb::SBEvent &event);
 
+  /// For eBroadcastBitNewTargetCreated events, returns the newly created
+  /// target. For other event types, returns an invalid SBTarget.
+  static lldb::SBTarget GetCreatedTargetFromEvent(const lldb::SBEvent &event);
+
   static uint32_t GetNumModulesFromEvent(const lldb::SBEvent &event);
 
   static lldb::SBModule GetModuleAtIndexFromEvent(const uint32_t idx,
@@ -365,6 +370,16 @@ class LLDB_API SBTarget {
   ///     LLDB_INVALID_GLOBALLY_UNIQUE_TARGET_ID if the target is invalid.
   lldb::user_id_t GetGloballyUniqueID() const;
 
+  /// Get the target session name for this target.
+  ///
+  /// The target session name provides a meaningful name for IDEs or tools to
+  /// display to help the user identify the origin and purpose of the target.
+  ///
+  /// \return
+  ///     The target session name for this target, or nullptr if the target is
+  ///     invalid or has no target session name.
+  const char *GetTargetSessionName() const;
+
   SBError SetLabel(const char *label);
 
   /// Architecture opcode byte size width accessor

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 +557,13 @@ class Target : public std::enable_shared_from_this<Target>,
     TargetEventData(const lldb::TargetSP &target_sp,
                     const ModuleList &module_list);
 
+    // Constructor for eBroadcastBitNewTargetCreated events. For this event
+    // type:
+    // - target_sp is the parent target (the subject/broadcaster of the event)
+    // - created_target_sp is the newly created target
+    TargetEventData(const lldb::TargetSP &target_sp,
+                    const lldb::TargetSP &created_target_sp);
+
     ~TargetEventData() override;
 
     static llvm::StringRef GetFlavorString();
@@ -570,14 +578,23 @@ class Target : public std::enable_shared_from_this<Target>,
 
     static lldb::TargetSP GetTargetFromEvent(const Event *event_ptr);
 
+    // For eBroadcastBitNewTargetCreated events, returns the newly created
+    // target. For other event types, returns an invalid target.
+    static lldb::TargetSP GetCreatedTargetFromEvent(const Event *event_ptr);
+
     static ModuleList GetModuleListFromEvent(const Event *event_ptr);
 
     const lldb::TargetSP &GetTarget() const { return m_target_sp; }
 
+    const lldb::TargetSP &GetCreatedTarget() const {
+      return m_created_target_sp;
+    }
+
     const ModuleList &GetModuleList() const { return m_module_list; }
 
   private:
     lldb::TargetSP m_target_sp;
+    lldb::TargetSP m_created_target_sp;
     ModuleList m_module_list;
 
     TargetEventData(const TargetEventData &) = delete;
@@ -622,6 +639,30 @@ class Target : public std::enable_shared_from_this<Target>,
   ///     requirements.
   llvm::Error SetLabel(llvm::StringRef label);
 
+  /// Get the target session name for this target.
+  ///
+  /// Provides a meaningful name for IDEs or tools to display for dynamically
+  /// created targets. Defaults to "Session {ID}" based on the globally unique
+  /// ID.
+  ///
+  /// \return
+  ///     The target session name for this target.
+  llvm::StringRef GetTargetSessionName() { return m_target_session_name; }
+
+  /// Set the target session name for this target.
+  ///
+  /// This should typically be set along with the event
+  /// eBroadcastBitNewTargetCreated. Useful for scripts or triggers that
+  /// automatically create targets and want to provide meaningful names that
+  /// IDEs or other tools can display to help users identify the origin and
+  /// purpose of each target.
+  ///
+  /// \param[in] target_session_name
+  ///     The target session name to set for this target.
+  void SetTargetSessionName(llvm::StringRef target_session_name) {
+    m_target_session_name = target_session_name.str();
+  }
+
   /// Find a binary on the system and return its Module,
   /// or return an existing Module that is already in the Target.
   ///
@@ -1719,8 +1760,11 @@ class Target : public std::enable_shared_from_this<Target>,
   bool m_is_dummy_target;
   unsigned m_next_persistent_variable_index = 0;
   lldb::user_id_t m_target_unique_id =
-      LLDB_INVALID_GLOBALLY_UNIQUE_TARGET_ID; /// The globally unique ID
+      LLDB_INVALID_GLOBALLY_UNIQUE_TARGET_ID; ///< The globally unique ID
                                               /// assigned to this target
+  std::string m_target_session_name; ///< The target session name for this
+                                     /// target, used to name debugging
+                                     /// sessions in DAP.
   /// An optional \a lldb_private::Trace object containing processor trace
   /// information of this target.
   lldb::TraceSP m_trace_sp;

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

@@ -785,6 +785,8 @@ def request_attach(
         *,
         program: Optional[str] = None,
         pid: Optional[int] = None,
+        debuggerId: Optional[int] = None,
+        targetId: Optional[int] = None,
         waitFor=False,
         initCommands: Optional[list[str]] = None,
         preRunCommands: Optional[list[str]] = None,
@@ -804,6 +806,10 @@ def request_attach(
             args_dict["pid"] = pid
         if program is not None:
             args_dict["program"] = program
+        if debuggerId is not None:
+            args_dict["debuggerId"] = debuggerId
+        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

@@ -128,6 +128,12 @@ SBTarget SBTarget::GetTargetFromEvent(const SBEvent &event) {
   return Target::TargetEventData::GetTargetFromEvent(event.get());
 }
 
+SBTarget SBTarget::GetCreatedTargetFromEvent(const SBEvent &event) {
+  LLDB_INSTRUMENT_VA(event);
+
+  return Target::TargetEventData::GetCreatedTargetFromEvent(event.get());
+}
+
 uint32_t SBTarget::GetNumModulesFromEvent(const SBEvent &event) {
   LLDB_INSTRUMENT_VA(event);
 
@@ -1641,6 +1647,14 @@ lldb::user_id_t SBTarget::GetGloballyUniqueID() const {
   return LLDB_INVALID_GLOBALLY_UNIQUE_TARGET_ID;
 }
 
+const char *SBTarget::GetTargetSessionName() const {
+  LLDB_INSTRUMENT_VA(this);
+
+  if (TargetSP target_sp = GetSP())
+    return ConstString(target_sp->GetTargetSessionName()).AsCString();
+  return nullptr;
+}
+
 SBError SBTarget::SetLabel(const char *label) {
   LLDB_INSTRUMENT_VA(this, label);
 

lldb/source/Target/Target.cpp

@@ -185,13 +185,16 @@ Target::Target(Debugger &debugger, const ArchSpec &target_arch,
       m_internal_stop_hooks(), m_latest_stop_hook_id(0), m_valid(true),
       m_suppress_stop_hooks(false), m_is_dummy_target(is_dummy_target),
       m_target_unique_id(g_target_unique_id++),
+      m_target_session_name(
+          llvm::formatv("Session {0}", m_target_unique_id).str()),
       m_frame_recognizer_manager_up(
           std::make_unique<StackFrameRecognizerManager>()) {
   SetEventName(eBroadcastBitBreakpointChanged, "breakpoint-changed");
   SetEventName(eBroadcastBitModulesLoaded, "modules-loaded");
   SetEventName(eBroadcastBitModulesUnloaded, "modules-unloaded");
   SetEventName(eBroadcastBitWatchpointChanged, "watchpoint-changed");
   SetEventName(eBroadcastBitSymbolsLoaded, "symbols-loaded");
+  SetEventName(eBroadcastBitNewTargetCreated, "new-target-created");
 
   CheckInWithManager();
 
@@ -5198,6 +5201,11 @@ Target::TargetEventData::TargetEventData(const lldb::TargetSP &target_sp,
                                          const ModuleList &module_list)
     : EventData(), m_target_sp(target_sp), m_module_list(module_list) {}
 
+Target::TargetEventData::TargetEventData(
+    const lldb::TargetSP &target_sp, const lldb::TargetSP &created_target_sp)
+    : EventData(), m_target_sp(target_sp),
+      m_created_target_sp(created_target_sp), m_module_list() {}
+
 Target::TargetEventData::~TargetEventData() = default;
 
 llvm::StringRef Target::TargetEventData::GetFlavorString() {
@@ -5232,6 +5240,15 @@ TargetSP Target::TargetEventData::GetTargetFromEvent(const Event *event_ptr) {
   return target_sp;
 }
 
+TargetSP
+Target::TargetEventData::GetCreatedTargetFromEvent(const Event *event_ptr) {
+  TargetSP created_target_sp;
+  const TargetEventData *event_data = GetEventDataFromEvent(event_ptr);
+  if (event_data)
+    created_target_sp = event_data->m_created_target_sp;
+  return created_target_sp;
+}
+
 ModuleList
 Target::TargetEventData::GetModuleListFromEvent(const Event *event_ptr) {
   ModuleList module_list;

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

@@ -75,3 +75,38 @@ 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_missing_debuggerId_or_targetId(self):
+        """
+        Test that attaching with only one of debuggerId/targetId specified
+        fails with the expected error message.
+        """
+        self.build_and_create_debug_adapter()
+
+        # Test with only targetId specified (no debuggerId)
+        resp = self.attach(targetId=99999, expectFailure=True)
+        self.assertFalse(resp["success"])
+        self.assertIn(
+            "Both debuggerId and targetId must be specified together",
+            resp["body"]["error"]["format"],
+        )
+
+    def test_attach_with_invalid_debuggerId_and_targetId(self):
+        """
+        Test that attaching with both debuggerId and targetId specified but
+        invalid fails with an appropriate error message.
+        """
+        self.build_and_create_debug_adapter()
+
+        # Attach with both debuggerId=9999 and targetId=99999 (both invalid).
+        # Since debugger ID 9999 likely doesn't exist in the global registry,
+        # we expect a validation error.
+        resp = self.attach(debuggerId=9999, targetId=99999, expectFailure=True)
+        self.assertFalse(resp["success"])
+        error_msg = resp["body"]["error"]["format"]
+        # Either error is acceptable - both indicate the debugger reuse
+        # validation is working correctly
+        self.assertTrue(
+            "Unable to find existing debugger" in error_msg
+            or f"Expected debugger/target not found error, got: {error_msg}"
+        )

lldb/test/API/tools/lldb-dap/startDebugging/TestDAP_startDebugging.py

@@ -36,3 +36,54 @@ def test_startDebugging(self):
         request = self.dap_server.reverse_requests[0]
         self.assertEqual(request["arguments"]["configuration"]["pid"], 321)
         self.assertEqual(request["arguments"]["request"], "attach")
+
+    def test_startDebugging_debugger_reuse(self):
+        """
+        Tests that debugger and target IDs can be passed through startDebugging
+        for debugger reuse. This verifies the infrastructure for child DAP
+        sessions to reuse the parent's debugger and attach to an existing target.
+        """
+        program = self.getBuildArtifact("a.out")
+        source = "main.c"
+        self.build_and_launch(program)
+
+        breakpoint_line = line_number(source, "// breakpoint")
+        self.set_source_breakpoints(source, [breakpoint_line])
+        self.continue_to_next_stop()
+
+        # Use mock IDs to test the infrastructure
+        # In a real scenario, these would come from the parent session
+        test_debugger_id = 1
+        test_target_id = 100
+
+        # Send a startDebugging request with debuggerId and targetId
+        # This simulates creating a child DAP session that reuses the debugger
+        self.dap_server.request_evaluate(
+            f'`lldb-dap start-debugging attach \'{{"debuggerId":{test_debugger_id},"targetId":{test_target_id}}}\'',
+            context="repl",
+        )
+
+        self.continue_to_exit()
+
+        # Verify the reverse request was sent with the correct IDs
+        self.assertEqual(
+            len(self.dap_server.reverse_requests),
+            1,
+            "Should have received one startDebugging reverse request",
+        )
+
+        request = self.dap_server.reverse_requests[0]
+        self.assertEqual(request["command"], "startDebugging")
+        self.assertEqual(request["arguments"]["request"], "attach")
+
+        config = request["arguments"]["configuration"]
+        self.assertEqual(
+            config["debuggerId"],
+            test_debugger_id,
+            "Reverse request should include debugger ID",
+        )
+        self.assertEqual(
+            config["targetId"],
+            test_target_id,
+            "Reverse request should include target ID",
+        )

lldb/tools/lldb-dap/CMakeLists.txt

@@ -10,6 +10,7 @@ add_lldb_library(lldbDAP
   DAP.cpp
   DAPError.cpp
   DAPLog.cpp
+  DAPSessionManager.cpp
   EventHelper.cpp
   ExceptionBreakpoint.cpp
   FifoFiles.cpp