inspector: allow concurrent inspector sessions

This change enables concurrent inspector sessions, through WebSocket
interface as well as JS interface, in any combination.

PR-URL: https://github.com/nodejs/node/pull/20137
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

15 files changed, 290 insertions, 254 deletions

diff --git a/src/inspector_agent.cc b/src/inspector_agent.cc
index fffaecdef3..1530b20645 100644
--- a/src/inspector_agent.cc
+++ b/src/inspector_agent.cc
@@ -189,9 +189,9 @@ const int CONTEXT_GROUP_ID = 1;
 
 class ChannelImpl final : public v8_inspector::V8Inspector::Channel {
  public:
-  explicit ChannelImpl(V8Inspector* inspector,
-                       InspectorSessionDelegate* delegate)
-                       : delegate_(delegate) {
+  explicit ChannelImpl(const std::unique_ptr<V8Inspector>& inspector,
+                       std::unique_ptr<InspectorSessionDelegate> delegate)
+                       : delegate_(std::move(delegate)) {
     session_ = inspector->connect(1, this, StringView());
   }
 
@@ -201,19 +201,11 @@ class ChannelImpl final : public v8_inspector::V8Inspector::Channel {
     session_->dispatchProtocolMessage(message);
   }
 
-  bool waitForFrontendMessage() {
-    return delegate_->WaitForFrontendMessageWhilePaused();
-  }
-
   void schedulePauseOnNextStatement(const std::string& reason) {
     std::unique_ptr<StringBuffer> buffer = Utf8ToStringView(reason);
     session_->schedulePauseOnNextStatement(buffer->string(), buffer->string());
   }
 
-  InspectorSessionDelegate* delegate() {
-    return delegate_;
-  }
-
  private:
   void sendResponse(
       int callId,
@@ -232,7 +224,7 @@ class ChannelImpl final : public v8_inspector::V8Inspector::Channel {
     delegate_->SendMessageToFrontend(message);
   }
 
-  InspectorSessionDelegate* const delegate_;
+  std::unique_ptr<InspectorSessionDelegate> delegate_;
   std::unique_ptr<v8_inspector::V8InspectorSession> session_;
 };
 
@@ -300,8 +292,7 @@ class InspectorTimerHandle {
 class NodeInspectorClient : public V8InspectorClient {
  public:
   NodeInspectorClient(node::Environment* env, node::NodePlatform* platform)
-      : env_(env), platform_(platform), terminated_(false),
-        running_nested_loop_(false) {
+      : env_(env), platform_(platform) {
     client_ = V8Inspector::create(env->isolate(), this);
     // TODO(bnoordhuis) Make name configurable from src/node.cc.
     ContextInfo info(GetHumanReadableProcessName());
@@ -310,18 +301,28 @@ class NodeInspectorClient : public V8InspectorClient {
   }
 
   void runMessageLoopOnPause(int context_group_id) override {
-    CHECK_NE(channel_, nullptr);
+    runMessageLoop(false);
+  }
+
+  void runMessageLoop(bool ignore_terminated) {
     if (running_nested_loop_)
       return;
     terminated_ = false;
     running_nested_loop_ = true;
-    while (!terminated_ && channel_->waitForFrontendMessage()) {
+    while ((ignore_terminated || !terminated_) && waitForFrontendEvent()) {
       while (platform_->FlushForegroundTasks(env_->isolate())) {}
     }
     terminated_ = false;
     running_nested_loop_ = false;
   }
 
+  bool waitForFrontendEvent() {
+    InspectorIo* io = env_->inspector_agent()->io();
+    if (io == nullptr)
+      return false;
+    return io->WaitForFrontendEvent();
+  }
+
   double currentTimeMS() override {
     return uv_hrtime() * 1.0 / NANOS_PER_MSEC;
   }
@@ -363,20 +364,22 @@ class NodeInspectorClient : public V8InspectorClient {
     terminated_ = true;
   }
 
-  void connectFrontend(InspectorSessionDelegate* delegate) {
-    CHECK_EQ(channel_, nullptr);
-    channel_ = std::unique_ptr<ChannelImpl>(
-        new ChannelImpl(client_.get(), delegate));
+  int connectFrontend(std::unique_ptr<InspectorSessionDelegate> delegate) {
+    events_dispatched_ = true;
+    int session_id = next_session_id_++;
+    channels_[session_id] =
+        std::make_unique<ChannelImpl>(client_, std::move(delegate));
+    return session_id;
   }
 
-  void disconnectFrontend() {
-    quitMessageLoopOnPause();
-    channel_.reset();
+  void disconnectFrontend(int session_id) {
+    events_dispatched_ = true;
+    channels_.erase(session_id);
   }
 
-  void dispatchMessageFromFrontend(const StringView& message) {
-    CHECK_NE(channel_, nullptr);
-    channel_->dispatchProtocolMessage(message);
+  void dispatchMessageFromFrontend(int session_id, const StringView& message) {
+    events_dispatched_ = true;
+    channels_[session_id]->dispatchProtocolMessage(message);
   }
 
   Local<Context> ensureDefaultContextInGroup(int contextGroupId) override {
@@ -426,10 +429,6 @@ class NodeInspectorClient : public V8InspectorClient {
         script_id);
   }
 
-  ChannelImpl* channel() {
-    return channel_.get();
-  }
-
   void startRepeatingTimer(double interval_s,
                            TimerCallback callback,
                            void* data) override {
@@ -464,20 +463,31 @@ class NodeInspectorClient : public V8InspectorClient {
     client_->allAsyncTasksCanceled();
   }
 
+  void schedulePauseOnNextStatement(const std::string& reason) {
+    for (const auto& id_channel : channels_) {
+      id_channel.second->schedulePauseOnNextStatement(reason);
+    }
+  }
+
+  bool hasConnectedSessions() {
+    return !channels_.empty();
+  }
+
  private:
   node::Environment* env_;
   node::NodePlatform* platform_;
-  bool terminated_;
-  bool running_nested_loop_;
+  bool terminated_ = false;
+  bool running_nested_loop_ = false;
   std::unique_ptr<V8Inspector> client_;
-  std::unique_ptr<ChannelImpl> channel_;
+  std::unordered_map<int, std::unique_ptr<ChannelImpl>> channels_;
   std::unordered_map<void*, InspectorTimerHandle> timers_;
+  int next_session_id_ = 1;
+  bool events_dispatched_ = false;
 };
 
 Agent::Agent(Environment* env) : parent_env_(env),
                                  client_(nullptr),
                                  platform_(nullptr),
-                                 enabled_(false),
                                  pending_enable_async_hook_(false),
                                  pending_disable_async_hook_(false) {}
 
@@ -491,7 +501,7 @@ bool Agent::Start(node::NodePlatform* platform, const char* path,
   path_ = path == nullptr ? "" : path;
   debug_options_ = options;
   client_ =
-      std::unique_ptr<NodeInspectorClient>(
+      std::shared_ptr<NodeInspectorClient>(
           new NodeInspectorClient(parent_env_, platform));
   platform_ = platform;
   CHECK_EQ(0, uv_async_init(uv_default_loop(),
@@ -515,7 +525,6 @@ bool Agent::StartIoThread(bool wait_for_connect) {
 
   CHECK_NE(client_, nullptr);
 
-  enabled_ = true;
   io_ = std::unique_ptr<InspectorIo>(
       new InspectorIo(parent_env_, platform_, path_, debug_options_,
                       wait_for_connect));
@@ -554,13 +563,13 @@ void Agent::Stop() {
   if (io_ != nullptr) {
     io_->Stop();
     io_.reset();
-    enabled_ = false;
   }
 }
 
-void Agent::Connect(InspectorSessionDelegate* delegate) {
-  enabled_ = true;
-  client_->connectFrontend(delegate);
+std::unique_ptr<InspectorSession> Agent::Connect(
+    std::unique_ptr<InspectorSessionDelegate> delegate) {
+  int session_id = client_->connectFrontend(std::move(delegate));
+  return std::make_unique<InspectorSession>(session_id, client_);
 }
 
 void Agent::WaitForDisconnect() {
@@ -568,6 +577,11 @@ void Agent::WaitForDisconnect() {
   client_->contextDestroyed(parent_env_->context());
   if (io_ != nullptr) {
     io_->WaitForDisconnect();
+    // There is a bug in V8 Inspector (https://crbug.com/834056) that
+    // calls V8InspectorClient::quitMessageLoopOnPause when a session
+    // disconnects. We are using this flag to ignore those calls so the message
+    // loop is spinning as long as there's a reason to expect inspector messages
+    client_->runMessageLoop(true);
   }
 }
 
@@ -578,33 +592,8 @@ void Agent::FatalException(Local<Value> error, Local<v8::Message> message) {
   WaitForDisconnect();
 }
 
-void Agent::Dispatch(const StringView& message) {
-  CHECK_NE(client_, nullptr);
-  client_->dispatchMessageFromFrontend(message);
-}
-
-void Agent::Disconnect() {
-  CHECK_NE(client_, nullptr);
-  client_->disconnectFrontend();
-}
-
-void Agent::RunMessageLoop() {
-  CHECK_NE(client_, nullptr);
-  client_->runMessageLoopOnPause(CONTEXT_GROUP_ID);
-}
-
-InspectorSessionDelegate* Agent::delegate() {
-  CHECK_NE(client_, nullptr);
-  ChannelImpl* channel = client_->channel();
-  if (channel == nullptr)
-    return nullptr;
-  return channel->delegate();
-}
-
 void Agent::PauseOnNextJavascriptStatement(const std::string& reason) {
-  ChannelImpl* channel = client_->channel();
-  if (channel != nullptr)
-    channel->schedulePauseOnNextStatement(reason);
+  client_->schedulePauseOnNextStatement(reason);
 }
 
 void Agent::RegisterAsyncHook(Isolate* isolate,
@@ -699,5 +688,20 @@ bool Agent::IsWaitingForConnect() {
   return debug_options_.wait_for_connect();
 }
 
+bool Agent::HasConnectedSessions() {
+  return client_->hasConnectedSessions();
+}
+
+InspectorSession::InspectorSession(int session_id,
+                                   std::shared_ptr<NodeInspectorClient> client)
+                                   : session_id_(session_id), client_(client) {}
+
+InspectorSession::~InspectorSession() {
+  client_->disconnectFrontend(session_id_);
+}
+
+void InspectorSession::Dispatch(const StringView& message) {
+  client_->dispatchMessageFromFrontend(session_id_, message);
+}
 }  // namespace inspector
 }  // namespace node
diff --git a/src/inspector_agent.h b/src/inspector_agent.h
index 56fb407930..64e4202ee8 100644
--- a/src/inspector_agent.h
+++ b/src/inspector_agent.h
@@ -23,18 +23,26 @@ class Environment;
 struct ContextInfo;
 
 namespace inspector {
+class InspectorIo;
+class NodeInspectorClient;
+
+class InspectorSession {
+ public:
+  InspectorSession(int session_id, std::shared_ptr<NodeInspectorClient> client);
+  ~InspectorSession();
+  void Dispatch(const v8_inspector::StringView& message);
+ private:
+  int session_id_;
+  std::shared_ptr<NodeInspectorClient> client_;
+};
 
 class InspectorSessionDelegate {
  public:
   virtual ~InspectorSessionDelegate() = default;
-  virtual bool WaitForFrontendMessageWhilePaused() = 0;
   virtual void SendMessageToFrontend(const v8_inspector::StringView& message)
                                      = 0;
 };
 
-class InspectorIo;
-class NodeInspectorClient;
-
 class Agent {
  public:
   explicit Agent(node::Environment* env);
@@ -66,19 +74,19 @@ class Agent {
   void RegisterAsyncHook(v8::Isolate* isolate,
     v8::Local<v8::Function> enable_function,
     v8::Local<v8::Function> disable_function);
+  void EnableAsyncHook();
+  void DisableAsyncHook();
 
-  // These methods are called by the WS protocol and JS binding to create
-  // inspector sessions.  The inspector responds by using the delegate to send
-  // messages back.
-  void Connect(InspectorSessionDelegate* delegate);
-  void Disconnect();
-  void Dispatch(const v8_inspector::StringView& message);
-  InspectorSessionDelegate* delegate();
+  // Called by the WS protocol and JS binding to create inspector sessions.
+  // The inspector responds by using the delegate to send messages back.
+  std::unique_ptr<InspectorSession> Connect(
+      std::unique_ptr<InspectorSessionDelegate> delegate);
 
-  void RunMessageLoop();
-  bool enabled() { return enabled_; }
   void PauseOnNextJavascriptStatement(const std::string& reason);
 
+  // Returns true as long as there is at least one connected session.
+  bool HasConnectedSessions();
+
   InspectorIo* io() {
     return io_.get();
   }
@@ -92,18 +100,14 @@ class Agent {
   DebugOptions& options() { return debug_options_; }
   void ContextCreated(v8::Local<v8::Context> context, const ContextInfo& info);
 
-  void EnableAsyncHook();
-  void DisableAsyncHook();
-
  private:
   void ToggleAsyncHook(v8::Isolate* isolate,
                        const Persistent<v8::Function>& fn);
 
   node::Environment* parent_env_;
-  std::unique_ptr<NodeInspectorClient> client_;
+  std::shared_ptr<NodeInspectorClient> client_;
   std::unique_ptr<InspectorIo> io_;
   v8::Platform* platform_;
-  bool enabled_;
   std::string path_;
   DebugOptions debug_options_;
 
diff --git a/src/inspector_io.cc b/src/inspector_io.cc
index 01ddc296b0..5e0d29d3ca 100644
--- a/src/inspector_io.cc
+++ b/src/inspector_io.cc
@@ -122,11 +122,11 @@ std::unique_ptr<StringBuffer> Utf8ToStringView(const std::string& message) {
 
 class IoSessionDelegate : public InspectorSessionDelegate {
  public:
-  explicit IoSessionDelegate(InspectorIo* io) : io_(io) { }
-  bool WaitForFrontendMessageWhilePaused() override;
+  explicit IoSessionDelegate(InspectorIo* io, int id) : io_(io), id_(id) { }
   void SendMessageToFrontend(const v8_inspector::StringView& message) override;
  private:
   InspectorIo* io_;
+  int id_;
 };
 
 // Passed to InspectorSocketServer to handle WS inspector protocol events,
@@ -190,8 +190,7 @@ InspectorIo::InspectorIo(Environment* env, v8::Platform* platform,
                          : options_(options), thread_(), delegate_(nullptr),
                            state_(State::kNew), parent_env_(env),
                            thread_req_(), platform_(platform),
-                           dispatching_messages_(false), session_id_(0),
-                           script_name_(path),
+                           dispatching_messages_(false), script_name_(path),
                            wait_for_connect_(wait_for_connect), port_(-1) {
   main_thread_req_ = new AsyncAndAgent({uv_async_t(), env->inspector_agent()});
   CHECK_EQ(0, uv_async_init(env->event_loop(), &main_thread_req_->first,
@@ -222,7 +221,7 @@ bool InspectorIo::Start() {
 }
 
 void InspectorIo::Stop() {
-  CHECK(state_ == State::kAccepting || state_ == State::kConnected);
+  CHECK(state_ == State::kAccepting || !sessions_.empty());
   Write(TransportAction::kKill, 0, StringView());
   int err = uv_thread_join(&thread_);
   CHECK_EQ(err, 0);
@@ -237,12 +236,11 @@ bool InspectorIo::IsStarted() {
 void InspectorIo::WaitForDisconnect() {
   if (state_ == State::kAccepting)
     state_ = State::kDone;
-  if (state_ == State::kConnected) {
+  if (!sessions_.empty()) {
     state_ = State::kShutDown;
     Write(TransportAction::kStop, 0, StringView());
     fprintf(stderr, "Waiting for the debugger to disconnect...\n");
     fflush(stderr);
-    parent_env_->inspector_agent()->RunMessageLoop();
   }
 }
 
@@ -348,45 +346,23 @@ void InspectorIo::PostIncomingMessage(InspectorAction action, int session_id,
     isolate->RequestInterrupt(InterruptCallback, agent);
     CHECK_EQ(0, uv_async_send(&main_thread_req_->first));
   }
-  NotifyMessageReceived();
+  Mutex::ScopedLock scoped_lock(state_lock_);
+  incoming_message_cond_.Broadcast(scoped_lock);
 }
 
 std::vector<std::string> InspectorIo::GetTargetIds() const {
   return delegate_ ? delegate_->GetTargetIds() : std::vector<std::string>();
 }
 
-void InspectorIo::WaitForFrontendMessageWhilePaused() {
-  dispatching_messages_ = false;
-  Mutex::ScopedLock scoped_lock(state_lock_);
-  if (incoming_message_queue_.empty())
-    incoming_message_cond_.Wait(scoped_lock);
-}
-
-void InspectorIo::NotifyMessageReceived() {
-  Mutex::ScopedLock scoped_lock(state_lock_);
-  incoming_message_cond_.Broadcast(scoped_lock);
-}
-
 TransportAction InspectorIo::Attach(int session_id) {
   Agent* agent = parent_env_->inspector_agent();
-  if (agent->delegate() != nullptr)
-    return TransportAction::kDeclineSession;
-
-  CHECK_EQ(session_delegate_, nullptr);
-  session_id_ = session_id;
-  state_ = State::kConnected;
   fprintf(stderr, "Debugger attached.\n");
-  session_delegate_ = std::unique_ptr<InspectorSessionDelegate>(
-      new IoSessionDelegate(this));
-  agent->Connect(session_delegate_.get());
+  sessions_[session_id] =
+      agent->Connect(std::make_unique<IoSessionDelegate>(this, session_id));
   return TransportAction::kAcceptSession;
 }
 
 void InspectorIo::DispatchMessages() {
-  // This function can be reentered if there was an incoming message while
-  // V8 was processing another inspector request (e.g. if the user is
-  // evaluating a long-running JS code snippet). This can happen only at
-  // specific points (e.g. the lines that call inspector_ methods)
   if (dispatching_messages_)
     return;
   dispatching_messages_ = true;
@@ -409,17 +385,20 @@ void InspectorIo::DispatchMessages() {
         Attach(id);
         break;
       case InspectorAction::kEndSession:
-        CHECK_NE(session_delegate_, nullptr);
+        sessions_.erase(id);
+        if (!sessions_.empty())
+          continue;
         if (state_ == State::kShutDown) {
           state_ = State::kDone;
         } else {
           state_ = State::kAccepting;
         }
-        parent_env_->inspector_agent()->Disconnect();
-        session_delegate_.reset();
         break;
       case InspectorAction::kSendMessage:
-        parent_env_->inspector_agent()->Dispatch(message);
+        auto session = sessions_.find(id);
+        if (session != sessions_.end() && session->second) {
+          session->second->Dispatch(message);
+        }
         break;
       }
     }
@@ -445,6 +424,20 @@ void InspectorIo::Write(TransportAction action, int session_id,
   CHECK_EQ(0, err);
 }
 
+bool InspectorIo::WaitForFrontendEvent() {
+  // We allow DispatchMessages reentry as we enter the pause. This is important
+  // to support debugging the code invoked by an inspector call, such
+  // as Runtime.evaluate
+  dispatching_messages_ = false;
+  Mutex::ScopedLock scoped_lock(state_lock_);
+  if (sessions_.empty())
+    return false;
+  if (dispatching_message_queue_.empty() && incoming_message_queue_.empty()) {
+    incoming_message_cond_.Wait(scoped_lock);
+  }
+  return true;
+}
+
 InspectorIoDelegate::InspectorIoDelegate(InspectorIo* io,
                                          const std::string& script_path,
                                          const std::string& script_name,
@@ -502,14 +495,9 @@ std::string InspectorIoDelegate::GetTargetUrl(const std::string& id) {
   return "file://" + script_path_;
 }
 
-bool IoSessionDelegate::WaitForFrontendMessageWhilePaused() {
-  io_->WaitForFrontendMessageWhilePaused();
-  return true;
-}
-
 void IoSessionDelegate::SendMessageToFrontend(
     const v8_inspector::StringView& message) {
-  io_->Write(TransportAction::kSendMessage, io_->session_id_, message);
+  io_->Write(TransportAction::kSendMessage, id_, message);
 }
 
 }  // namespace inspector
diff --git a/src/inspector_io.h b/src/inspector_io.h
index 79ccc6095f..276c78056c 100644
--- a/src/inspector_io.h
+++ b/src/inspector_io.h
@@ -7,6 +7,7 @@
 #include "uv.h"
 
 #include <deque>
+#include <map>
 #include <memory>
 #include <stddef.h>
 
@@ -76,6 +77,7 @@ class InspectorIo {
   void ServerDone() {
     uv_close(reinterpret_cast<uv_handle_t*>(&thread_req_), nullptr);
   }
+  bool WaitForFrontendEvent();
 
   int port() const { return port_; }
   std::string host() const { return options_.host_name(); }
@@ -89,7 +91,6 @@ class InspectorIo {
   enum class State {
     kNew,
     kAccepting,
-    kConnected,
     kDone,
     kError,
     kShutDown
@@ -107,7 +108,6 @@ class InspectorIo {
   // messages from outgoing_message_queue to the InspectorSockerServer
   template <typename Transport> static void IoThreadAsyncCb(uv_async_t* async);
 
-  void SetConnected(bool connected);
   void DispatchMessages();
   // Write action to outgoing_message_queue, and wake the thread
   void Write(TransportAction action, int session_id,
@@ -122,10 +122,6 @@ class InspectorIo {
   template <typename ActionType>
   void SwapBehindLock(MessageQueue<ActionType>* vector1,
                       MessageQueue<ActionType>* vector2);
-  // Wait on incoming_message_cond_
-  void WaitForFrontendMessageWhilePaused();
-  // Broadcast incoming_message_cond_
-  void NotifyMessageReceived();
   // Attach session to an inspector. Either kAcceptSession or kDeclineSession
   TransportAction Attach(int session_id);
 
@@ -147,7 +143,6 @@ class InspectorIo {
   // Note that this will live while the async is being closed - likely, past
   // the parent object lifespan
   std::pair<uv_async_t, Agent*>* main_thread_req_;
-  std::unique_ptr<InspectorSessionDelegate> session_delegate_;
   v8::Platform* platform_;
 
   // Message queues
@@ -155,15 +150,17 @@ class InspectorIo {
   Mutex state_lock_;  // Locked before mutating either queue.
   MessageQueue<InspectorAction> incoming_message_queue_;
   MessageQueue<TransportAction> outgoing_message_queue_;
+  // This queue is to maintain the order of the messages for the cases
+  // when we reenter the DispatchMessages function.
   MessageQueue<InspectorAction> dispatching_message_queue_;
 
   bool dispatching_messages_;
-  int session_id_;
 
   std::string script_name_;
   std::string script_path_;
   const bool wait_for_connect_;
   int port_;
+  std::unordered_map<int, std::unique_ptr<InspectorSession>> sessions_;
 
   friend class DispatchMessagesTask;
   friend class IoSessionDelegate;
diff --git a/src/inspector_js_api.cc b/src/inspector_js_api.cc
index 9a380bb2df..3d05b2f6fe 100644
--- a/src/inspector_js_api.cc
+++ b/src/inspector_js_api.cc
@@ -42,10 +42,6 @@ class JSBindingsConnection : public AsyncWrap {
                                 connection_(connection) {
     }
 
-    bool WaitForFrontendMessageWhilePaused() override {
-      return false;
-    }
-
     void SendMessageToFrontend(const v8_inspector::StringView& message)
         override {
       Isolate* isolate = env_->isolate();
@@ -58,12 +54,6 @@ class JSBindingsConnection : public AsyncWrap {
       connection_->OnMessage(argument);
     }
 
-    void Disconnect() {
-      Agent* agent = env_->inspector_agent();
-      if (agent->delegate() == this)
-        agent->Disconnect();
-    }
-
    private:
     Environment* env_;
     JSBindingsConnection* connection_;
@@ -73,31 +63,17 @@ class JSBindingsConnection : public AsyncWrap {
                        Local<Object> wrap,
                        Local<Function> callback)
                        : AsyncWrap(env, wrap, PROVIDER_INSPECTORJSBINDING),
-                         delegate_(env, this),
                          callback_(env->isolate(), callback) {
     Wrap(wrap, this);
-
     Agent* inspector = env->inspector_agent();
-    if (inspector->delegate() != nullptr) {
-      // This signals JS code that it has to throw an error.
-      Local<String> session_attached =
-          FIXED_ONE_BYTE_STRING(env->isolate(), "sessionAttached");
-      wrap->Set(env->context(), session_attached,
-                Boolean::New(env->isolate(), true)).ToChecked();
-      return;
-    }
-    inspector->Connect(&delegate_);
+    session_ = inspector->Connect(
+        std::make_unique<JSBindingsSessionDelegate>(env, this));
   }
 
   void OnMessage(Local<Value> value) {
     MakeCallback(callback_.Get(env()->isolate()), 1, &value);
   }
 
-  void CheckIsCurrent() {
-    Agent* inspector = env()->inspector_agent();
-    CHECK_EQ(&delegate_, inspector->delegate());
-  }
-
   static void New(const FunctionCallbackInfo<Value>& info) {
     Environment* env = Environment::GetCurrent(info);
     CHECK(info[0]->IsFunction());
@@ -106,7 +82,7 @@ class JSBindingsConnection : public AsyncWrap {
   }
 
   void Disconnect() {
-    delegate_.Disconnect();
+    session_.reset();
     if (!persistent().IsEmpty()) {
       ClearWrap(object());
     }
@@ -125,18 +101,23 @@ class JSBindingsConnection : public AsyncWrap {
     ASSIGN_OR_RETURN_UNWRAP(&session, info.Holder());
     CHECK(info[0]->IsString());
 
-    session->CheckIsCurrent();
-    Agent* inspector = env->inspector_agent();
-    inspector->Dispatch(ToProtocolString(env->isolate(), info[0])->string());
+    if (session->session_) {
+      session->session_->Dispatch(
+          ToProtocolString(env->isolate(), info[0])->string());
+    }
   }
 
   size_t self_size() const override { return sizeof(*this); }
 
  private:
-  JSBindingsSessionDelegate delegate_;
+  std::unique_ptr<InspectorSession> session_;
   Persistent<Function> callback_;
 };
 
+static bool InspectorEnabled(Environment* env) {
+  Agent* agent = env->inspector_agent();
+  return agent->io() != nullptr || agent->HasConnectedSessions();
+}
 
 void AddCommandLineAPI(const FunctionCallbackInfo<Value>& info) {
   auto env = Environment::GetCurrent(info);
@@ -178,7 +159,7 @@ void InspectorConsoleCall(const FunctionCallbackInfo<Value>& info) {
     call_args.push_back(info[i]);
   }
   Environment* env = Environment::GetCurrent(isolate);
-  if (env->inspector_agent()->enabled()) {
+  if (InspectorEnabled(env)) {
     Local<Value> inspector_method = info[0];
     CHECK(inspector_method->IsFunction());
     Local<Value> config_value = info[2];
@@ -256,7 +237,7 @@ static void RegisterAsyncHookWrapper(const FunctionCallbackInfo<Value>& args) {
 
 void IsEnabled(const FunctionCallbackInfo<Value>& args) {
   Environment* env = Environment::GetCurrent(args);
-  args.GetReturnValue().Set(env->inspector_agent()->enabled());
+  args.GetReturnValue().Set(InspectorEnabled(env));
 }
 
 void Open(const FunctionCallbackInfo<Value>& args) {
diff --git a/src/inspector_socket_server.cc b/src/inspector_socket_server.cc
index e69c8d768b..e890f66a38 100644
--- a/src/inspector_socket_server.cc
+++ b/src/inspector_socket_server.cc
@@ -373,27 +373,17 @@ void InspectorSocketServer::SendListResponse(InspectorSocket* socket,
     target_map["url"] = delegate_->GetTargetUrl(id);
     Escape(&target_map["url"]);
 
-    bool connected = false;
-    for (const auto& session : connected_sessions_) {
-      if (session.second.first == id) {
-        connected = true;
-        break;
-      }
-    }
-    if (!connected) {
-      std::string detected_host = host;
-      if (detected_host.empty()) {
-        detected_host = FormatHostPort(socket->GetHost(),
-                                       session->server_port());
-      }
-      std::ostringstream frontend_url;
-      frontend_url << "chrome-devtools://devtools/bundled";
-      frontend_url << "/inspector.html?experiments=true&v8only=true&ws=";
-      frontend_url << FormatAddress(detected_host, id, false);
-      target_map["devtoolsFrontendUrl"] += frontend_url.str();
-      target_map["webSocketDebuggerUrl"] =
-          FormatAddress(detected_host, id, true);
+    std::string detected_host = host;
+    if (detected_host.empty()) {
+      detected_host = FormatHostPort(socket->GetHost(),
+                                     session->server_port());
     }
+    std::ostringstream frontend_url;
+    frontend_url << "chrome-devtools://devtools/bundled";
+    frontend_url << "/inspector.html?experiments=true&v8only=true&ws=";
+    frontend_url << FormatAddress(detected_host, id, false);
+    target_map["devtoolsFrontendUrl"] += frontend_url.str();
+    target_map["webSocketDebuggerUrl"] = FormatAddress(detected_host, id, true);
   }
   SendHttpResponse(socket, MapsToString(response));
 }
diff --git a/src/node.cc b/src/node.cc
index 07dc4f13fd..d8c0bded38 100644
--- a/src/node.cc
+++ b/src/node.cc
@@ -1959,7 +1959,7 @@ static void InitGroups(const FunctionCallbackInfo<Value>& args) {
 
 static void WaitForInspectorDisconnect(Environment* env) {
 #if HAVE_INSPECTOR
-  if (env->inspector_agent()->delegate() != nullptr) {
+  if (env->inspector_agent()->HasConnectedSessions()) {
     // Restore signal dispositions, the app is done and is no longer
     // capable of handling signals.
 #if defined(__POSIX__) && !defined(NODE_SHARED_MODE)
diff --git a/test/common/inspector-helper.js b/test/common/inspector-helper.js
index 1bc16e3714..e590349f9c 100644
--- a/test/common/inspector-helper.js
+++ b/test/common/inspector-helper.js
@@ -154,8 +154,9 @@ class InspectorSession {
     return this._terminationPromise;
   }
 
-  disconnect() {
+  async disconnect() {
     this._socket.destroy();
+    return this.waitForServerDisconnect();
   }
 
   _onMessage(message) {
@@ -444,6 +445,7 @@ class NodeInstance {
 
   kill() {
     this._process.kill();
+    return this.expectShutdown();
   }
 
   scriptPath() {
diff --git a/test/parallel/test-inspector-esm.js b/test/parallel/test-inspector-esm.js
index 3171da58cf..62f0feefad 100644
--- a/test/parallel/test-inspector-esm.js
+++ b/test/parallel/test-inspector-esm.js
@@ -9,12 +9,6 @@ const { resolve: UrlResolve } = require('url');
 const fixtures = require('../common/fixtures');
 const { NodeInstance } = require('../common/inspector-helper.js');
 
-function assertNoUrlsWhileConnected(response) {
-  assert.strictEqual(response.length, 1);
-  assert.ok(!response[0].hasOwnProperty('devtoolsFrontendUrl'));
-  assert.ok(!response[0].hasOwnProperty('webSocketDebuggerUrl'));
-}
-
 function assertScopeValues({ result }, expected) {
   const unmatched = new Set(Object.keys(expected));
   for (const actual of result) {
@@ -110,7 +104,6 @@ async function runTest() {
                                  '', fixtures.path('es-modules/loop.mjs'));
 
   const session = await child.connectInspectorSession();
-  assertNoUrlsWhileConnected(await child.httpGet(null, '/json/list'));
   await testBreakpointOnStart(session);
   await testBreakpoint(session);
   await session.runToCompletion();
diff --git a/test/parallel/test-inspector-multisession-js.js b/test/parallel/test-inspector-multisession-js.js
new file mode 100644
index 0000000000..58533f4cd6
--- /dev/null
+++ b/test/parallel/test-inspector-multisession-js.js
@@ -0,0 +1,59 @@
+'use strict';
+const common = require('../common');
+
+common.skipIfInspectorDisabled();
+
+const assert = require('assert');
+const { Session } = require('inspector');
+const path = require('path');
+
+function debugged() {
+  return 42;
+}
+
+async function test() {
+  const session1 = new Session();
+  const session2 = new Session();
+
+  session1.connect();
+  session2.connect();
+
+  let session1Paused = false;
+  let session2Paused = false;
+
+  session1.on('Debugger.paused', () => session1Paused = true);
+  session2.on('Debugger.paused', () => session2Paused = true);
+
+  console.log('Connected');
+
+  session1.post('Debugger.enable');
+  session2.post('Debugger.enable');
+  console.log('Debugger was enabled');
+
+  await new Promise((resolve, reject) => {
+    session1.post('Debugger.setBreakpointByUrl', {
+      'lineNumber': 9,
+      'url': path.resolve(__dirname, __filename),
+      'columnNumber': 0,
+      'condition': ''
+    }, (error, result) => {
+      return error ? reject(error) : resolve(result);
+    });
+  });
+  console.log('Breakpoint was set');
+
+  debugged();
+
+  // Both sessions will receive the paused event
+  assert(session1Paused);
+  assert(session2Paused);
+  console.log('Breakpoint was hit');
+
+  session1.disconnect();
+  session2.disconnect();
+  console.log('Sessions were disconnected');
+}
+
+common.crashOnUnhandledRejection();
+
+test();
diff --git a/test/parallel/test-inspector-multisession-ws.js b/test/parallel/test-inspector-multisession-ws.js
new file mode 100644
index 0000000000..d17adab2f4
--- /dev/null
+++ b/test/parallel/test-inspector-multisession-ws.js
@@ -0,0 +1,75 @@
+// Flags: --expose-internals
+'use strict';
+const common = require('../common');
+
+common.skipIfInspectorDisabled();
+
+const { NodeInstance } = require('../common/inspector-helper.js');
+
+// Sets up JS bindings session and runs till the "paused" event
+const script = `
+const { Session } = require('inspector');
+const session = new Session();
+let done = false;
+const interval = setInterval(() => {
+  if (done)
+    clearInterval(interval);
+}, 150);
+session.on('Debugger.paused', () => {
+  done = true;
+});
+session.connect();
+session.post('Debugger.enable');
+console.log('Ready');
+`;
+
+async function setupSession(node) {
+  const session = await node.connectInspectorSession();
+  await session.send([
+    { 'method': 'Runtime.enable' },
+    { 'method': 'Debugger.enable' },
+    { 'method': 'Debugger.setPauseOnExceptions',
+      'params': { 'state': 'none' } },
+    { 'method': 'Debugger.setAsyncCallStackDepth',
+      'params': { 'maxDepth': 0 } },
+    { 'method': 'Profiler.enable' },
+    { 'method': 'Profiler.setSamplingInterval',
+      'params': { 'interval': 100 } },
+    { 'method': 'Debugger.setBlackboxPatterns',
+      'params': { 'patterns': [] } },
+    { 'method': 'Runtime.runIfWaitingForDebugger' }
+  ]);
+  return session;
+}
+
+async function testSuspend(sessionA, sessionB) {
+  console.log('[test]', 'Breaking in code and verifying events are fired');
+  await sessionA.waitForNotification('Debugger.paused', 'Initial pause');
+  sessionA.send({ 'method': 'Debugger.resume' });
+
+  await sessionA.waitForNotification('Runtime.consoleAPICalled',
+                                     'Console output');
+  sessionA.send({ 'method': 'Debugger.pause' });
+  return Promise.all([
+    sessionA.waitForNotification('Debugger.paused', 'SessionA paused'),
+    sessionB.waitForNotification('Debugger.paused', 'SessionB paused')
+  ]);
+}
+
+async function runTest() {
+  const child = new NodeInstance(undefined, script);
+
+  const [session1, session2] =
+      await Promise.all([setupSession(child), setupSession(child)]);
+  await testSuspend(session2, session1);
+  console.log('[test]', 'Should shut down after both sessions disconnect');
+
+  await session1.runToCompletion();
+  await session2.send({ 'method': 'Debugger.disable' });
+  await session2.disconnect();
+  return child.expectShutdown();
+}
+
+common.crashOnUnhandledRejection();
+
+runTest();
diff --git a/test/parallel/test-inspector-no-crash-ws-after-bindings.js b/test/parallel/test-inspector-no-crash-ws-after-bindings.js
deleted file mode 100644
index 15fa96952e..0000000000
--- a/test/parallel/test-inspector-no-crash-ws-after-bindings.js
+++ /dev/null
@@ -1,31 +0,0 @@
-// Flags: --expose-internals
-'use strict';
-const common = require('../common');
-common.skipIfInspectorDisabled();
-common.crashOnUnhandledRejection();
-const { NodeInstance } = require('../common/inspector-helper.js');
-const assert = require('assert');
-
-const expected = 'Can connect now!';
-
-const script = `
-  'use strict';
-  const { Session } = require('inspector');
-
-  const s = new Session();
-  s.connect();
-  console.error('${expected}');
-  process.stdin.on('data', () => process.exit(0));
-`;
-
-async function runTests() {
-  const instance = new NodeInstance(['--inspect=0', '--expose-internals'],
-                                    script);
-  while (await instance.nextStderrString() !== expected);
-  assert.strictEqual(400, await instance.expectConnectionDeclined());
-  instance.write('Stop!\n');
-  assert.deepStrictEqual({ exitCode: 0, signal: null },
-                         await instance.expectShutdown());
-}
-
-runTests();
diff --git a/test/sequential/test-inspector-bindings.js b/test/sequential/test-inspector-bindings.js
index bf97d8b512..c23c8520e0 100644
--- a/test/sequential/test-inspector-bindings.js
+++ b/test/sequential/test-inspector-bindings.js
@@ -76,15 +76,6 @@ function testSampleDebugSession() {
   };
   const session = new inspector.Session();
   session.connect();
-  let secondSessionOpened = false;
-  const secondSession = new inspector.Session();
-  try {
-    secondSession.connect();
-    secondSessionOpened = true;
-  } catch (error) {
-    // expected as the session already exists
-  }
-  assert.strictEqual(secondSessionOpened, false);
   session.on('Debugger.paused',
              (notification) => debuggerPausedCallback(session, notification));
   let cbAsSecondArgCalled = false;
diff --git a/test/sequential/test-inspector-module.js b/test/sequential/test-inspector-module.js
index 2435347e79..eaecd49c98 100644
--- a/test/sequential/test-inspector-module.js
+++ b/test/sequential/test-inspector-module.js
@@ -55,16 +55,6 @@ common.expectsError(
   }
 );
 
-const session2 = new Session();
-common.expectsError(
-  () => session2.connect(),
-  {
-    code: 'ERR_INSPECTOR_ALREADY_CONNECTED',
-    type: Error,
-    message: 'Another inspector session is already connected'
-  }
-);
-
 session.disconnect();
 // Calling disconnect twice should not throw.
 session.disconnect();
diff --git a/test/sequential/test-inspector.js b/test/sequential/test-inspector.js
index 4e0315807b..0a9b2ccd1a 100644
--- a/test/sequential/test-inspector.js
+++ b/test/sequential/test-inspector.js
@@ -37,12 +37,6 @@ function checkException(message) {
                      'An exception occurred during execution');
 }
 
-function assertNoUrlsWhileConnected(response) {
-  assert.strictEqual(1, response.length);
-  assert.ok(!response[0].hasOwnProperty('devtoolsFrontendUrl'));
-  assert.ok(!response[0].hasOwnProperty('webSocketDebuggerUrl'));
-}
-
 function assertScopeValues({ result }, expected) {
   const unmatched = new Set(Object.keys(expected));
   for (const actual of result) {
@@ -290,7 +284,6 @@ async function runTest() {
   await child.httpGet(null, '/json/badpath').catch(checkBadPath);
 
   const session = await child.connectInspectorSession();
-  assertNoUrlsWhileConnected(await child.httpGet(null, '/json/list'));
   await testBreakpointOnStart(session);
   await testBreakpoint(session);
   await testI18NCharacters(session);