inspector: workers debugging

Introduce a NodeTarget inspector domain modelled after ChromeDevTools
Target domain. It notifies inspector frontend attached to a main V8
isolate when workers are starting and allows passing messages to
inspectors on their isolates. All inspector functionality is enabled on
worker isolates.

PR-URL: https://github.com/nodejs/node/pull/21364
Reviewed-By: Aleksei Koziatinskii <ak239spb@gmail.com>
Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

11 files changed, 775 insertions, 10 deletions

diff --git a/node.gyp b/node.gyp
index fb1c674e8e..d52748ec2b 100644
--- a/node.gyp
+++ b/node.gyp
@@ -509,14 +509,18 @@
             'src/inspector_socket.cc',
             'src/inspector_socket_server.cc',
             'src/inspector/main_thread_interface.cc',
+            'src/inspector/worker_inspector.cc',
             'src/inspector/node_string.cc',
+            'src/inspector/worker_agent.cc',
             'src/inspector/tracing_agent.cc',
             'src/inspector_agent.h',
             'src/inspector_io.h',
             'src/inspector_socket.h',
             'src/inspector_socket_server.h',
             'src/inspector/main_thread_interface.h',
+            'src/inspector/worker_inspector.h',
             'src/inspector/node_string.h',
+            'src/inspector/worker_agent.h',
             'src/inspector/tracing_agent.h',
             '<@(node_inspector_generated_sources)'
           ],
@@ -1066,6 +1070,8 @@
           '<(SHARED_INTERMEDIATE_DIR)/src/node/inspector/protocol/Forward.h',
           '<(SHARED_INTERMEDIATE_DIR)/src/node/inspector/protocol/Protocol.cpp',
           '<(SHARED_INTERMEDIATE_DIR)/src/node/inspector/protocol/Protocol.h',
+          '<(SHARED_INTERMEDIATE_DIR)/src/node/inspector/protocol/NodeWorker.cpp',
+          '<(SHARED_INTERMEDIATE_DIR)/src/node/inspector/protocol/NodeWorker.h',
           '<(SHARED_INTERMEDIATE_DIR)/src/node/inspector/protocol/NodeTracing.cpp',
           '<(SHARED_INTERMEDIATE_DIR)/src/node/inspector/protocol/NodeTracing.h',
         ],
diff --git a/src/inspector/node_protocol.pdl b/src/inspector/node_protocol.pdl
index 27b3d814c8..9fb9f1c55f 100644
--- a/src/inspector/node_protocol.pdl
+++ b/src/inspector/node_protocol.pdl
@@ -37,3 +37,58 @@ experimental domain NodeTracing
   # Signals that tracing is stopped and there is no trace buffers pending flush, all data were
   # delivered via dataCollected events.
   event tracingComplete
+
+# Support for sending messages to Node worker Inspector instances.
+experimental domain NodeWorker
+
+  type WorkerID extends string
+
+  # Unique identifier of attached debugging session.
+  type SessionID extends string
+
+  type WorkerInfo extends object
+    properties
+      WorkerID workerId
+      string type
+      string title
+      string url
+
+  # Sends protocol message over session with given id.
+  command sendMessageToWorker
+    parameters
+      string message
+      # Identifier of the session.
+      SessionID sessionId
+
+  # Instructs the inspector to attach to running workers. Will also attach to new workers
+  # as they start
+  command enable
+    parameters
+      # Whether to new workers should be paused until the frontend sends `Runtime.runIfWaitingForDebugger`
+      # message to run them.
+      boolean waitForDebuggerOnStart
+
+  # Detaches from all running workers and disables attaching to new workers as they are started.
+  command disable
+
+  # Issued when attached to a worker.
+  event attachedToWorker
+    parameters
+      # Identifier assigned to the session used to send/receive messages.
+      SessionID sessionId
+      WorkerInfo workerInfo
+      boolean waitingForDebugger
+
+  # Issued when detached from the worker.
+  event detachedFromWorker
+    parameters
+      # Detached session identifier.
+      SessionID sessionId
+
+  # Notifies about a new protocol message received from the session
+  # (session ID is provided in attachedToWorker notification).
+  event receivedMessageFromWorker
+    parameters
+      # Identifier of a session which sends a message.
+      SessionID sessionId
+      string message
diff --git a/src/inspector/node_protocol_config.json b/src/inspector/node_protocol_config.json
index 7cea20ae93..4ef3785606 100644
--- a/src/inspector/node_protocol_config.json
+++ b/src/inspector/node_protocol_config.json
@@ -3,12 +3,7 @@
         "path": "node_protocol.json",
         "package": "src/node/inspector/protocol",
         "output": "node/inspector/protocol",
-        "namespace": ["node", "inspector", "protocol"],
-        "options": [
-            {
-                "domain": "NodeTracing"
-            }
-        ]
+        "namespace": ["node", "inspector", "protocol"]
     },
     "exported": {
         "package": "include/inspector",
diff --git a/src/inspector/worker_agent.cc b/src/inspector/worker_agent.cc
new file mode 100644
index 0000000000..fccd6d57a5
--- /dev/null
+++ b/src/inspector/worker_agent.cc
@@ -0,0 +1,154 @@
+#include "worker_agent.h"
+
+#include "main_thread_interface.h"
+#include "worker_inspector.h"
+
+namespace node {
+namespace inspector {
+namespace protocol {
+
+class NodeWorkers
+    : public std::enable_shared_from_this<NodeWorkers> {
+ public:
+  explicit NodeWorkers(std::weak_ptr<NodeWorker::Frontend> frontend,
+                      std::shared_ptr<MainThreadHandle> thread)
+                      : frontend_(frontend), thread_(thread) {}
+  void WorkerCreated(const std::string& title,
+                     const std::string& url,
+                     bool waiting,
+                     std::shared_ptr<MainThreadHandle> target);
+  void Receive(const std::string& id, const std::string& message);
+  void Send(const std::string& id, const std::string& message);
+  void Detached(const std::string& id);
+
+ private:
+  std::weak_ptr<NodeWorker::Frontend> frontend_;
+  std::shared_ptr<MainThreadHandle> thread_;
+  std::unordered_map<std::string, std::unique_ptr<InspectorSession>> sessions_;
+  int next_target_id_ = 0;
+};
+
+namespace {
+class AgentWorkerInspectorDelegate : public WorkerDelegate {
+ public:
+  explicit AgentWorkerInspectorDelegate(std::shared_ptr<NodeWorkers> workers)
+                                        : workers_(workers) {}
+
+  void WorkerCreated(const std::string& title,
+                     const std::string& url,
+                     bool waiting,
+                     std::shared_ptr<MainThreadHandle> target) override {
+    workers_->WorkerCreated(title, url, waiting, target);
+  }
+
+ private:
+  std::shared_ptr<NodeWorkers> workers_;
+};
+
+class ParentInspectorSessionDelegate : public InspectorSessionDelegate {
+ public:
+  ParentInspectorSessionDelegate(const std::string& id,
+                                 std::shared_ptr<NodeWorkers> workers)
+                                 : id_(id), workers_(workers) {}
+
+  ~ParentInspectorSessionDelegate() override {
+    workers_->Detached(id_);
+  }
+
+  void SendMessageToFrontend(const v8_inspector::StringView& msg) override {
+    std::string message = protocol::StringUtil::StringViewToUtf8(msg);
+    workers_->Send(id_, message);
+  }
+
+ private:
+  std::string id_;
+  std::shared_ptr<NodeWorkers> workers_;
+};
+
+std::unique_ptr<NodeWorker::WorkerInfo> WorkerInfo(const std::string& id,
+                                                   const std::string& title,
+                                                   const std::string& url) {
+  return NodeWorker::WorkerInfo::create()
+      .setWorkerId(id)
+      .setTitle(title)
+      .setUrl(url)
+      .setType("worker").build();
+}
+}  // namespace
+
+WorkerAgent::WorkerAgent(std::weak_ptr<WorkerManager> manager)
+                         : manager_(manager) {}
+
+
+void WorkerAgent::Wire(UberDispatcher* dispatcher) {
+  frontend_.reset(new NodeWorker::Frontend(dispatcher->channel()));
+  NodeWorker::Dispatcher::wire(dispatcher, this);
+  auto manager = manager_.lock();
+  CHECK_NOT_NULL(manager);
+  workers_ =
+      std::make_shared<NodeWorkers>(frontend_, manager->MainThread());
+}
+
+DispatchResponse WorkerAgent::sendMessageToWorker(const String& message,
+                                                  const String& sessionId) {
+  workers_->Receive(sessionId, message);
+  return DispatchResponse::OK();
+}
+
+DispatchResponse WorkerAgent::enable(bool waitForDebuggerOnStart) {
+  auto manager = manager_.lock();
+  if (!manager) {
+    return DispatchResponse::OK();
+  }
+  if (!event_handle_) {
+    std::unique_ptr<AgentWorkerInspectorDelegate> delegate(
+            new AgentWorkerInspectorDelegate(workers_));
+    event_handle_ = manager->SetAutoAttach(std::move(delegate));
+  }
+  event_handle_->SetWaitOnStart(waitForDebuggerOnStart);
+  return DispatchResponse::OK();
+}
+
+DispatchResponse WorkerAgent::disable() {
+  event_handle_.reset();
+  return DispatchResponse::OK();
+}
+
+void NodeWorkers::WorkerCreated(const std::string& title,
+                                const std::string& url,
+                                bool waiting,
+                                std::shared_ptr<MainThreadHandle> target) {
+  auto frontend = frontend_.lock();
+  if (!frontend)
+    return;
+  std::string id = std::to_string(++next_target_id_);
+  auto delegate = thread_->MakeDelegateThreadSafe(
+      std::unique_ptr<InspectorSessionDelegate>(
+          new ParentInspectorSessionDelegate(id, shared_from_this())));
+  sessions_[id] = target->Connect(std::move(delegate), true);
+  frontend->attachedToWorker(id, WorkerInfo(id, title, url), waiting);
+}
+
+void NodeWorkers::Send(const std::string& id, const std::string& message) {
+  auto frontend = frontend_.lock();
+  if (frontend)
+    frontend->receivedMessageFromWorker(id, message);
+}
+
+void NodeWorkers::Receive(const std::string& id, const std::string& message) {
+  auto it = sessions_.find(id);
+  if (it != sessions_.end())
+    it->second->Dispatch(Utf8ToStringView(message)->string());
+}
+
+void NodeWorkers::Detached(const std::string& id) {
+  if (sessions_.erase(id) == 0)
+    return;
+  auto frontend = frontend_.lock();
+  if (frontend) {
+    frontend->detachedFromWorker(id);
+  }
+}
+}  // namespace protocol
+}  // namespace inspector
+}  // namespace node
diff --git a/src/inspector/worker_agent.h b/src/inspector/worker_agent.h
new file mode 100644
index 0000000000..402c719416
--- /dev/null
+++ b/src/inspector/worker_agent.h
@@ -0,0 +1,39 @@
+#ifndef SRC_INSPECTOR_WORKER_AGENT_H_
+#define SRC_INSPECTOR_WORKER_AGENT_H_
+
+#include "node/inspector/protocol/NodeWorker.h"
+#include "v8.h"
+
+
+namespace node {
+namespace inspector {
+class WorkerManagerEventHandle;
+class WorkerManager;
+
+namespace protocol {
+class NodeWorkers;
+
+class WorkerAgent : public NodeWorker::Backend {
+ public:
+  explicit WorkerAgent(std::weak_ptr<WorkerManager> manager);
+  ~WorkerAgent() override = default;
+
+  void Wire(UberDispatcher* dispatcher);
+
+  DispatchResponse sendMessageToWorker(const String& message,
+                                       const String& sessionId) override;
+
+  DispatchResponse enable(bool waitForDebuggerOnStart) override;
+  DispatchResponse disable() override;
+
+ private:
+  std::shared_ptr<NodeWorker::Frontend> frontend_;
+  std::weak_ptr<WorkerManager> manager_;
+  std::unique_ptr<WorkerManagerEventHandle> event_handle_;
+  std::shared_ptr<NodeWorkers> workers_;
+};
+}  // namespace protocol
+}  // namespace inspector
+}  // namespace node
+
+#endif  // SRC_INSPECTOR_WORKER_AGENT_H_
diff --git a/src/inspector/worker_inspector.cc b/src/inspector/worker_inspector.cc
new file mode 100644
index 0000000000..52e71a562d
--- /dev/null
+++ b/src/inspector/worker_inspector.cc
@@ -0,0 +1,128 @@
+#include "worker_inspector.h"
+
+#include "main_thread_interface.h"
+
+namespace node {
+namespace inspector {
+namespace {
+
+class WorkerStartedRequest : public Request {
+ public:
+  WorkerStartedRequest(
+      int id,
+      const std::string& url,
+      std::shared_ptr<node::inspector::MainThreadHandle> worker_thread,
+      bool waiting)
+      : id_(id),
+        info_(BuildWorkerTitle(id), url, worker_thread),
+        waiting_(waiting) {}
+  void Call(MainThreadInterface* thread) override {
+    auto manager = thread->inspector_agent()->GetWorkerManager();
+    manager->WorkerStarted(id_, info_, waiting_);
+  }
+
+ private:
+  static std::string BuildWorkerTitle(int id) {
+    return "Worker " + std::to_string(id);
+  }
+
+  int id_;
+  WorkerInfo info_;
+  bool waiting_;
+};
+
+
+void Report(const std::unique_ptr<WorkerDelegate>& delegate,
+            const WorkerInfo& info, bool waiting) {
+  if (info.worker_thread)
+    delegate->WorkerCreated(info.title, info.url, waiting, info.worker_thread);
+}
+
+class WorkerFinishedRequest : public Request {
+ public:
+  explicit WorkerFinishedRequest(int worker_id) : worker_id_(worker_id) {}
+
+  void Call(MainThreadInterface* thread) override {
+    thread->inspector_agent()->GetWorkerManager()->WorkerFinished(worker_id_);
+  }
+
+ private:
+  int worker_id_;
+};
+}  // namespace
+
+
+ParentInspectorHandle::ParentInspectorHandle(
+    int id, const std::string& url,
+    std::shared_ptr<MainThreadHandle> parent_thread, bool wait_for_connect)
+    : id_(id), url_(url), parent_thread_(parent_thread),
+      wait_(wait_for_connect) {}
+
+ParentInspectorHandle::~ParentInspectorHandle() {
+  parent_thread_->Post(
+      std::unique_ptr<Request>(new WorkerFinishedRequest(id_)));
+}
+
+void ParentInspectorHandle::WorkerStarted(
+    std::shared_ptr<MainThreadHandle> worker_thread, bool waiting) {
+  std::unique_ptr<Request> request(
+      new WorkerStartedRequest(id_, url_, worker_thread, waiting));
+  parent_thread_->Post(std::move(request));
+}
+
+void WorkerManager::WorkerFinished(int session_id) {
+  children_.erase(session_id);
+}
+
+void WorkerManager::WorkerStarted(int session_id,
+                                  const WorkerInfo& info,
+                                  bool waiting) {
+  if (info.worker_thread->Expired())
+    return;
+  children_.emplace(session_id, info);
+  for (const auto& delegate : delegates_) {
+    Report(delegate.second, info, waiting);
+  }
+}
+
+std::unique_ptr<ParentInspectorHandle>
+WorkerManager::NewParentHandle(int thread_id, const std::string& url) {
+  bool wait = !delegates_waiting_on_start_.empty();
+  return std::unique_ptr<ParentInspectorHandle>(
+      new ParentInspectorHandle(thread_id, url, thread_, wait));
+}
+
+void WorkerManager::RemoveAttachDelegate(int id) {
+  delegates_.erase(id);
+  delegates_waiting_on_start_.erase(id);
+}
+
+std::unique_ptr<WorkerManagerEventHandle> WorkerManager::SetAutoAttach(
+    std::unique_ptr<WorkerDelegate> attach_delegate) {
+  int id = ++next_delegate_id_;
+  delegates_[id] = std::move(attach_delegate);
+  const auto& delegate = delegates_[id];
+  for (const auto& worker : children_) {
+    // Waiting is only reported when a worker is started, same as browser
+    Report(delegate, worker.second, false);
+  }
+  return std::unique_ptr<WorkerManagerEventHandle>(
+      new WorkerManagerEventHandle(shared_from_this(), id));
+}
+
+void WorkerManager::SetWaitOnStartForDelegate(int id, bool wait) {
+  if (wait)
+    delegates_waiting_on_start_.insert(id);
+  else
+    delegates_waiting_on_start_.erase(id);
+}
+
+void WorkerManagerEventHandle::SetWaitOnStart(bool wait_on_start) {
+    manager_->SetWaitOnStartForDelegate(id_, wait_on_start);
+}
+
+WorkerManagerEventHandle::~WorkerManagerEventHandle() {
+  manager_->RemoveAttachDelegate(id_);
+}
+}  // namespace inspector
+}  // namespace node
diff --git a/src/inspector/worker_inspector.h b/src/inspector/worker_inspector.h
new file mode 100644
index 0000000000..e3c96cf62f
--- /dev/null
+++ b/src/inspector/worker_inspector.h
@@ -0,0 +1,98 @@
+#ifndef SRC_INSPECTOR_WORKER_INSPECTOR_H_
+#define SRC_INSPECTOR_WORKER_INSPECTOR_H_
+
+#if !HAVE_INSPECTOR
+#error("This header can only be used when inspector is enabled")
+#endif
+
+#include <memory>
+#include <string>
+#include <unordered_map>
+#include <unordered_set>
+
+namespace node {
+namespace inspector {
+class MainThreadHandle;
+class WorkerManager;
+
+class WorkerDelegate {
+ public:
+  virtual void WorkerCreated(const std::string& title,
+                             const std::string& url,
+                             bool waiting,
+                             std::shared_ptr<MainThreadHandle> worker) = 0;
+};
+
+class WorkerManagerEventHandle {
+ public:
+  explicit WorkerManagerEventHandle(std::shared_ptr<WorkerManager> manager,
+                                    int id)
+                                    : manager_(manager), id_(id) {}
+  void SetWaitOnStart(bool wait_on_start);
+  ~WorkerManagerEventHandle();
+
+ private:
+  std::shared_ptr<WorkerManager> manager_;
+  int id_;
+};
+
+struct WorkerInfo {
+  WorkerInfo(const std::string& target_title,
+             const std::string& target_url,
+             std::shared_ptr<MainThreadHandle> worker_thread)
+             : title(target_title),
+               url(target_url),
+               worker_thread(worker_thread) {}
+  std::string title;
+  std::string url;
+  std::shared_ptr<MainThreadHandle> worker_thread;
+};
+
+class ParentInspectorHandle {
+ public:
+  ParentInspectorHandle(int id, const std::string& url,
+                        std::shared_ptr<MainThreadHandle> parent_thread,
+                        bool wait_for_connect);
+  ~ParentInspectorHandle();
+  void WorkerStarted(std::shared_ptr<MainThreadHandle> worker_thread,
+                     bool waiting);
+  bool WaitForConnect() {
+    return wait_;
+  }
+
+ private:
+  int id_;
+  std::string url_;
+  std::shared_ptr<MainThreadHandle> parent_thread_;
+  bool wait_;
+};
+
+class WorkerManager : public std::enable_shared_from_this<WorkerManager> {
+ public:
+  explicit WorkerManager(std::shared_ptr<MainThreadHandle> thread)
+                         : thread_(thread) {}
+
+  std::unique_ptr<ParentInspectorHandle> NewParentHandle(
+      int thread_id, const std::string& url);
+  void WorkerStarted(int session_id, const WorkerInfo& info, bool waiting);
+  void WorkerFinished(int session_id);
+  std::unique_ptr<WorkerManagerEventHandle> SetAutoAttach(
+      std::unique_ptr<WorkerDelegate> attach_delegate);
+  void SetWaitOnStartForDelegate(int id, bool wait);
+  void RemoveAttachDelegate(int id);
+  std::shared_ptr<MainThreadHandle> MainThread() {
+    return thread_;
+  }
+
+ private:
+  std::shared_ptr<MainThreadHandle> thread_;
+  std::unordered_map<int, WorkerInfo> children_;
+  std::unordered_map<int, std::unique_ptr<WorkerDelegate>> delegates_;
+  // If any one needs it, workers stop for all
+  std::unordered_set<int> delegates_waiting_on_start_;
+  int next_delegate_id_ = 0;
+};
+}  // namespace inspector
+}  // namespace node
+
+#endif  // SRC_INSPECTOR_WORKER_INSPECTOR_H_
diff --git a/src/inspector_agent.cc b/src/inspector_agent.cc
index 63b9266353..ebb7b7d5bc 100644
--- a/src/inspector_agent.cc
+++ b/src/inspector_agent.cc
@@ -4,6 +4,8 @@
 #include "inspector/main_thread_interface.h"
 #include "inspector/node_string.h"
 #include "inspector/tracing_agent.h"
+#include "inspector/worker_agent.h"
+#include "inspector/worker_inspector.h"
 #include "node/inspector/protocol/Protocol.h"
 #include "node_internals.h"
 #include "node_url.h"
@@ -201,6 +203,7 @@ class ChannelImpl final : public v8_inspector::V8Inspector::Channel,
  public:
   explicit ChannelImpl(Environment* env,
                        const std::unique_ptr<V8Inspector>& inspector,
+                       std::shared_ptr<WorkerManager> worker_manager,
                        std::unique_ptr<InspectorSessionDelegate> delegate,
                        bool prevent_shutdown)
                        : delegate_(std::move(delegate)),
@@ -209,11 +212,15 @@ class ChannelImpl final : public v8_inspector::V8Inspector::Channel,
     node_dispatcher_.reset(new protocol::UberDispatcher(this));
     tracing_agent_.reset(new protocol::TracingAgent(env));
     tracing_agent_->Wire(node_dispatcher_.get());
+    worker_agent_.reset(new protocol::WorkerAgent(worker_manager));
+    worker_agent_->Wire(node_dispatcher_.get());
   }
 
   virtual ~ChannelImpl() {
     tracing_agent_->disable();
     tracing_agent_.reset();  // Dispose before the dispatchers
+    worker_agent_->disable();
+    worker_agent_.reset();  // Dispose before the dispatchers
   }
 
   std::string dispatchProtocolMessage(const StringView& message) {
@@ -273,6 +280,7 @@ class ChannelImpl final : public v8_inspector::V8Inspector::Channel,
   }
 
   std::unique_ptr<protocol::TracingAgent> tracing_agent_;
+  std::unique_ptr<protocol::WorkerAgent> worker_agent_;
   std::unique_ptr<InspectorSessionDelegate> delegate_;
   std::unique_ptr<v8_inspector::V8InspectorSession> session_;
   std::unique_ptr<protocol::UberDispatcher> node_dispatcher_;
@@ -469,7 +477,8 @@ class NodeInspectorClient : public V8InspectorClient {
     // TODO(addaleax): Revert back to using make_unique once we get issues
     // with CI resolved (i.e. revert the patch that added this comment).
     channels_[session_id].reset(
-        new ChannelImpl(env_, client_, std::move(delegate), prevent_shutdown));
+        new ChannelImpl(env_, client_, getWorkerManager(),
+                        std::move(delegate), prevent_shutdown));
     return session_id;
   }
 
@@ -589,6 +598,14 @@ class NodeInspectorClient : public V8InspectorClient {
     return interface_->GetHandle();
   }
 
+  std::shared_ptr<WorkerManager> getWorkerManager() {
+    if (worker_manager_ == nullptr) {
+      worker_manager_ =
+          std::make_shared<WorkerManager>(getThreadHandle());
+    }
+    return worker_manager_;
+  }
+
   bool IsActive() {
     return !channels_.empty();
   }
@@ -646,6 +663,7 @@ class NodeInspectorClient : public V8InspectorClient {
   bool waiting_for_io_shutdown_ = false;
   // Allows accessing Inspector from non-main threads
   std::unique_ptr<MainThreadInterface> interface_;
+  std::shared_ptr<WorkerManager> worker_manager_;
 };
 
 Agent::Agent(Environment* env)
@@ -680,7 +698,10 @@ bool Agent::Start(const std::string& path,
   }
 
   bool wait_for_connect = options->wait_for_connect();
-  if (!options->inspector_enabled || !StartIoThread()) {
+  if (parent_handle_) {
+    wait_for_connect = parent_handle_->WaitForConnect();
+    parent_handle_->WorkerStarted(client_->getThreadHandle(), wait_for_connect);
+  } else if (!options->inspector_enabled || !StartIoThread()) {
     return false;
   }
   if (wait_for_connect) {
@@ -727,7 +748,9 @@ std::unique_ptr<InspectorSession> Agent::Connect(
 
 void Agent::WaitForDisconnect() {
   CHECK_NOT_NULL(client_);
-  if (client_->hasConnectedSessions()) {
+  bool is_worker = parent_handle_ != nullptr;
+  parent_handle_.reset();
+  if (client_->hasConnectedSessions() && !is_worker) {
     fprintf(stderr, "Waiting for the debugger to disconnect...\n");
     fflush(stderr);
   }
@@ -842,7 +865,11 @@ void Agent::ContextCreated(Local<Context> context, const ContextInfo& info) {
 }
 
 bool Agent::WillWaitForConnect() {
-  return debug_options_->wait_for_connect();
+  if (debug_options_->wait_for_connect())
+    return true;
+  if (parent_handle_)
+    return parent_handle_->WaitForConnect();
+  return false;
 }
 
 bool Agent::IsActive() {
@@ -851,11 +878,24 @@ bool Agent::IsActive() {
   return io_ != nullptr || client_->IsActive();
 }
 
+void Agent::AddWorkerInspector(int thread_id,
+                               const std::string& url,
+                               Agent* agent) {
+  CHECK_NOT_NULL(client_);
+  agent->parent_handle_ =
+      client_->getWorkerManager()->NewParentHandle(thread_id, url);
+}
+
 void Agent::WaitForConnect() {
   CHECK_NOT_NULL(client_);
   client_->waitForFrontend();
 }
 
+std::shared_ptr<WorkerManager> Agent::GetWorkerManager() {
+  CHECK_NOT_NULL(client_);
+  return client_->getWorkerManager();
+}
+
 SameThreadInspectorSession::~SameThreadInspectorSession() {
   auto client = client_.lock();
   if (client)
diff --git a/src/inspector_agent.h b/src/inspector_agent.h
index 79ae8d4cd9..e926ccaa92 100644
--- a/src/inspector_agent.h
+++ b/src/inspector_agent.h
@@ -26,7 +26,9 @@ struct ContextInfo;
 
 namespace inspector {
 class InspectorIo;
+class ParentInspectorHandle;
 class NodeInspectorClient;
+class WorkerManager;
 
 class InspectorSession {
  public:
@@ -82,6 +84,8 @@ class Agent {
   void EnableAsyncHook();
   void DisableAsyncHook();
 
+  void AddWorkerInspector(int thread_id, const std::string& url, Agent* agent);
+
   // Called to create inspector sessions that can be used from the main thread.
   // The inspector responds by using the delegate to send messages back.
   std::unique_ptr<InspectorSession> Connect(
@@ -103,6 +107,9 @@ class Agent {
   std::shared_ptr<DebugOptions> options() { return debug_options_; }
   void ContextCreated(v8::Local<v8::Context> context, const ContextInfo& info);
 
+  // Interface for interacting with inspectors in worker threads
+  std::shared_ptr<WorkerManager> GetWorkerManager();
+
  private:
   void ToggleAsyncHook(v8::Isolate* isolate,
                        const node::Persistent<v8::Function>& fn);
@@ -112,6 +119,7 @@ class Agent {
   std::shared_ptr<NodeInspectorClient> client_;
   // Interface for transports, e.g. WebSocket server
   std::unique_ptr<InspectorIo> io_;
+  std::unique_ptr<ParentInspectorHandle> parent_handle_;
   std::string path_;
   std::shared_ptr<DebugOptions> debug_options_;
 
diff --git a/src/node_worker.cc b/src/node_worker.cc
index 1a90e3a64f..209b7a4d09 100644
--- a/src/node_worker.cc
+++ b/src/node_worker.cc
@@ -40,6 +40,14 @@ void StartWorkerInspector(Environment* child, const std::string& url) {
   child->inspector_agent()->Start(url, nullptr, false);
 }
 
+void AddWorkerInspector(Environment* parent,
+                        Environment* child,
+                        int id,
+                        const std::string& url) {
+  parent->inspector_agent()->AddWorkerInspector(id, url,
+                                                child->inspector_agent());
+}
+
 void WaitForWorkerInspectorToStop(Environment* child) {
   child->inspector_agent()->WaitForDisconnect();
   child->inspector_agent()->Stop();
@@ -48,6 +56,10 @@ void WaitForWorkerInspectorToStop(Environment* child) {
 #else
 // No-ops
 void StartWorkerInspector(Environment* child, const std::string& url) {}
+void AddWorkerInspector(Environment* parent,
+                        Environment* child,
+                        int id,
+                        const std::string& url) {}
 void WaitForWorkerInspectorToStop(Environment* child) {}
 #endif
 
@@ -115,6 +127,8 @@ Worker::Worker(Environment* env, Local<Object> wrap, const std::string& url)
     env_->Start(std::vector<std::string>{},
                 std::vector<std::string>{},
                 env->profiler_idle_notifier_started());
+    // Done while on the parent thread
+    AddWorkerInspector(env, env_.get(), thread_id_, url_);
   }
 
   // The new isolate won't be bothered on this thread again.
diff --git a/test/parallel/test-worker-debug.js b/test/parallel/test-worker-debug.js
new file mode 100644
index 0000000000..8691879f14
--- /dev/null
+++ b/test/parallel/test-worker-debug.js
@@ -0,0 +1,228 @@
+// Flags: --experimental-worker
+'use strict';
+const common = require('../common');
+
+common.skipIfInspectorDisabled();
+
+const assert = require('assert');
+const EventEmitter = require('events');
+const { Session } = require('inspector');
+const { pathToFileURL } = require('url');
+const {
+  Worker, isMainThread, parentPort, workerData
+} = require('worker_threads');
+
+
+const workerMessage = 'This is a message from a worker';
+
+function waitForMessage() {
+  return new Promise((resolve) => {
+    parentPort.once('message', resolve);
+  });
+}
+
+// This is at the top so line numbers change less often
+if (!isMainThread) {
+  if (workerData === 1) {
+    console.log(workerMessage);
+    debugger;  // eslint-disable-line no-debugger
+  } else if (workerData === 2) {
+    parentPort.postMessage('running');
+    waitForMessage();
+  }
+  return;
+}
+
+function doPost(session, method, params) {
+  return new Promise((resolve, reject) => {
+    session.post(method, params, (error, result) => {
+      if (error)
+        reject(JSON.stringify(error));
+      else
+        resolve(result);
+    });
+  });
+}
+
+function waitForEvent(emitter, event) {
+  return new Promise((resolve) => emitter.once(event, resolve));
+}
+
+function waitForWorkerAttach(session) {
+  return waitForEvent(session, 'NodeWorker.attachedToWorker')
+      .then(({ params }) => params);
+}
+
+async function waitForWorkerDetach(session, id) {
+  let sessionId;
+  do {
+    const { params } =
+        await waitForEvent(session, 'NodeWorker.detachedFromWorker');
+    sessionId = params.sessionId;
+  } while (sessionId !== id);
+}
+
+function runWorker(id, workerCallback = () => {}) {
+  return new Promise((resolve, reject) => {
+    const worker = new Worker(__filename, { workerData: id });
+    workerCallback(worker);
+    worker.on('error', reject);
+    worker.on('exit', resolve);
+  });
+}
+
+class WorkerSession extends EventEmitter {
+  constructor(parentSession, id) {
+    super();
+    this._parentSession = parentSession;
+    this._id = id;
+    this._requestCallbacks = new Map();
+    this._nextCommandId = 1;
+    this._parentSession.on('NodeWorker.receivedMessageFromWorker',
+                           ({ params }) => {
+                             if (params.sessionId === this._id)
+                               this._processMessage(JSON.parse(params.message));
+                           });
+  }
+
+  _processMessage(message) {
+    if (message.id === undefined) {
+      // console.log(JSON.stringify(message));
+      this.emit('inspectorNotification', message);
+      this.emit(message.method, message);
+      return;
+    }
+    const callback = this._requestCallbacks.get(message.id);
+    if (callback) {
+      this._requestCallbacks.delete(message.id);
+      if (message.error)
+        callback[1](message.error.message);
+      else
+        callback[0](message.result);
+    }
+  }
+
+  async waitForBreakAfterCommand(command, script, line) {
+    const notificationPromise = waitForEvent(this, 'Debugger.paused');
+    this.post(command);
+    const notification = await notificationPromise;
+    const callFrame = notification.params.callFrames[0];
+    assert.strictEqual(callFrame.url, pathToFileURL(script).toString());
+    assert.strictEqual(callFrame.location.lineNumber, line);
+  }
+
+  post(method, parameters) {
+    const msg = {
+      id: this._nextCommandId++,
+      method
+    };
+    if (parameters)
+      msg.params = parameters;
+
+    return new Promise((resolve, reject) => {
+      this._requestCallbacks.set(msg.id, [resolve, reject]);
+      this._parentSession.post('NodeWorker.sendMessageToWorker', {
+        sessionId: this._id, message: JSON.stringify(msg)
+      });
+    });
+  }
+}
+
+async function testBasicWorkerDebug(session, post) {
+  /*
+    1. Do 'enble' with waitForDebuggerOnStart = true
+    2. Run worker. It should break on start.
+    3. Enable Runtime (to get console message) and Debugger. Resume.
+    4. Breaks on the 'debugger' statement. Resume.
+    5. Console message recieved, worker runs to a completion.
+    6. contextCreated/contextDestroyed had been properly dispatched
+  */
+  console.log('Test basic debug scenario');
+  await post('NodeWorker.enable', { waitForDebuggerOnStart: true });
+  const attached = waitForWorkerAttach(session);
+  const worker = runWorker(1);
+  const { sessionId, waitingForDebugger } = await attached;
+  assert.strictEqual(waitingForDebugger, true);
+  const detached = waitForWorkerDetach(session, sessionId);
+  const workerSession = new WorkerSession(session, sessionId);
+  const contextEvents = Promise.all([
+    waitForEvent(workerSession, 'Runtime.executionContextCreated'),
+    waitForEvent(workerSession, 'Runtime.executionContextDestroyed')
+  ]);
+  const consolePromise = waitForEvent(workerSession, 'Runtime.consoleAPICalled')
+      .then((notification) => notification.params.args[0].value);
+  await workerSession.post('Debugger.enable');
+  await workerSession.post('Runtime.enable');
+  await workerSession.waitForBreakAfterCommand(
+    'Runtime.runIfWaitingForDebugger', __filename, 2);
+  await workerSession.waitForBreakAfterCommand(
+    'Debugger.resume', __filename, 27);  // V8 line number is zero-based
+  assert.strictEqual(await consolePromise, workerMessage);
+  workerSession.post('Debugger.resume');
+  await Promise.all([worker, detached, contextEvents]);
+}
+
+async function testNoWaitOnStart(session, post) {
+  console.log('Test disabled waitForDebuggerOnStart');
+  await post('NodeWorker.enable', { waitForDebuggerOnStart: false });
+  let worker;
+  const promise = waitForWorkerAttach(session);
+  const exitPromise = runWorker(2, (w) => { worker = w; });
+  const { waitingForDebugger } = await promise;
+  assert.strictEqual(waitingForDebugger, false);
+  worker.postMessage('resume');
+  await exitPromise;
+}
+
+async function testTwoWorkers(session, post) {
+  console.log('Test attach to a running worker and then start a new one');
+  await post('NodeWorker.disable');
+  let okToAttach = false;
+  const worker1attached = waitForWorkerAttach(session).then((notification) => {
+    assert.strictEqual(okToAttach, true);
+    return notification;
+  });
+
+  let worker1Exited;
+  const worker = await new Promise((resolve, reject) => {
+    worker1Exited = runWorker(2, resolve);
+  }).then((worker) => new Promise(
+    (resolve) => worker.once('message', () => resolve(worker))));
+  okToAttach = true;
+  await post('NodeWorker.enable', { waitForDebuggerOnStart: true });
+  const { waitingForDebugger: worker1Waiting } = await worker1attached;
+  assert.strictEqual(worker1Waiting, false);
+
+  const worker2Attached = waitForWorkerAttach(session);
+  let worker2Done = false;
+  const worker2Exited = runWorker(1)
+      .then(() => assert.strictEqual(worker2Done, true));
+  const worker2AttachInfo = await worker2Attached;
+  assert.strictEqual(worker2AttachInfo.waitingForDebugger, true);
+  worker2Done = true;
+
+  const workerSession = new WorkerSession(session, worker2AttachInfo.sessionId);
+  workerSession.post('Runtime.runIfWaitingForDebugger');
+  worker.postMessage('resume');
+  await Promise.all([worker1Exited, worker2Exited]);
+}
+
+async function test() {
+  const session = new Session();
+  session.connect();
+  const post = doPost.bind(null, session);
+
+  await testBasicWorkerDebug(session, post);
+
+  console.log('Test disabling attach to workers');
+  await post('NodeWorker.disable');
+  await runWorker(1);
+
+  await testNoWaitOnStart(session, post);
+  await testTwoWorkers(session, post);
+
+  session.disconnect();
+  console.log('Test done');
+}
+
+test();