From 36ab7afb9e1ce60d373a278b63887f2214f14556 Mon Sep 17 00:00:00 2001
From: Ingvar Stepanyan <rreverser@google.com>
Date: Fri, 27 Sep 2019 19:07:58 +0100
Subject: [PATCH] Improve JavaScript debugging in d8

Adds ability to pause JavaScript debugger from d8 by defining a global function
`handleInspectorMessage` which should block waiting for a new inspector message,
and `send` it afterwards.

Additionally, adds a simple helper script that, when invoked via `websocketd`
as per instructions, can be used for debugging `d8` using Chrome DevTools
(inspecting script sources, pausing, stepping over, etc.).

Change-Id: Iee75fb4e3f2ccc8c8552c804fefaefb233d6b089
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1829221
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Ingvar Stepanyan <rreverser@google.com>
Cr-Commit-Position: refs/heads/master@{#64040}
---
 src/d8/d8.cc        | 28 ++++++++++++++++++++++++++++
 tools/inspect-d8.js | 30 ++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+)
 create mode 100644 tools/inspect-d8.js

diff --git a/src/d8/d8.cc b/src/d8/d8.cc
index 9fd67013d0..80beb851b8 100644
--- a/src/d8/d8.cc
+++ b/src/d8/d8.cc
@@ -2502,6 +2502,33 @@ class InspectorClient : public v8_inspector::V8InspectorClient {
     context_.Reset(isolate_, context);
   }
 
+  void runMessageLoopOnPause(int contextGroupId) override {
+    v8::Isolate::AllowJavascriptExecutionScope allow_script(isolate_);
+    v8::HandleScope handle_scope(isolate_);
+    Local<String> callback_name =
+        v8::String::NewFromUtf8(isolate_, "handleInspectorMessage",
+                                v8::NewStringType::kNormal)
+            .ToLocalChecked();
+    Local<Context> context = context_.Get(isolate_);
+    Local<Value> callback =
+        context->Global()->Get(context, callback_name).ToLocalChecked();
+    if (!callback->IsFunction()) return;
+
+    v8::TryCatch try_catch(isolate_);
+    is_paused = true;
+
+    while (is_paused) {
+      USE(Local<Function>::Cast(callback)->Call(context, Undefined(isolate_), 0,
+                                                {}));
+      if (try_catch.HasCaught()) {
+        Shell::ReportException(isolate_, &try_catch);
+        is_paused = false;
+      }
+    }
+  }
+
+  void quitMessageLoopOnPause() override { is_paused = false; }
+
  private:
   static v8_inspector::V8InspectorSession* GetSession(Local<Context> context) {
     InspectorClient* inspector_client = static_cast<InspectorClient*>(
@@ -2540,6 +2567,7 @@ class InspectorClient : public v8_inspector::V8InspectorClient {
   std::unique_ptr<v8_inspector::V8Inspector> inspector_;
   std::unique_ptr<v8_inspector::V8InspectorSession> session_;
   std::unique_ptr<v8_inspector::V8Inspector::Channel> channel_;
+  bool is_paused = false;
   Global<Context> context_;
   Isolate* isolate_;
 };
diff --git a/tools/inspect-d8.js b/tools/inspect-d8.js
new file mode 100644
index 0000000000..b87a7586b2
--- /dev/null
+++ b/tools/inspect-d8.js
@@ -0,0 +1,30 @@
+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// This helper allows to debug d8 using Chrome DevTools.
+//
+// It runs a simple REPL for inspector messages and relies on
+// websocketd (https://github.com/joewalnes/websocketd) for the WebSocket
+// communication.
+//
+// You can start a session with a debug build of d8 like:
+//
+// $ websocketd out/x64.debug/d8 YOUR_SCRIPT.js tools/inspect-d8.js
+//
+// After that, copy the URL from console and pass it as `ws=` parameter to
+// the Chrome DevTools frontend like:
+//
+// chrome-devtools://devtools/bundled/js_app.html?ws=localhost:80
+
+function receive(msg) {
+  print(msg);
+}
+
+function handleInspectorMessage() {
+  send(readline());
+}
+
+while (true) {
+  handleInspectorMessage();
+}