[debug] Fully implement Debug::ArchiveDebug and Debug::RestoreDebug.

I have a project that embeds V8 and uses a single `Isolate` from multiple
threads. The program runs just fine, but sometimes the inspector doesn't
stop on the correct line after stepping over a statement that switches
threads behind the scenes, even though the original thread is restored by
the time the next statement is executed.

After some digging, I discovered that the `Debug::ArchiveDebug` and
`Debug::RestoreDebug` methods, which should be responsible for
saving/restoring this `ThreadLocal` information when switching threads,
currently don't do anything.

This commit implements those methods using MemCopy, in the style of other
Archive/Restore methods in the V8 codebase.

Related: https://groups.google.com/forum/#!topic/v8-users/_Qf2rwljRk8

Note: I believe my employer, Meteor Development Group, has previously
signed the CLA using the group email address google-contrib@meteor.com.

R=yangguo@chromium.org,jgruber@chromium.org
CC=info@bnoordhuis.nl

Bug: v8:7230
Change-Id: Id517c873eb81cd53f7216c7efd441b956cf7f943
Reviewed-on: https://chromium-review.googlesource.com/833260
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54902}

AUTHORS

@@ -33,6 +33,7 @@ Facebook, Inc. <*@fb.com>
 Facebook, Inc. <*@oculus.com>
 Vewd Software AS <*@vewd.com>
 Groupon <*@groupon.com>
+Meteor Development Group <*@meteor.com>
 Cloudflare, Inc. <*@cloudflare.com>
 
 Aaron Bieber <deftly@gmail.com>
@@ -50,6 +51,7 @@ Andrei Kashcha <anvaka@gmail.com>
 Anna Henningsen <anna@addaleax.net>
 Bangfu Tao <bangfu.tao@samsung.com>
 Ben Coe <ben@npmjs.com>
+Ben Newman <ben@meteor.com>
 Ben Noordhuis <info@bnoordhuis.nl>
 Benjamin Tan <demoneaux@gmail.com>
 Bert Belder <bertbelder@gmail.com>

src/debug/debug.cc

@@ -357,19 +357,31 @@ void Debug::ThreadInit() {
 
 
 char* Debug::ArchiveDebug(char* storage) {
-  // Simply reset state. Don't archive anything.
-  ThreadInit();
+  MemCopy(storage, reinterpret_cast<char*>(&thread_local_),
+          ArchiveSpacePerThread());
   return storage + ArchiveSpacePerThread();
 }
 
-
 char* Debug::RestoreDebug(char* storage) {
-  // Simply reset state. Don't restore anything.
-  ThreadInit();
+  MemCopy(reinterpret_cast<char*>(&thread_local_), storage,
+          ArchiveSpacePerThread());
+
+  // Enter the debugger.
+  DebugScope debug_scope(this);
+
+  // Clear any one-shot breakpoints that may have been set by the other
+  // thread, and reapply breakpoints for this thread.
+  ClearOneShot();
+
+  if (thread_local_.last_step_action_ != StepNone) {
+    // Reset the previous step action for this thread.
+    PrepareStep(thread_local_.last_step_action_);
+  }
+
   return storage + ArchiveSpacePerThread();
 }
 
-int Debug::ArchiveSpacePerThread() { return 0; }
+int Debug::ArchiveSpacePerThread() { return sizeof(ThreadLocal); }
 
 void Debug::Iterate(RootVisitor* v) {
   v->VisitRootPointer(Root::kDebug, nullptr, &thread_local_.return_value_);

src/debug/debug.h

@@ -315,6 +315,7 @@ class Debug {
   static int ArchiveSpacePerThread();
   void FreeThreadResources() { }
   void Iterate(RootVisitor* v);
+  void InitThread(const ExecutionAccess& lock) { ThreadInit(); }
 
   bool CheckExecutionState() { return is_active() && break_id() != 0; }
 

src/v8threads.cc

@@ -45,7 +45,7 @@ void Locker::Initialize(v8::Isolate* isolate) {
     } else {
       internal::ExecutionAccess access(isolate_);
       isolate_->stack_guard()->ClearThread(access);
-      isolate_->stack_guard()->InitThread(access);
+      isolate_->thread_manager()->InitThread(access);
     }
   }
   DCHECK(isolate_->thread_manager()->IsLockedByCurrentThread());
@@ -95,6 +95,10 @@ Unlocker::~Unlocker() {
 
 namespace internal {
 
+void ThreadManager::InitThread(const ExecutionAccess& lock) {
+  isolate_->stack_guard()->InitThread(lock);
+  isolate_->debug()->InitThread(lock);
+}
 
 bool ThreadManager::RestoreThread() {
   DCHECK(IsLockedByCurrentThread());
@@ -127,7 +131,7 @@ bool ThreadManager::RestoreThread() {
       isolate_->FindPerThreadDataForThisThread();
   if (per_thread == nullptr || per_thread->thread_state() == nullptr) {
     // This is a new thread.
-    isolate_->stack_guard()->InitThread(access);
+    InitThread(access);
     return false;
   }
   ThreadState* state = per_thread->thread_state();

src/v8threads.h

@@ -67,6 +67,7 @@ class ThreadManager {
   void Lock();
   void Unlock();
 
+  void InitThread(const ExecutionAccess&);
   void ArchiveThread();
   bool RestoreThread();
   void FreeThreadResources();

test/cctest/test-debug.cc

@@ -3717,6 +3717,133 @@ TEST(DebugBreakOffThreadTerminate) {
   CHECK(try_catch.HasTerminated());
 }
 
+class ArchiveRestoreThread : public v8::base::Thread,
+                             public v8::debug::DebugDelegate {
+ public:
+  ArchiveRestoreThread(v8::Isolate* isolate, int spawn_count)
+      : Thread(Options("ArchiveRestoreThread")),
+        isolate_(isolate),
+        debug_(reinterpret_cast<i::Isolate*>(isolate_)->debug()),
+        spawn_count_(spawn_count),
+        break_count_(0) {}
+
+  virtual void Run() {
+    v8::Locker locker(isolate_);
+    isolate_->Enter();
+
+    v8::HandleScope scope(isolate_);
+    v8::Local<v8::Context> context = v8::Context::New(isolate_);
+    v8::Context::Scope context_scope(context);
+
+    v8::Local<v8::Function> test = CompileFunction(isolate_,
+                                                   "function test(n) {\n"
+                                                   "  debugger;\n"
+                                                   "  return n + 1;\n"
+                                                   "}\n",
+                                                   "test");
+
+    debug_->SetDebugDelegate(this);
+    v8::internal::DisableBreak enable_break(debug_, false);
+
+    v8::Local<v8::Value> args[1] = {v8::Integer::New(isolate_, spawn_count_)};
+
+    int result = test->Call(context, context->Global(), 1, args)
+                     .ToLocalChecked()
+                     ->Int32Value(context)
+                     .FromJust();
+
+    // Verify that test(spawn_count_) returned spawn_count_ + 1.
+    CHECK_EQ(spawn_count_ + 1, result);
+
+    isolate_->Exit();
+  }
+
+  void BreakProgramRequested(v8::Local<v8::Context> context,
+                             const std::vector<v8::debug::BreakpointId>&) {
+    auto stack_traces = v8::debug::StackTraceIterator::Create(isolate_);
+    if (!stack_traces->Done()) {
+      v8::debug::Location location = stack_traces->GetSourceLocation();
+
+      i::PrintF("ArchiveRestoreThread #%d hit breakpoint at line %d\n",
+                spawn_count_, location.GetLineNumber());
+
+      switch (location.GetLineNumber()) {
+        case 1:  // debugger;
+          CHECK_EQ(break_count_, 0);
+
+          // Attempt to stop on the next line after the first debugger
+          // statement. If debug->{Archive,Restore}Debug() improperly reset
+          // thread-local debug information, the debugger will fail to stop
+          // before the test function returns.
+          debug_->PrepareStep(StepNext);
+
+          // Spawning threads while handling the current breakpoint verifies
+          // that the parent thread correctly archived and restored the
+          // state necessary to stop on the next line. If not, then control
+          // will simply continue past the `return n + 1` statement.
+          MaybeSpawnChildThread();
+
+          break;
+
+        case 2:  // return n + 1;
+          CHECK_EQ(break_count_, 1);
+          break;
+
+        default:
+          CHECK(false);
+      }
+    }
+
+    ++break_count_;
+  }
+
+  void MaybeSpawnChildThread() {
+    if (spawn_count_ > 1) {
+      v8::Unlocker unlocker(isolate_);
+
+      // Spawn a thread that spawns a thread that spawns a thread (and so
+      // on) so that the ThreadManager is forced to archive and restore
+      // the current thread.
+      ArchiveRestoreThread child(isolate_, spawn_count_ - 1);
+      child.Start();
+      child.Join();
+
+      // The child thread sets itself as the debug delegate, so we need to
+      // usurp it after the child finishes, or else future breakpoints
+      // will be delegated to a destroyed ArchiveRestoreThread object.
+      debug_->SetDebugDelegate(this);
+
+      // This is the most important check in this test, since
+      // child.GetBreakCount() will return 1 if the debugger fails to stop
+      // on the `return n + 1` line after the grandchild thread returns.
+      CHECK_EQ(child.GetBreakCount(), 2);
+    }
+  }
+
+  int GetBreakCount() { return break_count_; }
+
+ private:
+  v8::Isolate* isolate_;
+  v8::internal::Debug* debug_;
+  const int spawn_count_;
+  int break_count_;
+};
+
+TEST(DebugArchiveRestore) {
+  v8::Isolate::CreateParams create_params;
+  create_params.array_buffer_allocator = CcTest::array_buffer_allocator();
+  v8::Isolate* isolate = v8::Isolate::New(create_params);
+
+  ArchiveRestoreThread thread(isolate, 5);
+  // Instead of calling thread.Start() and thread.Join() here, we call
+  // thread.Run() directly, to make sure we exercise archive/restore
+  // logic on the *current* thread as well as other threads.
+  thread.Run();
+  CHECK_EQ(thread.GetBreakCount(), 2);
+
+  isolate->Dispose();
+}
+
 class DebugEventExpectNoException : public v8::debug::DebugDelegate {
  public:
   void ExceptionThrown(v8::Local<v8::Context> paused_context,