[debugger] step-next across yield should not leave the generator.

Stepping in a generator now behaves similar to stepping inside an
async function. Stepping in or next at a yield expression will result in
a break inside the same generator when we return to the generator.
Behavior of step-out does not change.

R=jgruber@chromium.org, neis@chromium.org
BUG=chromium:496865

Review-Url: https://codereview.chromium.org/2519853002
Cr-Commit-Position: refs/heads/master@{#41132}

src/debug/debug.cc

@@ -1430,9 +1430,16 @@ bool Debug::GetPossibleBreakpoints(Handle<Script> script, int start_position,
   return false;
 }
 
-void Debug::RecordAsyncFunction(Handle<JSGeneratorObject> generator_object) {
+void Debug::RecordGenerator(Handle<JSGeneratorObject> generator_object) {
   if (last_step_action() <= StepOut) return;
-  if (!IsAsyncFunction(generator_object->function()->shared()->kind())) return;
+
+  if (last_step_action() == StepNext) {
+    // Only consider this generator a step-next target if not stepping in.
+    JavaScriptFrameIterator stack_iterator(isolate_);
+    JavaScriptFrame* frame = stack_iterator.frame();
+    if (frame->UnpaddedFP() < thread_local_.target_fp_) return;
+  }
+
   DCHECK(!has_suspended_generator());
   thread_local_.suspended_generator_ = *generator_object;
   ClearStepping();

src/debug/debug.h

@@ -464,7 +464,7 @@ class Debug {
   bool GetPossibleBreakpoints(Handle<Script> script, int start_position,
                               int end_position, std::set<int>* positions);
 
-  void RecordAsyncFunction(Handle<JSGeneratorObject> generator_object);
+  void RecordGenerator(Handle<JSGeneratorObject> generator_object);
 
   // Returns whether the operation succeeded. Compilation can only be triggered
   // if a valid closure is passed as the second argument, otherwise the shared

src/interpreter/interpreter.cc

@@ -2766,7 +2766,7 @@ void Interpreter::DoSuspendGenerator(InterpreterAssembler* assembler) {
   __ Bind(&if_stepping);
   {
     Node* context = __ GetContext();
-    __ CallRuntime(Runtime::kDebugRecordAsyncFunction, context, generator);
+    __ CallRuntime(Runtime::kDebugRecordGenerator, context, generator);
     __ Goto(&ok);
   }
 }

src/runtime/runtime-debug.cc

@@ -1888,12 +1888,12 @@ RUNTIME_FUNCTION(Runtime_DebugPrepareStepInSuspendedGenerator) {
   return isolate->heap()->undefined_value();
 }
 
-RUNTIME_FUNCTION(Runtime_DebugRecordAsyncFunction) {
+RUNTIME_FUNCTION(Runtime_DebugRecordGenerator) {
   HandleScope scope(isolate);
   DCHECK_EQ(1, args.length());
   CONVERT_ARG_HANDLE_CHECKED(JSGeneratorObject, generator, 0);
   CHECK(isolate->debug()->last_step_action() >= StepNext);
-  isolate->debug()->RecordAsyncFunction(generator);
+  isolate->debug()->RecordGenerator(generator);
   return isolate->heap()->undefined_value();
 }
 

src/runtime/runtime-generator.cc

@@ -54,7 +54,7 @@ RUNTIME_FUNCTION(Runtime_SuspendJSGeneratorObject) {
   DCHECK(frame->function()->shared()->is_compiled());
   DCHECK(!frame->function()->IsOptimized());
 
-  isolate->debug()->RecordAsyncFunction(generator_object);
+  isolate->debug()->RecordGenerator(generator_object);
 
   // The caller should have saved the context and continuation already.
   DCHECK_EQ(generator_object->context(), Context::cast(frame->context()));

src/runtime/runtime.h

@@ -194,7 +194,7 @@ namespace internal {
   F(ScriptSourceLine, 2, 1)                     \
   F(DebugPrepareStepInIfStepping, 1, 1)         \
   F(DebugPrepareStepInSuspendedGenerator, 0, 1) \
-  F(DebugRecordAsyncFunction, 1, 1)             \
+  F(DebugRecordGenerator, 1, 1)                 \
   F(DebugPushPromise, 1, 1)                     \
   F(DebugPopPromise, 0, 1)                      \
   F(DebugNextMicrotaskId, 0, 1)                 \

test/debugger/debug/es6/debug-stepin-generators.js

@@ -14,7 +14,11 @@ function listener(event, exec_state, event_data, data) {
     print(source);
     if (/stop stepping/.test(source)) return;
     if (/yield/.test(source)) yields++;
-    exec_state.prepareStep(Debug.StepAction.StepIn);
+    if (yields == 4) {
+      exec_state.prepareStep(Debug.StepAction.StepOut);
+    } else {
+      exec_state.prepareStep(Debug.StepAction.StepIn);
+    }
   } catch (e) {
     print(e, e.stack);
     exception = e;

test/debugger/debug/es6/debug-stepnext-generators.js

@@ -0,0 +1,48 @@
+// Copyright 2016 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.
+
+Debug = debug.Debug
+var exception = null;
+var breaks = 0;
+
+function listener(event, exec_state, event_data, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var source = exec_state.frame(0).sourceLineText();
+    assertTrue(RegExp(`B${breaks++}`).test(source));
+    if (/stop/.test(source)) return;
+    if (/step out/.test(source)) {
+      exec_state.prepareStep(Debug.StepAction.StepOut);
+    } else if (/step in/.test(source)) {
+      exec_state.prepareStep(Debug.StepAction.StepIn);
+    } else {
+      exec_state.prepareStep(Debug.StepAction.StepNext);
+    }
+  } catch (e) {
+    print(e, e.stack);
+    exception = e;
+  }
+}
+
+Debug.setListener(listener);
+
+function * g() {
+  debugger;  // B0
+  yield 1;   // B1
+  yield 2;   // B2 step out
+  yield 3;   // B5
+  yield 4;   // B6 step out
+  return 2 * (yield 5);
+}
+
+var i = g();
+assertEquals(1, i.next().value);
+assertEquals(2, i.next().value);   // B3
+assertEquals(3, i.next().value);   // B4 step in
+assertEquals(4, i.next().value);   // B7
+assertEquals(5, i.next().value);   // B8
+assertEquals(6, i.next(3).value);  // B9 stop
+
+assertNull(exception);
+assertEquals(10, breaks);