Handle missing context when getting frame details

This bug was triggered by a very specific combination:

* A context-allocated variable at script scope.
* OSR optimization.
* A scheduled breakpoint, which triggers at stack checks.

Stack checks differ from other possible breakpoint locations in that
the context (among other things) may be in a register and not on the
stack, making it impossible to recover during deoptimization. The
frame_inspector then returns undefined when asked for the context.

In GetFrameDetails, handle this case by omitting all context-allocated
variables.

BUG=v8:5279

Review-Url: https://codereview.chromium.org/2245603002
Cr-Commit-Position: refs/heads/master@{#38611}

src/runtime/runtime-debug.cc

@@ -606,8 +606,12 @@ RUNTIME_FUNCTION(Runtime_GetFrameDetails) {
   DCHECK(*scope_info != ScopeInfo::Empty(isolate));
 
   // Get the locals names and values into a temporary array.
-  int local_count = scope_info->LocalCount();
-  for (int slot = 0; slot < scope_info->LocalCount(); ++slot) {
+  Handle<Object> maybe_context = frame_inspector.GetContext();
+  const int local_count_with_synthetic = maybe_context->IsContext()
+                                             ? scope_info->LocalCount()
+                                             : scope_info->StackLocalCount();
+  int local_count = local_count_with_synthetic;
+  for (int slot = 0; slot < local_count_with_synthetic; ++slot) {
     // Hide compiler-introduced temporary variables, whether on the stack or on
     // the context.
     if (ScopeInfo::VariableIsSynthetic(scope_info->LocalName(slot))) {
@@ -633,8 +637,9 @@ RUNTIME_FUNCTION(Runtime_GetFrameDetails) {
   }
   if (locals.length() < local_count * 2) {
     // Get the context containing declarations.
-    Handle<Context> context(
-        Handle<Context>::cast(frame_inspector.GetContext())->closure_context());
+    DCHECK(maybe_context->IsContext());
+    Handle<Context> context(Context::cast(*maybe_context)->closure_context());
+
     for (; i < scope_info->LocalCount(); ++i) {
       Handle<String> name(scope_info->LocalName(i));
       if (ScopeInfo::VariableIsSynthetic(*name)) continue;

test/mjsunit/regress/regress-5279.js

+// 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.
+
+// Flags: --allow-natives-syntax --expose-debug-as debug
+
+var Debug = debug.Debug;
+
+Debug.setListener(() => undefined);
+
+const myObj = {};
+
+for (let i = 0; i < 10; i++) {
+  %OptimizeOsr();
+  %ScheduleBreak();
+}