src/debug.h

// Copyright 2006-2008 the V8 project authors. All rights reserved.
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
//       notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
//       copyright notice, this list of conditions and the following
//       disclaimer in the documentation and/or other materials provided
//       with the distribution.
//     * Neither the name of Google Inc. nor the names of its
//       contributors may be used to endorse or promote products derived
//       from this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

#ifndef V8_DEBUG_H_
#define V8_DEBUG_H_

#include "assembler.h"
#include "code-stubs.h"
#include "debug-agent.h"
#include "execution.h"
#include "factory.h"
#include "hashmap.h"
#include "platform.h"
#include "string-stream.h"
#include "v8threads.h"

#ifdef ENABLE_DEBUGGER_SUPPORT
#include "../include/v8-debug.h"

namespace v8 {
namespace internal {


// Forward declarations.
class EnterDebugger;


// Step actions. NOTE: These values are in macros.py as well.
enum StepAction {
  StepNone = -1,  // Stepping not prepared.
  StepOut = 0,   // Step out of the current function.
  StepNext = 1,  // Step to the next statement in the current function.
  StepIn = 2,    // Step into new functions invoked or the next statement
                 // in the current function.
  StepMin = 3,   // Perform a minimum step in the current function.
  StepInMin = 4  // Step into new functions invoked or perform a minimum step
                 // in the current function.
};


// Type of exception break. NOTE: These values are in macros.py as well.
enum ExceptionBreakType {
  BreakException = 0,
  BreakUncaughtException = 1
};


// Type of exception break. NOTE: These values are in macros.py as well.
enum BreakLocatorType {
  ALL_BREAK_LOCATIONS = 0,
  SOURCE_BREAK_LOCATIONS = 1
};


// Class for iterating through the break points in a function and changing
// them.
class BreakLocationIterator {
 public:
  explicit BreakLocationIterator(Handle<DebugInfo> debug_info,
                                 BreakLocatorType type);
  virtual ~BreakLocationIterator();

  void Next();
  void Next(int count);
  void FindBreakLocationFromAddress(Address pc);
  void FindBreakLocationFromPosition(int position);
  void Reset();
  bool Done() const;
  void SetBreakPoint(Handle<Object> break_point_object);
  void ClearBreakPoint(Handle<Object> break_point_object);
  void SetOneShot();
  void ClearOneShot();
  void PrepareStepIn();
  bool IsExit() const;
  bool HasBreakPoint();
  bool IsDebugBreak();
  Object* BreakPointObjects();
  void ClearAllDebugBreak();


  inline int code_position() {
    return static_cast<int>(pc() - debug_info_->code()->entry());
  }
  inline int break_point() { return break_point_; }
  inline int position() { return position_; }
  inline int statement_position() { return statement_position_; }
  inline Address pc() { return reloc_iterator_->rinfo()->pc(); }
  inline Code* code() { return debug_info_->code(); }
  inline RelocInfo* rinfo() { return reloc_iterator_->rinfo(); }
  inline RelocInfo::Mode rmode() const {
    return reloc_iterator_->rinfo()->rmode();
  }
  inline RelocInfo* original_rinfo() {
    return reloc_iterator_original_->rinfo();
  }
  inline RelocInfo::Mode original_rmode() const {
    return reloc_iterator_original_->rinfo()->rmode();
  }

  bool IsDebuggerStatement();

 protected:
  bool RinfoDone() const;
  void RinfoNext();

  BreakLocatorType type_;
  int break_point_;
  int position_;
  int statement_position_;
  Handle<DebugInfo> debug_info_;
  Handle<Code> debug_break_stub_;
  RelocIterator* reloc_iterator_;
  RelocIterator* reloc_iterator_original_;

 private:
  void SetDebugBreak();
  void ClearDebugBreak();

  void SetDebugBreakAtIC();
  void ClearDebugBreakAtIC();

  bool IsDebugBreakAtReturn();
  void SetDebugBreakAtReturn();
  void ClearDebugBreakAtReturn();

  DISALLOW_COPY_AND_ASSIGN(BreakLocationIterator);
};


// Cache of all script objects in the heap. When a script is added a weak handle
// to it is created and that weak handle is stored in the cache. The weak handle
// callback takes care of removing the script from the cache. The key used in
// the cache is the script id.
class ScriptCache : private HashMap {
 public:
  ScriptCache() : HashMap(ScriptMatch), collected_scripts_(10) {}
  virtual ~ScriptCache() { Clear(); }

  // Add script to the cache.
  void Add(Handle<Script> script);

  // Return the scripts in the cache.
  Handle<FixedArray> GetScripts();

  // Generate debugger events for collected scripts.
  void ProcessCollectedScripts();

 private:
  // Calculate the hash value from the key (script id).
  static uint32_t Hash(int key) { return ComputeIntegerHash(key); }

  // Scripts match if their keys (script id) match.
  static bool ScriptMatch(void* key1, void* key2) { return key1 == key2; }

  // Clear the cache releasing all the weak handles.
  void Clear();

  // Weak handle callback for scripts in the cache.
  static void HandleWeakScript(v8::Persistent<v8::Value> obj, void* data);

  // List used during GC to temporarily store id's of collected scripts.
  List<int> collected_scripts_;
};


// Linked list holding debug info objects. The debug info objects are kept as
// weak handles to avoid a debug info object to keep a function alive.
class DebugInfoListNode {
 public:
  explicit DebugInfoListNode(DebugInfo* debug_info);
  virtual ~DebugInfoListNode();

  DebugInfoListNode* next() { return next_; }
  void set_next(DebugInfoListNode* next) { next_ = next; }
  Handle<DebugInfo> debug_info() { return debug_info_; }

 private:
  // Global (weak) handle to the debug info object.
  Handle<DebugInfo> debug_info_;

  // Next pointer for linked list.
  DebugInfoListNode* next_;
};


// This class contains the debugger support. The main purpose is to handle
// setting break points in the code.
//
// This class controls the debug info for all functions which currently have
// active breakpoints in them. This debug info is held in the heap root object
// debug_info which is a FixedArray. Each entry in this list is of class
// DebugInfo.
class Debug {
 public:
  static void Setup(bool create_heap_objects);
  static bool Load();
  static void Unload();
  static bool IsLoaded() { return !debug_context_.is_null(); }
  static bool InDebugger() { return thread_local_.debugger_entry_ != NULL; }
  static void PreemptionWhileInDebugger();
  static void Iterate(ObjectVisitor* v);

  static Object* Break(Arguments args);
  static void SetBreakPoint(Handle<SharedFunctionInfo> shared,
                            int source_position,
                            Handle<Object> break_point_object);
  static void ClearBreakPoint(Handle<Object> break_point_object);
  static void ClearAllBreakPoints();
  static void FloodWithOneShot(Handle<SharedFunctionInfo> shared);
  static void FloodHandlerWithOneShot();
  static void ChangeBreakOnException(ExceptionBreakType type, bool enable);
  static void PrepareStep(StepAction step_action, int step_count);
  static void ClearStepping();
  static bool StepNextContinue(BreakLocationIterator* break_location_iterator,
                               JavaScriptFrame* frame);
  static Handle<DebugInfo> GetDebugInfo(Handle<SharedFunctionInfo> shared);
  static bool HasDebugInfo(Handle<SharedFunctionInfo> shared);

  // Returns whether the operation succeeded.
  static bool EnsureDebugInfo(Handle<SharedFunctionInfo> shared);

  // Returns true if the current stub call is patched to call the debugger.
  static bool IsDebugBreak(Address addr);
  // Returns true if the current return statement has been patched to be
  // a debugger breakpoint.
  static bool IsDebugBreakAtReturn(RelocInfo* rinfo);

  // Check whether a code stub with the specified major key is a possible break
  // point location.
  static bool IsSourceBreakStub(Code* code);
  static bool IsBreakStub(Code* code);

  // Find the builtin to use for invoking the debug break
  static Handle<Code> FindDebugBreak(Handle<Code> code, RelocInfo::Mode mode);

  static Handle<Object> GetSourceBreakLocations(
      Handle<SharedFunctionInfo> shared);

  // Getter for the debug_context.
  inline static Handle<Context> debug_context() { return debug_context_; }

  // Check whether a global object is the debug global object.
  static bool IsDebugGlobal(GlobalObject* global);

  // Fast check to see if any break points are active.
  inline static bool has_break_points() { return has_break_points_; }

  static void NewBreak(StackFrame::Id break_frame_id);
  static void SetBreak(StackFrame::Id break_frame_id, int break_id);
  static StackFrame::Id break_frame_id() {
    return thread_local_.break_frame_id_;
  }
  static int break_id() { return thread_local_.break_id_; }

  static bool StepInActive() { return thread_local_.step_into_fp_ != 0; }
  static void HandleStepIn(Handle<JSFunction> function,
                           Handle<Object> holder,
                           Address fp,
                           bool is_constructor);
  static Address step_in_fp() { return thread_local_.step_into_fp_; }
  static Address* step_in_fp_addr() { return &thread_local_.step_into_fp_; }

  static bool StepOutActive() { return thread_local_.step_out_fp_ != 0; }
  static Address step_out_fp() { return thread_local_.step_out_fp_; }

  static EnterDebugger* debugger_entry() {
    return thread_local_.debugger_entry_;
  }
  static void set_debugger_entry(EnterDebugger* entry) {
    thread_local_.debugger_entry_ = entry;
  }

  // Check whether any of the specified interrupts are pending.
  static bool is_interrupt_pending(InterruptFlag what) {
    return (thread_local_.pending_interrupts_ & what) != 0;
  }

  // Set specified interrupts as pending.
  static void set_interrupts_pending(InterruptFlag what) {
    thread_local_.pending_interrupts_ |= what;
  }

  // Clear specified interrupts from pending.
  static void clear_interrupt_pending(InterruptFlag what) {
    thread_local_.pending_interrupts_ &= ~static_cast<int>(what);
  }

  // Getter and setter for the disable break state.
  static bool disable_break() { return disable_break_; }
  static void set_disable_break(bool disable_break) {
    disable_break_ = disable_break;
  }

  // Getters for the current exception break state.
  static bool break_on_exception() { return break_on_exception_; }
  static bool break_on_uncaught_exception() {
    return break_on_uncaught_exception_;
  }

  enum AddressId {
    k_after_break_target_address,
    k_debug_break_return_address,
    k_register_address
  };

  // Support for setting the address to jump to when returning from break point.
  static Address* after_break_target_address() {
    return reinterpret_cast<Address*>(&thread_local_.after_break_target_);
  }

  // Support for saving/restoring registers when handling debug break calls.
  static Object** register_address(int r) {
    return &registers_[r];
  }

  // Access to the debug break on return code.
  static Code* debug_break_return() { return debug_break_return_; }
  static Code** debug_break_return_address() {
    return &debug_break_return_;
  }

  static const int kEstimatedNofDebugInfoEntries = 16;
  static const int kEstimatedNofBreakPointsInFunction = 16;

  static void HandleWeakDebugInfo(v8::Persistent<v8::Value> obj, void* data);

  friend class Debugger;
  friend Handle<FixedArray> GetDebuggedFunctions();  // In test-debug.cc
  friend void CheckDebuggerUnloaded(bool check_functions);  // In test-debug.cc

  // Threading support.
  static char* ArchiveDebug(char* to);
  static char* RestoreDebug(char* from);
  static int ArchiveSpacePerThread();
  static void FreeThreadResources() { }

  // Mirror cache handling.
  static void ClearMirrorCache();

  // Script cache handling.
  static void CreateScriptCache();
  static void DestroyScriptCache();
  static void AddScriptToScriptCache(Handle<Script> script);
  static Handle<FixedArray> GetLoadedScripts();

  // Garbage collection notifications.
  static void AfterGarbageCollection();

  // Code generator routines.
  static void GenerateLoadICDebugBreak(MacroAssembler* masm);
  static void GenerateStoreICDebugBreak(MacroAssembler* masm);
  static void GenerateKeyedLoadICDebugBreak(MacroAssembler* masm);
  static void GenerateKeyedStoreICDebugBreak(MacroAssembler* masm);
  static void GenerateConstructCallDebugBreak(MacroAssembler* masm);
  static void GenerateReturnDebugBreak(MacroAssembler* masm);
  static void GenerateStubNoRegistersDebugBreak(MacroAssembler* masm);

  // Called from stub-cache.cc.
  static void GenerateCallICDebugBreak(MacroAssembler* masm);

 private:
  static bool CompileDebuggerScript(int index);
  static void ClearOneShot();
  static void ActivateStepIn(StackFrame* frame);
  static void ClearStepIn();
  static void ActivateStepOut(StackFrame* frame);
  static void ClearStepOut();
  static void ClearStepNext();
  // Returns whether the compile succeeded.
  static bool EnsureCompiled(Handle<SharedFunctionInfo> shared);
  static void RemoveDebugInfo(Handle<DebugInfo> debug_info);
  static void SetAfterBreakTarget(JavaScriptFrame* frame);
  static Handle<Object> CheckBreakPoints(Handle<Object> break_point);
  static bool CheckBreakPoint(Handle<Object> break_point_object);

  // Global handle to debug context where all the debugger JavaScript code is
  // loaded.
  static Handle<Context> debug_context_;

  // Boolean state indicating whether any break points are set.
  static bool has_break_points_;

  // Cache of all scripts in the heap.
  static ScriptCache* script_cache_;

  // List of active debug info objects.
  static DebugInfoListNode* debug_info_list_;

  static bool disable_break_;
  static bool break_on_exception_;
  static bool break_on_uncaught_exception_;

  // Per-thread data.
  class ThreadLocal {
   public:
    // Counter for generating next break id.
    int break_count_;

    // Current break id.
    int break_id_;

    // Frame id for the frame of the current break.
    StackFrame::Id break_frame_id_;

    // Step action for last step performed.
    StepAction last_step_action_;

    // Source statement position from last step next action.
    int last_statement_position_;

    // Number of steps left to perform before debug event.
    int step_count_;

    // Frame pointer from last step next action.
    Address last_fp_;

    // Frame pointer for frame from which step in was performed.
    Address step_into_fp_;

    // Frame pointer for the frame where debugger should be called when current
    // step out action is completed.
    Address step_out_fp_;

    // Storage location for jump when exiting debug break calls.
    Address after_break_target_;

    // Top debugger entry.
    EnterDebugger* debugger_entry_;

    // Pending interrupts scheduled while debugging.
    int pending_interrupts_;
  };

  // Storage location for registers when handling debug break calls
  static JSCallerSavedBuffer registers_;
  static ThreadLocal thread_local_;
  static void ThreadInit();

  // Code to call for handling debug break on return.
  static Code* debug_break_return_;

  DISALLOW_COPY_AND_ASSIGN(Debug);
};


// Message delivered to the message handler callback. This is either a debugger
// event or the response to a command.
class MessageImpl: public v8::Debug::Message {
 public:
  // Create a message object for a debug event.
  static MessageImpl NewEvent(DebugEvent event,
                              bool running,
                              Handle<JSObject> exec_state,
                              Handle<JSObject> event_data);

  // Create a message object for the response to a debug command.
  static MessageImpl NewResponse(DebugEvent event,
                                 bool running,
                                 Handle<JSObject> exec_state,
                                 Handle<JSObject> event_data,
                                 Handle<String> response_json,
                                 v8::Debug::ClientData* client_data);

  // Implementation of interface v8::Debug::Message.
  virtual bool IsEvent() const;
  virtual bool IsResponse() const;
  virtual DebugEvent GetEvent() const;
  virtual bool WillStartRunning() const;
  virtual v8::Handle<v8::Object> GetExecutionState() const;
  virtual v8::Handle<v8::Object> GetEventData() const;
  virtual v8::Handle<v8::String> GetJSON() const;
  virtual v8::Handle<v8::Context> GetEventContext() const;
  virtual v8::Debug::ClientData* GetClientData() const;

 private:
  MessageImpl(bool is_event,
              DebugEvent event,
              bool running,
              Handle<JSObject> exec_state,
              Handle<JSObject> event_data,
              Handle<String> response_json,
              v8::Debug::ClientData* client_data);

  bool is_event_;  // Does this message represent a debug event?
  DebugEvent event_;  // Debug event causing the break.
  bool running_;  // Will the VM start running after this event?
  Handle<JSObject> exec_state_;  // Current execution state.
  Handle<JSObject> event_data_;  // Data associated with the event.
  Handle<String> response_json_;  // Response JSON if message holds a response.
  v8::Debug::ClientData* client_data_;  // Client data passed with the request.
};


// Message send by user to v8 debugger or debugger output message.
// In addition to command text it may contain a pointer to some user data
// which are expected to be passed along with the command reponse to message
// handler.
class CommandMessage {
 public:
  static CommandMessage New(const Vector<uint16_t>& command,
                            v8::Debug::ClientData* data);
  CommandMessage();
  ~CommandMessage();

  // Deletes user data and disposes of the text.
  void Dispose();
  Vector<uint16_t> text() const { return text_; }
  v8::Debug::ClientData* client_data() const { return client_data_; }
 private:
  CommandMessage(const Vector<uint16_t>& text,
                 v8::Debug::ClientData* data);

  Vector<uint16_t> text_;
  v8::Debug::ClientData* client_data_;
};

// A Queue of CommandMessage objects.  A thread-safe version is
// LockingCommandMessageQueue, based on this class.
class CommandMessageQueue BASE_EMBEDDED {
 public:
  explicit CommandMessageQueue(int size);
  ~CommandMessageQueue();
  bool IsEmpty() const { return start_ == end_; }
  CommandMessage Get();
  void Put(const CommandMessage& message);
  void Clear() { start_ = end_ = 0; }  // Queue is empty after Clear().
 private:
  // Doubles the size of the message queue, and copies the messages.
  void Expand();

  CommandMessage* messages_;
  int start_;
  int end_;
  int size_;  // The size of the queue buffer.  Queue can hold size-1 messages.
};


// LockingCommandMessageQueue is a thread-safe circular buffer of CommandMessage
// messages.  The message data is not managed by LockingCommandMessageQueue.
// Pointers to the data are passed in and out. Implemented by adding a
// Mutex to CommandMessageQueue.  Includes logging of all puts and gets.
class LockingCommandMessageQueue BASE_EMBEDDED {
 public:
  explicit LockingCommandMessageQueue(int size);
  ~LockingCommandMessageQueue();
  bool IsEmpty() const;
  CommandMessage Get();
  void Put(const CommandMessage& message);
  void Clear();
 private:
  CommandMessageQueue queue_;
  Mutex* lock_;
  DISALLOW_COPY_AND_ASSIGN(LockingCommandMessageQueue);
};


class Debugger {
 public:
  static void DebugRequest(const uint16_t* json_request, int length);

  static Handle<Object> MakeJSObject(Vector<const char> constructor_name,
                                     int argc, Object*** argv,
                                     bool* caught_exception);
  static Handle<Object> MakeExecutionState(bool* caught_exception);
  static Handle<Object> MakeBreakEvent(Handle<Object> exec_state,
                                       Handle<Object> break_points_hit,
                                       bool* caught_exception);
  static Handle<Object> MakeExceptionEvent(Handle<Object> exec_state,
                                           Handle<Object> exception,
                                           bool uncaught,
                                           bool* caught_exception);
  static Handle<Object> MakeNewFunctionEvent(Handle<Object> func,
                                             bool* caught_exception);
  static Handle<Object> MakeCompileEvent(Handle<Script> script,
                                         bool before,
                                         bool* caught_exception);
  static Handle<Object> MakeScriptCollectedEvent(int id,
                                                 bool* caught_exception);
  static void OnDebugBreak(Handle<Object> break_points_hit, bool auto_continue);
  static void OnException(Handle<Object> exception, bool uncaught);
  static void OnBeforeCompile(Handle<Script> script);
  static void OnAfterCompile(Handle<Script> script,
                           Handle<JSFunction> fun);
  static void OnNewFunction(Handle<JSFunction> fun);
  static void OnScriptCollected(int id);
  static void ProcessDebugEvent(v8::DebugEvent event,
                                Handle<JSObject> event_data,
                                bool auto_continue);
  static void NotifyMessageHandler(v8::DebugEvent event,
                                   Handle<JSObject> exec_state,
                                   Handle<JSObject> event_data,
                                   bool auto_continue);
  static void SetEventListener(Handle<Object> callback, Handle<Object> data);
  static void SetMessageHandler(v8::Debug::MessageHandler2 handler);
  static void SetHostDispatchHandler(v8::Debug::HostDispatchHandler handler,
                                     int period);
  static void SetDebugMessageDispatchHandler(
      v8::Debug::DebugMessageDispatchHandler handler);

  // Invoke the message handler function.
  static void InvokeMessageHandler(MessageImpl message);

  // Add a debugger command to the command queue.
  static void ProcessCommand(Vector<const uint16_t> command,
                             v8::Debug::ClientData* client_data = NULL);

  // Check whether there are commands in the command queue.
  static bool HasCommands();

  static Handle<Object> Call(Handle<JSFunction> fun,
                             Handle<Object> data,
                             bool* pending_exception);

  // Start the debugger agent listening on the provided port.
  static bool StartAgent(const char* name, int port);

  // Stop the debugger agent.
  static void StopAgent();

  // Blocks until the agent has started listening for connections
  static void WaitForAgent();

  // Unload the debugger if possible. Only called when no debugger is currently
  // active.
  static void UnloadDebugger();

  inline static bool EventActive(v8::DebugEvent event) {
    ScopedLock with(debugger_access_);

    // Check whether the message handler was been cleared.
    if (debugger_unload_pending_) {
      UnloadDebugger();
    }

    // Currently argument event is not used.
    return !compiling_natives_ && Debugger::IsDebuggerActive();
  }

  static void set_compiling_natives(bool compiling_natives) {
    Debugger::compiling_natives_ = compiling_natives;
  }
  static bool compiling_natives() { return Debugger::compiling_natives_; }
  static void set_loading_debugger(bool v) { is_loading_debugger_ = v; }
  static bool is_loading_debugger() { return Debugger::is_loading_debugger_; }

 private:
  static bool IsDebuggerActive();
  static void ListenersChanged();

  static Mutex* debugger_access_;  // Mutex guarding debugger variables.
  static Handle<Object> event_listener_;  // Global handle to listener.
  static Handle<Object> event_listener_data_;
  static bool compiling_natives_;  // Are we compiling natives?
  static bool is_loading_debugger_;  // Are we loading the debugger?
  static bool never_unload_debugger_;  // Can we unload the debugger?
  static v8::Debug::MessageHandler2 message_handler_;
  static bool debugger_unload_pending_;  // Was message handler cleared?
  static v8::Debug::HostDispatchHandler host_dispatch_handler_;
  static v8::Debug::DebugMessageDispatchHandler debug_message_dispatch_handler_;
  static int host_dispatch_micros_;

  static DebuggerAgent* agent_;

  static const int kQueueInitialSize = 4;
  static LockingCommandMessageQueue command_queue_;
  static Semaphore* command_received_;  // Signaled for each command received.

  friend class EnterDebugger;
};


// This class is used for entering the debugger. Create an instance in the stack
// to enter the debugger. This will set the current break state, make sure the
// debugger is loaded and switch to the debugger context. If the debugger for
// some reason could not be entered FailedToEnter will return true.
class EnterDebugger BASE_EMBEDDED {
 public:
  EnterDebugger()
      : prev_(Debug::debugger_entry()),
        has_js_frames_(!it_.done()) {
    ASSERT(prev_ != NULL || !Debug::is_interrupt_pending(PREEMPT));
    ASSERT(prev_ != NULL || !Debug::is_interrupt_pending(DEBUGBREAK));

    // Link recursive debugger entry.
    Debug::set_debugger_entry(this);

    // Store the previous break id and frame id.
    break_id_ = Debug::break_id();
    break_frame_id_ = Debug::break_frame_id();

    // Create the new break info. If there is no JavaScript frames there is no
    // break frame id.
    if (has_js_frames_) {
      Debug::NewBreak(it_.frame()->id());
    } else {
      Debug::NewBreak(StackFrame::NO_ID);
    }

    // Make sure that debugger is loaded and enter the debugger context.
    load_failed_ = !Debug::Load();
    if (!load_failed_) {
      // NOTE the member variable save which saves the previous context before
      // this change.
      Top::set_context(*Debug::debug_context());
    }
  }

  ~EnterDebugger() {
    // Restore to the previous break state.
    Debug::SetBreak(break_frame_id_, break_id_);

    // Check for leaving the debugger.
    if (prev_ == NULL) {
      // Clear mirror cache when leaving the debugger. Skip this if there is a
      // pending exception as clearing the mirror cache calls back into
      // JavaScript. This can happen if the v8::Debug::Call is used in which
      // case the exception should end up in the calling code.
      if (!Top::has_pending_exception()) {
        // Try to avoid any pending debug break breaking in the clear mirror
        // cache JavaScript code.
        if (StackGuard::IsDebugBreak()) {
          Debug::set_interrupts_pending(DEBUGBREAK);
          StackGuard::Continue(DEBUGBREAK);
        }
        Debug::ClearMirrorCache();
      }

      // Request preemption and debug break when leaving the last debugger entry
      // if any of these where recorded while debugging.
      if (Debug::is_interrupt_pending(PREEMPT)) {
        // This re-scheduling of preemption is to avoid starvation in some
        // debugging scenarios.
        Debug::clear_interrupt_pending(PREEMPT);
        StackGuard::Preempt();
      }
      if (Debug::is_interrupt_pending(DEBUGBREAK)) {
        Debug::clear_interrupt_pending(DEBUGBREAK);
        StackGuard::DebugBreak();
      }

      // If there are commands in the queue when leaving the debugger request
      // that these commands are processed.
      if (Debugger::HasCommands()) {
        StackGuard::DebugCommand();
      }

      // If leaving the debugger with the debugger no longer active unload it.
      if (!Debugger::IsDebuggerActive()) {
        Debugger::UnloadDebugger();
      }
    }

    // Leaving this debugger entry.
    Debug::set_debugger_entry(prev_);
  }

  // Check whether the debugger could be entered.
  inline bool FailedToEnter() { return load_failed_; }

  // Check whether there are any JavaScript frames on the stack.
  inline bool HasJavaScriptFrames() { return has_js_frames_; }

  // Get the active context from before entering the debugger.
  inline Handle<Context> GetContext() { return save_.context(); }

 private:
  EnterDebugger* prev_;  // Previous debugger entry if entered recursively.
  JavaScriptFrameIterator it_;
  const bool has_js_frames_;  // Were there any JavaScript frames?
  StackFrame::Id break_frame_id_;  // Previous break frame id.
  int break_id_;  // Previous break id.
  bool load_failed_;  // Did the debugger fail to load?
  SaveContext save_;  // Saves previous context.
};


// Stack allocated class for disabling break.
class DisableBreak BASE_EMBEDDED {
 public:
  // Enter the debugger by storing the previous top context and setting the
  // current top context to the debugger context.
  explicit DisableBreak(bool disable_break)  {
    prev_disable_break_ = Debug::disable_break();
    Debug::set_disable_break(disable_break);
  }
  ~DisableBreak() {
    Debug::set_disable_break(prev_disable_break_);
  }

 private:
  // The previous state of the disable break used to restore the value when this
  // object is destructed.
  bool prev_disable_break_;
};


// Debug_Address encapsulates the Address pointers used in generating debug
// code.
class Debug_Address {
 public:
  Debug_Address(Debug::AddressId id, int reg = 0)
    : id_(id), reg_(reg) {
    ASSERT(reg == 0 || id == Debug::k_register_address);
  }

  static Debug_Address AfterBreakTarget() {
    return Debug_Address(Debug::k_after_break_target_address);
  }

  static Debug_Address DebugBreakReturn() {
    return Debug_Address(Debug::k_debug_break_return_address);
  }

  static Debug_Address Register(int reg) {
    return Debug_Address(Debug::k_register_address, reg);
  }

  Address address() const {
    switch (id_) {
      case Debug::k_after_break_target_address:
        return reinterpret_cast<Address>(Debug::after_break_target_address());
      case Debug::k_debug_break_return_address:
        return reinterpret_cast<Address>(Debug::debug_break_return_address());
      case Debug::k_register_address:
        return reinterpret_cast<Address>(Debug::register_address(reg_));
      default:
        UNREACHABLE();
        return NULL;
    }
  }
 private:
  Debug::AddressId id_;
  int reg_;
};


} }  // namespace v8::internal

#endif  // ENABLE_DEBUGGER_SUPPORT

#endif  // V8_DEBUG_H_