First step towards support for being able to interrupt a running Dart Isolate

runtime/bin/dbg_connection.cc

Index: runtime/bin/dbg_connection.cc
===================================================================
--- runtime/bin/dbg_connection.cc	(revision 13147)
+++ runtime/bin/dbg_connection.cc	(working copy)
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 #include "bin/dbg_connection.h"
+#include "bin/dbg_message.h"
 #include "bin/dartutils.h"
 #include "bin/socket.h"
 #include "bin/thread.h"
@@ -17,34 +18,14 @@
 
 
 int DebuggerConnectionHandler::listener_fd_ = -1;
-int DebuggerConnectionHandler::debugger_fd_ = -1;
-dart::Monitor DebuggerConnectionHandler::is_connected_;
-MessageBuffer* DebuggerConnectionHandler::msgbuf_ = NULL;
+dart::Monitor DebuggerConnectionHandler::handler_lock_;
 
-bool DebuggerConnectionHandler::handler_started_ = false;
-bool DebuggerConnectionHandler::request_resume_ = false;
+// TODO(asiva): Remove this once we have support for multiple debugger
+// connections. For now we just store the single debugger connection
+// handler in a static variable.
+static DebuggerConnectionHandler* singleton_handler = NULL;
 
-dart::TextBuffer DebuggerConnectionHandler::queued_messages_(64);
 
-
-// TODO(hausner): Need better error handling.
-#define ASSERT_NOT_ERROR(handle)          \
-  ASSERT(!Dart_IsError(handle))
-
-#define RETURN_IF_ERROR(handle)           \
-  if (Dart_IsError(handle)) {             \
-    return Dart_GetError(handle);         \
-  }
-
-
-typedef void (*CommandHandler)(const char* json_cmd);
-
-struct JSONDebuggerCommand {
-  const char* cmd_string;
-  CommandHandler handler_function;
-};
-
-
 class MessageBuffer {
  public:
   explicit MessageBuffer(int fd);
@@ -53,11 +34,6 @@
   bool IsValidMessage() const;
   void PopMessage();
   int MessageId() const;
-  const char* Params() const;
-  intptr_t GetIntParam(const char* name) const;
-  intptr_t GetOptIntParam(const char* name, intptr_t default_val) const;
-  // GetStringParam mallocs the buffer that it returns. Caller must free.
-  char* GetStringParam(const char* name) const;
   char* buf() const { return buf_; }
   bool Alive() const { return connection_is_alive_; }
 
@@ -75,10 +51,10 @@
 
 MessageBuffer::MessageBuffer(int fd)
     :  buf_(NULL),
-        buf_length_(0),
-        fd_(fd),
-        data_length_(0),
-        connection_is_alive_(true) {
+       buf_length_(0),
+       fd_(fd),
+       data_length_(0),
+       connection_is_alive_(true) {
   buf_ = reinterpret_cast<char*>(malloc(kInitialBufferSize));
   if (buf_ == NULL) {
     FATAL("Failed to allocate message buffer\n");
@@ -116,56 +92,6 @@
 }
 
 
-const char* MessageBuffer::Params() const {
-  dart::JSONReader r(buf_);
-  r.Seek("params");
-  if (r.Type() == dart::JSONReader::kObject) {
-    return r.ValueChars();
-  } else {
-    return NULL;
-  }
-}
-
-
-intptr_t MessageBuffer::GetIntParam(const char* name) const {
-  const char* params = Params();
-  ASSERT(params != NULL);
-  dart::JSONReader r(params);
-  r.Seek(name);
-  ASSERT(r.Type() == dart::JSONReader::kInteger);
-  return strtol(r.ValueChars(), NULL, 10);
-}
-
-
-intptr_t MessageBuffer::GetOptIntParam(const char* name,
-                                       intptr_t default_val) const {
-  const char* params = Params();
-  ASSERT(params != NULL);
-  dart::JSONReader r(params);
-  r.Seek(name);
-  if (r.Type() == dart::JSONReader::kInteger) {
-    return strtol(r.ValueChars(), NULL, 10);
-  } else {
-    return default_val;
-  }
-}
-
-
-char* MessageBuffer::GetStringParam(const char* name) const {
-  const char* params = Params();
-  ASSERT(params != NULL);
-  dart::JSONReader pr(params);
-  pr.Seek(name);
-  if (pr.Type() != dart::JSONReader::kString) {
-    return NULL;
-  }
-  intptr_t buflen = pr.ValueLen() + 1;
-  char* param_chars = reinterpret_cast<char*>(malloc(buflen));
-  pr.GetValueChars(param_chars, buflen);
-  // TODO(hausner): Decode escape sequences.
-  return param_chars;
-}
-
 void MessageBuffer::ReadData() {
   ASSERT(data_length_ >= 0);
   ASSERT(data_length_ < buf_length_);
@@ -216,722 +142,57 @@
 }
 
 
-void DebuggerConnectionHandler::SendMsg(dart::TextBuffer* msg) {
-  ASSERT(debugger_fd_ >= 0);
-  ASSERT(IsValidJSON(msg->buf()));
-  // Sending messages in short pieces can be used to stress test the
-  // debugger front-end's message handling code.
-  const bool send_in_pieces = false;
-  if (send_in_pieces) {
-    intptr_t remaining = msg->length();
-    intptr_t sent = 0;
-    const intptr_t max_piece_len = 122;  // Pretty arbitrary, not a power of 2.
-    dart::Monitor sleep;
-    while (remaining > 0) {
-      intptr_t piece_len = remaining;
-      if (piece_len > max_piece_len) {
-        piece_len = max_piece_len;
-      }
-      intptr_t written =
-        Socket::Write(debugger_fd_, msg->buf() + sent, piece_len);
-      ASSERT(written == piece_len);
-      sent += written;
-      remaining -= written;
-      // Wait briefly so the OS does not coalesce message fragments.
-      {
-        MonitorLocker ml(&sleep);
-        ml.Wait(10);
-      }
-    }
-    return;
-  }
-  intptr_t bytes_written =
-      Socket::Write(debugger_fd_, msg->buf(), msg->length());
-  ASSERT(msg->length() == bytes_written);
-  // TODO(hausner): Error checking. Probably just shut down the debugger
-  // session if we there is an error while writing.
+DebuggerConnectionHandler::DebuggerConnectionHandler(int debug_fd)
+    : debug_fd_(debug_fd), msgbuf_(NULL) {
+  msgbuf_ = new MessageBuffer(debug_fd_);
 }
 
 
-void DebuggerConnectionHandler::QueueMsg(dart::TextBuffer* msg) {
-  queued_messages_.Printf("%s", msg->buf());
+DebuggerConnectionHandler::~DebuggerConnectionHandler() {
+  CloseDbgConnection();
+  DebuggerConnectionHandler::RemoveDebuggerConnection(debug_fd_);
 }
 
 
-void DebuggerConnectionHandler::SendQueuedMsgs() {
-  if (queued_messages_.length() > 0) {
-    SendMsg(&queued_messages_);
-    queued_messages_.Clear();
-  }
+int DebuggerConnectionHandler::MessageId() {
+  ASSERT(msgbuf_ != NULL);
+  return msgbuf_->MessageId();
 }
 
 
-void DebuggerConnectionHandler::SendError(int msg_id,
-                                          const char* err_msg) {
-  dart::TextBuffer msg(64);
-  msg.Printf("{\"id\": %d, \"error\": \"Error: %s\"}", msg_id, err_msg);
-  SendMsg(&msg);
-}
-
-
-static const char* GetStringChars(Dart_Handle str) {
-  ASSERT(Dart_IsString(str));
-  const char* chars;
-  Dart_Handle res = Dart_StringToCString(str, &chars);
-  ASSERT(!Dart_IsError(res));
-  return chars;
-}
-
-
-static int GetIntValue(Dart_Handle int_handle) {
-  int64_t int64_val = -1;
-  ASSERT(Dart_IsInteger(int_handle));
-  Dart_Handle res = Dart_IntegerToInt64(int_handle, &int64_val);
-  ASSERT_NOT_ERROR(res);
-  // TODO(hausner): Range check.
-  return int64_val;
-}
-
-
-void DebuggerConnectionHandler::HandleResumeCmd(const char* json_msg) {
+void DebuggerConnectionHandler::HandleUnknownMsg() {
   int msg_id = msgbuf_->MessageId();
-  dart::TextBuffer msg(64);
-  msg.Printf("{ \"id\": %d }", msg_id);
-  SendMsg(&msg);
-  request_resume_ = true;
+  ASSERT(msg_id >= 0);
+  SendError(debug_fd_, msg_id, "unknown debugger command");
 }
 
 
-void DebuggerConnectionHandler::HandleStepIntoCmd(const char* json_msg) {
-  Dart_Handle res = Dart_SetStepInto();
-  ASSERT_NOT_ERROR(res);
-  HandleResumeCmd(json_msg);
-}
+typedef void (*CommandHandler)(DebuggerConnectionHandler* handler);
 
+struct JSONDebuggerCommand {
+  const char* cmd_string;
+  CommandHandler handler_function;
+};
 
-void DebuggerConnectionHandler::HandleStepOutCmd(const char* json_msg) {
-  Dart_Handle res = Dart_SetStepOut();
-  ASSERT_NOT_ERROR(res);
-  HandleResumeCmd(json_msg);
-}
 
-
-void DebuggerConnectionHandler::HandleStepOverCmd(const char* json_msg) {
-  Dart_Handle res = Dart_SetStepOver();
-  ASSERT_NOT_ERROR(res);
-  HandleResumeCmd(json_msg);
-}
-
-
-static void FormatEncodedString(dart::TextBuffer* buf, Dart_Handle str) {
-  intptr_t str_len = 0;
-  Dart_Handle res = Dart_StringLength(str, &str_len);
-  ASSERT_NOT_ERROR(res);
-  uint32_t* codepoints =
-      reinterpret_cast<uint32_t*>(malloc(str_len * sizeof(uint32_t)));
-  ASSERT(codepoints != NULL);
-  intptr_t actual_len = str_len;
-  res = Dart_StringGet32(str, codepoints, &actual_len);
-  ASSERT_NOT_ERROR(res);
-  ASSERT(str_len == actual_len);
-  buf->AddChar('\"');
-  for (int i = 0; i < str_len; i++) {
-    buf->AddEscapedChar(codepoints[i]);
-  }
-  buf->AddChar('\"');
-  free(codepoints);
-}
-
-
-static void FormatErrorMsg(dart::TextBuffer* buf, Dart_Handle err) {
-  // TODO(hausner): Turn message into Dart string and
-  // properly encode the message.
-  ASSERT(Dart_IsError(err));
-  const char* msg = Dart_GetError(err);
-  buf->Printf("\"%s\"", msg);
-}
-
-
-void DebuggerConnectionHandler::HandleGetScriptURLsCmd(const char* json_msg) {
-  int msg_id = msgbuf_->MessageId();
-  dart::TextBuffer msg(64);
-  intptr_t lib_id = msgbuf_->GetIntParam("libraryId");
-  Dart_Handle lib_url = Dart_GetLibraryURL(lib_id);
-  ASSERT_NOT_ERROR(lib_url);
-  Dart_Handle urls = Dart_GetScriptURLs(lib_url);
-  if (Dart_IsError(urls)) {
-    SendError(msg_id, Dart_GetError(urls));
-    return;
-  }
-  ASSERT(Dart_IsList(urls));
-  intptr_t num_urls = 0;
-  Dart_ListLength(urls, &num_urls);
-  msg.Printf("{ \"id\": %d, ", msg_id);
-  msg.Printf("\"result\": { \"urls\": [");
-  for (int i = 0; i < num_urls; i++) {
-    Dart_Handle script_url = Dart_ListGetAt(urls, i);
-    if (i > 0) {
-      msg.Printf(",");
-    }
-    FormatEncodedString(&msg, script_url);
-  }
-  msg.Printf("]}}");
-  SendMsg(&msg);
-}
-
-
-void DebuggerConnectionHandler::HandleGetSourceCmd(const char* json_msg) {
-  int msg_id = msgbuf_->MessageId();
-  dart::TextBuffer msg(64);
-  intptr_t lib_id = msgbuf_->GetIntParam("libraryId");
-  char* url_chars = msgbuf_->GetStringParam("url");
-  ASSERT(url_chars != NULL);
-  Dart_Handle url = Dart_NewString(url_chars);
-  ASSERT_NOT_ERROR(url);
-  free(url_chars);
-  url_chars = NULL;
-  Dart_Handle source = Dart_ScriptGetSource(lib_id, url);
-  if (Dart_IsError(source)) {
-    SendError(msg_id, Dart_GetError(source));
-    return;
-  }
-  msg.Printf("{ \"id\": %d, ", msg_id);
-  msg.Printf("\"result\": { \"text\": ");
-  FormatEncodedString(&msg, source);
-  msg.Printf("}}");
-  SendMsg(&msg);
-}
-
-
-void DebuggerConnectionHandler::HandleGetLibrariesCmd(const char* json_msg) {
-  int msg_id = msgbuf_->MessageId();
-  dart::TextBuffer msg(64);
-  msg.Printf("{ \"id\": %d, \"result\": { \"libraries\": [", msg_id);
-  Dart_Handle lib_ids = Dart_GetLibraryIds();
-  ASSERT_NOT_ERROR(lib_ids);
-  intptr_t num_libs;
-  Dart_Handle res = Dart_ListLength(lib_ids, &num_libs);
-  ASSERT_NOT_ERROR(res);
-  for (int i = 0; i < num_libs; i++) {
-    Dart_Handle lib_id_handle = Dart_ListGetAt(lib_ids, i);
-    ASSERT(Dart_IsInteger(lib_id_handle));
-    int lib_id = GetIntValue(lib_id_handle);
-    Dart_Handle lib_url = Dart_GetLibraryURL(lib_id);
-    ASSERT_NOT_ERROR(lib_url);
-    ASSERT(Dart_IsString(lib_url));
-    msg.Printf("%s{\"id\":%d,\"url\":", (i == 0) ? "" : ", ", lib_id);
-    FormatEncodedString(&msg, lib_url);
-    msg.Printf("}");
-  }
-  msg.Printf("]}}");
-  SendMsg(&msg);
-}
-
-
-static void FormatTextualValue(dart::TextBuffer* buf, Dart_Handle object) {
-  Dart_Handle text;
-  if (Dart_IsNull(object)) {
-    text = Dart_Null();
-  } else {
-    Dart_ExceptionPauseInfo savedState = Dart_GetExceptionPauseInfo();
-
-    // TODO(hausner): Check whether recursive/reentrant pauses on exceptions
-    // should be prevented in Debugger::SignalExceptionThrown() instead.
-    if (savedState != kNoPauseOnExceptions) {
-      Dart_Handle res = Dart_SetExceptionPauseInfo(kNoPauseOnExceptions);
-      ASSERT_NOT_ERROR(res);
-    }
-
-    text = Dart_ToString(object);
-
-    if (savedState != kNoPauseOnExceptions) {
-      Dart_Handle res = Dart_SetExceptionPauseInfo(savedState);
-      ASSERT_NOT_ERROR(res);
-    }
-  }
-  buf->Printf("\"text\":");
-  if (Dart_IsNull(text)) {
-    buf->Printf("null");
-  } else if (Dart_IsError(text)) {
-    FormatErrorMsg(buf, text);
-  } else {
-    FormatEncodedString(buf, text);
-  }
-}
-
-
-static void FormatValue(dart::TextBuffer* buf, Dart_Handle object) {
-  if (Dart_IsInteger(object)) {
-    buf->Printf("\"kind\":\"integer\",");
-  } else if (Dart_IsString(object)) {
-    buf->Printf("\"kind\":\"string\",");
-  } else if (Dart_IsBoolean(object)) {
-    buf->Printf("\"kind\":\"boolean\",");
-  } else if (Dart_IsList(object)) {
-    intptr_t len = 0;
-    Dart_Handle res = Dart_ListLength(object, &len);
-    ASSERT_NOT_ERROR(res);
-    buf->Printf("\"kind\":\"list\",\"length\":%"Pd",", len);
-  } else {
-    buf->Printf("\"kind\":\"object\",");
-  }
-  FormatTextualValue(buf, object);
-}
-
-
-static void FormatValueObj(dart::TextBuffer* buf, Dart_Handle object) {
-  buf->Printf("{");
-  FormatValue(buf, object);
-  buf->Printf("}");
-}
-
-
-static void FormatRemoteObj(dart::TextBuffer* buf, Dart_Handle object) {
-  intptr_t obj_id = Dart_CacheObject(object);
-  ASSERT(obj_id >= 0);
-  buf->Printf("{\"objectId\":%"Pd",", obj_id);
-  FormatValue(buf, object);
-  buf->Printf("}");
-}
-
-
-static void FormatNamedValue(dart::TextBuffer* buf,
-                             Dart_Handle object_name,
-                             Dart_Handle object) {
-  ASSERT(Dart_IsString(object_name));
-  buf->Printf("{\"name\":\"%s\",", GetStringChars(object_name));
-  buf->Printf("\"value\":");
-  FormatRemoteObj(buf, object);
-  buf->Printf("}");
-}
-
-
-static void FormatNamedValueList(dart::TextBuffer* buf,
-                                 Dart_Handle obj_list) {
-  ASSERT(Dart_IsList(obj_list));
-  intptr_t list_length = 0;
-  Dart_Handle res = Dart_ListLength(obj_list, &list_length);
-  ASSERT_NOT_ERROR(res);
-  ASSERT(list_length % 2 == 0);
-  buf->Printf("[");
-  for (int i = 0; i + 1 < list_length; i += 2) {
-    Dart_Handle name_handle = Dart_ListGetAt(obj_list, i);
-    ASSERT_NOT_ERROR(name_handle);
-    Dart_Handle value_handle = Dart_ListGetAt(obj_list, i + 1);
-    ASSERT_NOT_ERROR(value_handle);
-    if (i > 0) {
-      buf->Printf(",");
-    }
-    FormatNamedValue(buf, name_handle, value_handle);
-  }
-  buf->Printf("]");
-}
-
-
-static const char* FormatClassProps(dart::TextBuffer* buf,
-                                    intptr_t cls_id) {
-  Dart_Handle name, static_fields;
-  intptr_t super_id = -1;
-  intptr_t library_id = -1;
-  Dart_Handle res =
-      Dart_GetClassInfo(cls_id, &name, &library_id, &super_id, &static_fields);
-  RETURN_IF_ERROR(res);
-  RETURN_IF_ERROR(name);
-  buf->Printf("{\"name\":\"%s\",", GetStringChars(name));
-  if (super_id > 0) {
-    buf->Printf("\"superclassId\":%"Pd",", super_id);
-  }
-  buf->Printf("\"libraryId\":%"Pd",", library_id);
-  RETURN_IF_ERROR(static_fields);
-  buf->Printf("\"fields\":");
-  FormatNamedValueList(buf, static_fields);
-  buf->Printf("}");
-  return NULL;
-}
-
-
-static const char* FormatLibraryProps(dart::TextBuffer* buf,
-                                      intptr_t lib_id) {
-  Dart_Handle url = Dart_GetLibraryURL(lib_id);
-  RETURN_IF_ERROR(url);
-  buf->Printf("{\"url\":");
-  FormatEncodedString(buf, url);
-
-  // Whether debugging is enabled.
-  bool is_debuggable = false;
-  Dart_Handle res = Dart_GetLibraryDebuggable(lib_id, &is_debuggable);
-  RETURN_IF_ERROR(res);
-  buf->Printf(",\"debuggingEnabled\":%s",
-              is_debuggable ? "\"true\"" : "\"false\"");
-
-  // Imports and prefixes.
-  Dart_Handle import_list = Dart_GetLibraryImports(lib_id);
-  RETURN_IF_ERROR(import_list);
-  ASSERT(Dart_IsList(import_list));
-  intptr_t list_length = 0;
-  res = Dart_ListLength(import_list, &list_length);
-  RETURN_IF_ERROR(res);
-  buf->Printf(",\"imports\":[");
-  for (int i = 0; i + 1 < list_length; i += 2) {
-    Dart_Handle lib_id = Dart_ListGetAt(import_list, i + 1);
-    ASSERT_NOT_ERROR(lib_id);
-    buf->Printf("%s{\"libraryId\":%d,",
-                (i > 0) ? ",": "",
-                GetIntValue(lib_id));
-
-    Dart_Handle name = Dart_ListGetAt(import_list, i);
-    ASSERT_NOT_ERROR(name);
-    buf->Printf("\"prefix\":\"%s\"}",
-                Dart_IsNull(name) ? "" : GetStringChars(name));
-  }
-  buf->Printf("],");
-
-  // Global variables in the library.
-  Dart_Handle global_vars = Dart_GetLibraryFields(lib_id);
-  RETURN_IF_ERROR(global_vars);
-  buf->Printf("\"globals\":");
-  FormatNamedValueList(buf, global_vars);
-  buf->Printf("}");
-  return NULL;
-}
-
-
-static const char* FormatObjProps(dart::TextBuffer* buf,
-                                  Dart_Handle object) {
-  intptr_t class_id;
-  if (Dart_IsNull(object)) {
-    buf->Printf("{\"classId\":-1,\"fields\":[]}");
-    return NULL;
-  }
-  Dart_Handle res = Dart_GetObjClassId(object, &class_id);
-  RETURN_IF_ERROR(res);
-  buf->Printf("{\"classId\": %"Pd",", class_id);
-  buf->Printf("\"kind\":\"object\",\"fields\":");
-  Dart_Handle fields = Dart_GetInstanceFields(object);
-  RETURN_IF_ERROR(fields);
-  FormatNamedValueList(buf, fields);
-  buf->Printf("}");
-  return NULL;
-}
-
-
-static const char* FormatListSlice(dart::TextBuffer* buf,
-                                   Dart_Handle list,
-                                   intptr_t list_length,
-                                   intptr_t index,
-                                   intptr_t slice_length) {
-  intptr_t end_index = index + slice_length;
-  ASSERT(end_index <= list_length);
-  buf->Printf("{\"index\":%"Pd",", index);
-  buf->Printf("\"length\":%"Pd",", slice_length);
-  buf->Printf("\"elements\":[");
-  for (intptr_t i = index; i < end_index; i++) {
-    Dart_Handle value = Dart_ListGetAt(list, i);
-    if (i > index) {
-      buf->Printf(",");
-    }
-    FormatValueObj(buf, value);
-  }
-  buf->Printf("]}");
-  return NULL;
-}
-
-
-static void FormatCallFrames(dart::TextBuffer* msg, Dart_StackTrace trace) {
-  intptr_t trace_len = 0;
-  Dart_Handle res = Dart_StackTraceLength(trace, &trace_len);
-  ASSERT_NOT_ERROR(res);
-  msg->Printf("\"callFrames\" : [ ");
-  for (int i = 0; i < trace_len; i++) {
-    Dart_ActivationFrame frame;
-    res = Dart_GetActivationFrame(trace, i, &frame);
-    ASSERT_NOT_ERROR(res);
-    Dart_Handle func_name;
-    Dart_Handle script_url;
-    intptr_t line_number = 0;
-    intptr_t library_id = 0;
-    res = Dart_ActivationFrameInfo(
-        frame, &func_name, &script_url, &line_number, &library_id);
-    ASSERT_NOT_ERROR(res);
-    ASSERT(Dart_IsString(func_name));
-    msg->Printf("%s{\"functionName\":", (i > 0) ? "," : "");
-    FormatEncodedString(msg, func_name);
-    msg->Printf(",\"libraryId\": %"Pd",", library_id);
-
-    ASSERT(Dart_IsString(script_url));
-    msg->Printf("\"location\": { \"url\":");
-    FormatEncodedString(msg, script_url);
-    msg->Printf(",\"lineNumber\":%"Pd"},", line_number);
-
-    Dart_Handle locals = Dart_GetLocalVariables(frame);
-    ASSERT_NOT_ERROR(locals);
-    msg->Printf("\"locals\":");
-    FormatNamedValueList(msg, locals);
-    msg->Printf("}");
-  }
-  msg->Printf("]");
-}
-
-
-void DebuggerConnectionHandler::HandleGetStackTraceCmd(const char* json_msg) {
-  int msg_id = msgbuf_->MessageId();
-  Dart_StackTrace trace;
-  Dart_Handle res = Dart_GetStackTrace(&trace);
-  ASSERT_NOT_ERROR(res);
-  dart::TextBuffer msg(128);
-  msg.Printf("{ \"id\": %d, \"result\": {", msg_id);
-  FormatCallFrames(&msg, trace);
-  msg.Printf("}}");
-  SendMsg(&msg);
-}
-
-
-void DebuggerConnectionHandler::HandleSetBpCmd(const char* json_msg) {
-  int msg_id = msgbuf_->MessageId();
-  char* url_chars = msgbuf_->GetStringParam("url");
-  ASSERT(url_chars != NULL);
-  Dart_Handle url = Dart_NewString(url_chars);
-  ASSERT_NOT_ERROR(url);
-  free(url_chars);
-  url_chars = NULL;
-  intptr_t line_number = msgbuf_->GetIntParam("line");
-  Dart_Handle bp_id = Dart_SetBreakpoint(url, line_number);
-  if (Dart_IsError(bp_id)) {
-    SendError(msg_id, Dart_GetError(bp_id));
-    return;
-  }
-  ASSERT(Dart_IsInteger(bp_id));
-  uint64_t bp_id_value;
-  Dart_Handle res = Dart_IntegerToUint64(bp_id, &bp_id_value);
-  ASSERT_NOT_ERROR(res);
-  dart::TextBuffer msg(64);
-  msg.Printf("{ \"id\": %d, \"result\": { \"breakpointId\": %"Pu64" }}",
-             msg_id, bp_id_value);
-  SendMsg(&msg);
-}
-
-
-void DebuggerConnectionHandler::HandlePauseOnExcCmd(const char* json_msg) {
-  int msg_id = msgbuf_->MessageId();
-  char* exc_chars = msgbuf_->GetStringParam("exceptions");
-  Dart_ExceptionPauseInfo info = kNoPauseOnExceptions;
-  if (strcmp(exc_chars, "none") == 0) {
-    info = kNoPauseOnExceptions;
-  } else if (strcmp(exc_chars, "all") == 0) {
-    info = kPauseOnAllExceptions;
-  } else if (strcmp(exc_chars, "unhandled") == 0) {
-    info = kPauseOnUnhandledExceptions;
-  } else {
-    SendError(msg_id, "illegal value for parameter 'exceptions'");
-    return;
-  }
-  Dart_Handle res = Dart_SetExceptionPauseInfo(info);
-  ASSERT_NOT_ERROR(res);
-  dart::TextBuffer msg(32);
-  msg.Printf("{ \"id\": %d }", msg_id);
-  SendMsg(&msg);
-}
-
-
-void DebuggerConnectionHandler::HandleRemBpCmd(const char* json_msg) {
-  int msg_id = msgbuf_->MessageId();
-  int bpt_id = msgbuf_->GetIntParam("breakpointId");
-  Dart_Handle res = Dart_RemoveBreakpoint(bpt_id);
-  if (Dart_IsError(res)) {
-    SendError(msg_id, Dart_GetError(res));
-    return;
-  }
-  dart::TextBuffer msg(32);
-  msg.Printf("{ \"id\": %d }", msg_id);
-  SendMsg(&msg);
-}
-
-
-void DebuggerConnectionHandler::HandleGetObjPropsCmd(const char* json_msg) {
-  int msg_id = msgbuf_->MessageId();
-  intptr_t obj_id = msgbuf_->GetIntParam("objectId");
-  Dart_Handle obj = Dart_GetCachedObject(obj_id);
-  if (Dart_IsError(obj)) {
-    SendError(msg_id, Dart_GetError(obj));
-    return;
-  }
-  dart::TextBuffer msg(64);
-  msg.Printf("{\"id\":%d, \"result\":", msg_id);
-  const char* err = FormatObjProps(&msg, obj);
-  if (err != NULL) {
-    SendError(msg_id, err);
-    return;
-  }
-  msg.Printf("}");
-  SendMsg(&msg);
-}
-
-
-void DebuggerConnectionHandler::HandleGetListCmd(const char* json_msg) {
-  const intptr_t kDefaultSliceLength = 100;
-  int msg_id = msgbuf_->MessageId();
-  intptr_t obj_id = msgbuf_->GetIntParam("objectId");
-  Dart_Handle list = Dart_GetCachedObject(obj_id);
-  if (Dart_IsError(list)) {
-    SendError(msg_id, Dart_GetError(list));
-    return;
-  }
-  if (!Dart_IsList(list)) {
-    SendError(msg_id, "object is not a list");
-    return;
-  }
-  intptr_t list_length = 0;
-  Dart_Handle res = Dart_ListLength(list, &list_length);
-  if (Dart_IsError(res)) {
-    SendError(msg_id, Dart_GetError(res));
-    return;
-  }
-
-  intptr_t index = msgbuf_->GetIntParam("index");
-  if (index < 0) {
-    index = 0;
-  } else if (index > list_length) {
-    index = list_length;
-  }
-
-  // If no slice length is given, get only one element. If slice length
-  // is given as 0, get entire list.
-  intptr_t slice_length = msgbuf_->GetOptIntParam("length", 1);
-  if (slice_length == 0) {
-    slice_length = list_length - index;
-  }
-  if ((index + slice_length) > list_length) {
-    slice_length = list_length - index;
-  }
-  ASSERT(slice_length >= 0);
-  if (slice_length > kDefaultSliceLength) {
-    slice_length = kDefaultSliceLength;
-  }
-  dart::TextBuffer msg(64);
-  msg.Printf("{\"id\":%d, \"result\":", msg_id);
-  if (slice_length == 1) {
-    Dart_Handle value = Dart_ListGetAt(list, index);
-    FormatRemoteObj(&msg, value);
-  } else {
-    FormatListSlice(&msg, list, list_length, index, slice_length);
-  }
-  msg.Printf("}");
-  SendMsg(&msg);
-}
-
-
-void DebuggerConnectionHandler::HandleGetClassPropsCmd(const char* json_msg) {
-  int msg_id = msgbuf_->MessageId();
-  intptr_t cls_id = msgbuf_->GetIntParam("classId");
-  dart::TextBuffer msg(64);
-  msg.Printf("{\"id\":%d, \"result\":", msg_id);
-  const char* err = FormatClassProps(&msg, cls_id);
-  if (err != NULL) {
-    SendError(msg_id, err);
-    return;
-  }
-  msg.Printf("}");
-  SendMsg(&msg);
-}
-
-
-void DebuggerConnectionHandler::HandleGetLibPropsCmd(const char* json_msg) {
-  int msg_id = msgbuf_->MessageId();
-  intptr_t lib_id = msgbuf_->GetIntParam("libraryId");
-  dart::TextBuffer msg(64);
-  msg.Printf("{\"id\":%d, \"result\":", msg_id);
-  const char* err = FormatLibraryProps(&msg, lib_id);
-  if (err != NULL) {
-    SendError(msg_id, err);
-    return;
-  }
-  msg.Printf("}");
-  SendMsg(&msg);
-}
-
-
-void DebuggerConnectionHandler::HandleSetLibPropsCmd(const char* json_msg) {
-  int msg_id = msgbuf_->MessageId();
-  intptr_t lib_id = msgbuf_->GetIntParam("libraryId");
-  const char* enable_request = msgbuf_->GetStringParam("debuggingEnabled");
-  bool enable;
-  if (strcmp(enable_request, "true") == 0) {
-    enable = true;
-  } else if (strcmp(enable_request, "false") == 0) {
-    enable = false;
-  } else {
-    SendError(msg_id, "illegal argument for 'debuggingEnabled'");
-    return;
-  }
-  Dart_Handle res = Dart_SetLibraryDebuggable(lib_id, enable);
-  if (Dart_IsError(res)) {
-    SendError(msg_id, Dart_GetError(res));
-    return;
-  }
-  bool enabled = false;
-  res = Dart_GetLibraryDebuggable(lib_id, &enabled);
-  if (Dart_IsError(res)) {
-    SendError(msg_id, Dart_GetError(res));
-    return;
-  }
-  dart::TextBuffer msg(64);
-  msg.Printf("{\"id\":%d, \"result\": {\"debuggingEnabled\": \"%s\"}}",
-             msg_id,
-             enabled ? "true" : "false");
-  SendMsg(&msg);
-}
-
-
-void DebuggerConnectionHandler::HandleGetGlobalsCmd(const char* json_msg) {
-  int msg_id = msgbuf_->MessageId();
-  intptr_t lib_id = msgbuf_->GetIntParam("libraryId");
-  dart::TextBuffer msg(64);
-  msg.Printf("{\"id\":%d, \"result\": { \"globals\":", msg_id);
-  Dart_Handle globals = Dart_GetGlobalVariables(lib_id);
-  ASSERT_NOT_ERROR(globals);
-  FormatNamedValueList(&msg, globals);
-  msg.Printf("}}");
-  SendMsg(&msg);
-}
-
-
-void DebuggerConnectionHandler::HandleUnknownMsg(const char* json_msg) {
-  int msg_id = msgbuf_->MessageId();
-  ASSERT(msg_id >= 0);
-  SendError(msg_id, "unknown debugger command");
-}
-
-
 void DebuggerConnectionHandler::HandleMessages() {
-  static JSONDebuggerCommand debugger_commands[] = {
-    { "resume", HandleResumeCmd },
-    { "getLibraries", HandleGetLibrariesCmd },
-    { "getClassProperties", HandleGetClassPropsCmd },
-    { "getLibraryProperties", HandleGetLibPropsCmd },
-    { "setLibraryProperties", HandleSetLibPropsCmd },
-    { "getObjectProperties", HandleGetObjPropsCmd },
-    { "getListElements", HandleGetListCmd },
-    { "getGlobalVariables", HandleGetGlobalsCmd },
-    { "getScriptURLs", HandleGetScriptURLsCmd },
-    { "getScriptSource", HandleGetSourceCmd },
-    { "getStackTrace", HandleGetStackTraceCmd },
-    { "setBreakpoint", HandleSetBpCmd },
-    { "setPauseOnException", HandlePauseOnExcCmd },
-    { "removeBreakpoint", HandleRemBpCmd },
-    { "stepInto", HandleStepIntoCmd },
-    { "stepOut", HandleStepOutCmd },
-    { "stepOver", HandleStepOverCmd },
+  static JSONDebuggerCommand generic_debugger_commands[] = {
+    { "interrupt", HandleInterruptCmd },
+    { "isolates", HandleIsolatesListCmd },
+    { "quit", HandleQuitCmd },
     { NULL, NULL }
   };
 
   for (;;) {
-    SendQueuedMsgs();
+    // Read a message.
     while (!msgbuf_->IsValidMessage() && msgbuf_->Alive()) {
       msgbuf_->ReadData();
     }
     if (!msgbuf_->Alive()) {
       return;
     }
+
+    // Parse out the command portion from the message.
     dart::JSONReader r(msgbuf_->buf());
     bool found = r.Seek("command");
     if (r.Error()) {
@@ -941,165 +202,217 @@
       printf("'command' not found in JSON message: '%s'\n", msgbuf_->buf());
       msgbuf_->PopMessage();
     }
+
+    // Check if this is a generic command (not isolate specific).
     int i = 0;
     bool is_handled = false;
-    request_resume_ = false;
-    while (debugger_commands[i].cmd_string != NULL) {
-      if (r.IsStringLiteral(debugger_commands[i].cmd_string)) {
+    while (generic_debugger_commands[i].cmd_string != NULL) {
+      if (r.IsStringLiteral(generic_debugger_commands[i].cmd_string)) {
+        (*generic_debugger_commands[i].handler_function)(this);
         is_handled = true;
-        (*debugger_commands[i].handler_function)(msgbuf_->buf());
         msgbuf_->PopMessage();
-        if (request_resume_) {
-          return;
-        }
         break;
       }
       i++;
     }
     if (!is_handled) {
+      // Check if this is an isolate specific command.
+      int32_t cmd_idx = DbgMessageQueue::LookupIsolateCommand(r.ValueChars(),
+                                                              r.ValueLen());
+      if (cmd_idx != DbgMessageQueue::kInvalidCommand) {
+        // Get debug message queue corresponding to isolate.
+        // TODO(asiva): Need to read the isolate id, map it to the appropriate
+        // isolate and pass it down to GetIsolateMessageQueue to get the
+        // appropriate debug message queue.
+        DbgMessageQueue* queue = DbgMessageQueue::GetIsolateMessageQueue(NULL);
+        ASSERT(queue != NULL);
+        queue->AddMessage(cmd_idx, msgbuf_->buf(), r.EndOfObject(), debug_fd_);
+        msgbuf_->PopMessage();
+        continue;
+      }
+
+      // This is an unrecognized command, report error and move on to next.
       printf("unrecognized command received: '%s'\n", msgbuf_->buf());
-      HandleUnknownMsg(msgbuf_->buf());
+      HandleUnknownMsg();
       msgbuf_->PopMessage();
     }
   }
 }
 
 
+void DebuggerConnectionHandler::SendError(int debug_fd,
+                                          int msg_id,
+                                          const char* err_msg) {
+  dart::TextBuffer msg(64);
+  msg.Printf("{\"id\": %d, \"error\": \"Error: %s\"}", msg_id, err_msg);
+  SendMsg(debug_fd, &msg);
+}
+
+
+void DebuggerConnectionHandler::CloseDbgConnection() {
+  if (debug_fd_ >= 0) {
+    // TODO(hausner): need a Socket::Close() function.
+  }
+  if (msgbuf_ != NULL) {
+    delete msgbuf_;
+    msgbuf_ = NULL;
+  }
+  // TODO(hausner): Need to tell the VM debugger object to remove all
+  // breakpoints.
+}
+
+
+void DebuggerConnectionHandler::StartHandler(const char* address,
+                                             int port_number) {
+  MonitorLocker ml(&handler_lock_);
+  if (listener_fd_ != -1) {
+    return;  // The debugger connection handler was already started.
+  }
+
+  // First setup breakpoint, exception and delayed breakpoint handlers.
+  DbgMessageQueue::Initialize();
+
+  // Now setup a listener socket and start a thread which will
+  // listen, accept connections from debuggers, read and handle/dispatch
+  // debugger commands received on these connections.
+  ASSERT(listener_fd_ == -1);
+  listener_fd_ = ServerSocket::CreateBindListen(address, port_number, 1);
+  DebuggerConnectionImpl::StartHandler(port_number);
+}
+
+
 void DebuggerConnectionHandler::WaitForConnection() {
-  MonitorLocker ml(&is_connected_);
+  MonitorLocker ml(&handler_lock_);
   while (!IsConnected()) {
-    dart::Monitor::WaitResult res = ml.Wait(dart::Monitor::kNoTimeout);
+    dart::Monitor::WaitResult res = ml.Wait();
     ASSERT(res == dart::Monitor::kNotified);
   }
 }
 
 
-void DebuggerConnectionHandler::SendBreakpointEvent(Dart_StackTrace trace) {
-  dart::TextBuffer msg(128);
-  msg.Printf("{ \"event\": \"paused\", \"params\": { ");
-  msg.Printf("\"reason\": \"breakpoint\", ");
-  FormatCallFrames(&msg, trace);
-  msg.Printf("}}");
-  SendMsg(&msg);
+void DebuggerConnectionHandler::SendMsg(int debug_fd, dart::TextBuffer* msg) {
+  MonitorLocker ml(&handler_lock_);
+  SendMsgHelper(debug_fd, msg);
 }
 
 
-void DebuggerConnectionHandler::BreakpointHandler(Dart_Breakpoint bpt,
-                                                  Dart_StackTrace trace) {
-  WaitForConnection();
-  Dart_EnterScope();
-  SendQueuedMsgs();
-  SendBreakpointEvent(trace);
-  HandleMessages();
-  if (!msgbuf_->Alive()) {
-    CloseDbgConnection();
-  }
-  Dart_ExitScope();
+void DebuggerConnectionHandler::BroadcastMsg(dart::TextBuffer* msg) {
+  MonitorLocker ml(&handler_lock_);
+  // TODO(asiva): Once we support connection to multiple debuggers
+  // we need to send the message to all of them.
+  ASSERT(singleton_handler != NULL);
+  SendMsgHelper(singleton_handler->debug_fd(), msg);
 }
 
 
-void DebuggerConnectionHandler::IsolateEventHandler(Dart_Isolate isolate,
-                                                    Dart_IsolateEvent kind) {
-  WaitForConnection();
-#if 0
-  if (kind == kCreated) {
-    printf("Isolate created %p\n", isolate);
-  } else if (kind == kInterrupted) {
-    printf("Isolate interrupted %p\n", isolate);
-  } else if (kind == kShutdown) {
-    printf("Isolate shutdown %p\n", isolate);
+void DebuggerConnectionHandler::SendMsgHelper(int debug_fd,
+                                              dart::TextBuffer* msg) {
+  ASSERT(debug_fd >= 0);
+  ASSERT(IsValidJSON(msg->buf()));
+  // Sending messages in short pieces can be used to stress test the
+  // debugger front-end's message handling code.
+  const bool send_in_pieces = false;
+  if (send_in_pieces) {
+    intptr_t remaining = msg->length();
+    intptr_t sent = 0;
+    const intptr_t max_piece_len = 122;  // Pretty arbitrary, not a power of 2.
+    dart::Monitor sleep;
+    while (remaining > 0) {
+      intptr_t piece_len = remaining;
+      if (piece_len > max_piece_len) {
+        piece_len = max_piece_len;
+      }
+      intptr_t written =
+        Socket::Write(debug_fd, msg->buf() + sent, piece_len);
+      ASSERT(written == piece_len);
+      sent += written;
+      remaining -= written;
+      // Wait briefly so the OS does not coalesce message fragments.
+      {
+        MonitorLocker ml(&sleep);
+        ml.Wait(10);
+      }
+    }
+    return;
   }
-#endif
+  intptr_t bytes_written = Socket::Write(debug_fd, msg->buf(), msg->length());
+  ASSERT(msg->length() == bytes_written);
+  // TODO(hausner): Error checking. Probably just shut down the debugger
+  // session if we there is an error while writing.
 }
 
 
-void DebuggerConnectionHandler::SendExceptionEvent(
-                                    Dart_Handle exception,
-                                    Dart_StackTrace stack_trace) {
-  intptr_t exception_id = Dart_CacheObject(exception);
-  ASSERT(exception_id >= 0);
-  dart::TextBuffer msg(128);
-  msg.Printf("{ \"event\": \"paused\", \"params\": {");
-  msg.Printf("\"reason\": \"exception\", ");
-  msg.Printf("\"exception\":");
-  FormatRemoteObj(&msg, exception);
-  msg.Printf(", ");
-  FormatCallFrames(&msg, stack_trace);
-  msg.Printf("}}");
-  SendMsg(&msg);
+void DebuggerConnectionHandler::AcceptDbgConnection(int debug_fd) {
+  AddNewDebuggerConnection(debug_fd);
+  {
+    MonitorLocker ml(&handler_lock_);
+    ml.NotifyAll();
+  }
+  // TODO(asiva): Once we implement support for multiple connections
+  // we should have a different callback for wakeups on fds which
+  // are not the listener_fd_.
+  // In that callback we would lookup the handler object
+  // corresponding to that fd and invoke HandleMessages on it.
+  // For now we run that code here.
+  DebuggerConnectionHandler* handler = GetDebuggerConnectionHandler(debug_fd);
+  if (handler != NULL) {
+    handler->HandleMessages();
+    delete handler;
+  }
 }
 
 
-void DebuggerConnectionHandler::ExceptionThrownHandler(
-                                    Dart_Handle exception,
-                                    Dart_StackTrace stack_trace) {
-  WaitForConnection();
-  Dart_EnterScope();
-  SendQueuedMsgs();
-  SendExceptionEvent(exception, stack_trace);
-  HandleMessages();
-  if (!msgbuf_->Alive()) {
-    CloseDbgConnection();
-  }
-  Dart_ExitScope();
+void DebuggerConnectionHandler::HandleInterruptCmd(
+    DebuggerConnectionHandler* handler) {
+  int msg_id = handler->MessageId();
+  ASSERT(msg_id >= 0);
+  SendError(handler->debug_fd(), msg_id, "interrupt command unimplemented");
 }
 
 
-void DebuggerConnectionHandler::BptResolvedHandler(intptr_t bp_id,
-                                                   Dart_Handle url,
-                                                   intptr_t line_number) {
-  Dart_EnterScope();
-  dart::TextBuffer msg(128);
-  msg.Printf("{ \"event\": \"breakpointResolved\", \"params\": {");
-  msg.Printf("\"breakpointId\": %"Pd", \"url\":", bp_id);
-  FormatEncodedString(&msg, url);
-  msg.Printf(",\"line\": %"Pd" }}", line_number);
-  QueueMsg(&msg);
-  Dart_ExitScope();
+void DebuggerConnectionHandler::HandleIsolatesListCmd(
+    DebuggerConnectionHandler* handler) {
+  int msg_id = handler->MessageId();
+  ASSERT(msg_id >= 0);
+  SendError(handler->debug_fd(), msg_id, "isolate list command unimplemented");
 }
 
 
-void DebuggerConnectionHandler::AcceptDbgConnection(int debugger_fd) {
-  debugger_fd_ = debugger_fd;
-  ASSERT(msgbuf_ == NULL);
-  msgbuf_ = new MessageBuffer(debugger_fd_);
-  {
-    MonitorLocker ml(&is_connected_);
-    ml.Notify();
-  }
+void DebuggerConnectionHandler::HandleQuitCmd(
+    DebuggerConnectionHandler* handler) {
+  int msg_id = handler->MessageId();
+  ASSERT(msg_id >= 0);
+  SendError(handler->debug_fd(), msg_id, "quit command unimplemented");
 }
 
 
-void DebuggerConnectionHandler::CloseDbgConnection() {
-  if (debugger_fd_ >= 0) {
-    // TODO(hausner): need a Socket::Close() function.
-  }
-  if (msgbuf_ != NULL) {
-    delete msgbuf_;
-    msgbuf_ = NULL;
-  }
-  // TODO(hausner): Need to tell the VM debugger object to remove all
-  // breakpoints.
+void DebuggerConnectionHandler::AddNewDebuggerConnection(int debug_fd) {
+  // TODO(asiva): Support multiple debugger connections, for now we just
+  // create one handler, store it in a static variable and use it.
+  ASSERT(singleton_handler == NULL);
+  singleton_handler = new DebuggerConnectionHandler(debug_fd);
 }
 
 
-void DebuggerConnectionHandler::StartHandler(const char* address,
-                                             int port_number) {
-  if (handler_started_) {
-    return;
-  }
-  ASSERT(listener_fd_ == -1);
-  listener_fd_ = ServerSocket::CreateBindListen(address, port_number, 1);
+void DebuggerConnectionHandler::RemoveDebuggerConnection(int debug_fd) {
+  // TODO(asiva): Support multiple debugger connections, for now we just
+  // set the static handler back to NULL.
+  ASSERT(singleton_handler != NULL);
+  singleton_handler = NULL;
+}
 
-  handler_started_ = true;
-  DebuggerConnectionImpl::StartHandler(port_number);
-  Dart_SetIsolateEventHandler(IsolateEventHandler);
-  Dart_SetBreakpointHandler(BreakpointHandler);
-  Dart_SetBreakpointResolvedHandler(BptResolvedHandler);
-  Dart_SetExceptionThrownHandler(ExceptionThrownHandler);
+
+DebuggerConnectionHandler*
+DebuggerConnectionHandler::GetDebuggerConnectionHandler(int debug_fd) {
+  // TODO(asiva): Support multiple debugger connections, for now we just
+  // return the one static handler that was created.
+  ASSERT(singleton_handler != NULL);
+  return singleton_handler;
 }
 
 
-DebuggerConnectionHandler::~DebuggerConnectionHandler() {
-  CloseDbgConnection();
+bool DebuggerConnectionHandler::IsConnected() {
+  // TODO(asiva): Support multiple debugger connections.
+  // Return true if a connection has been established.
+  return singleton_handler != NULL;
 }