Add --warn-on-pause-with-no-debugger flag. Add this to --observe behavior.

runtime/vm/service.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/service.h"
 
 #include "include/dart_api.h"
 #include "include/dart_native_api.h"
 #include "platform/globals.h"
 
@@ skipping 29 matching lines @@
 
 #define Z (T->zone())
 
 
 DECLARE_FLAG(bool, trace_service);
 DECLARE_FLAG(bool, trace_service_pause_events);
 DEFINE_FLAG(charp, vm_name, "vm",
             "The default name of this vm as reported by the VM service "
             "protocol");
 
+DEFINE_FLAG(bool, warn_on_pause_with_no_debugger, false,
+            "Print a message when an isolate is paused but there is no "
+            "debugger attached.");
+
 // The name of this of this vm as reported by the VM service protocol.
 static char* vm_name = NULL;
 
 
 static const char* GetVMName() {
   if (vm_name == NULL) {
     return FLAG_vm_name;
   }
   return vm_name;
 }
@@ skipping 855 matching lines @@
   {
     NoSafepointScope no_safepoint;
     memmove(message.DataAddr(offset), data, size);
     offset += size;
   }
   ASSERT(offset == total_bytes);
   SendEvent(stream_id, event_type, message);
 }
 
 
+static void ReportPauseOnConsole(ServiceEvent* event) {
+  const char* name = event->isolate()->debugger_name();
+  switch (event->kind())  {
+    case ServiceEvent::kPauseStart:
+      OS::PrintErr(
+          "vm-service: isolate '%s' has no debugger attached and is paused at "
+          "start. Connect to Observatory to debug.\n", name);
+      break;
+    case ServiceEvent::kPauseExit:
+      OS::PrintErr(
+          "vm-service: isolate '%s' has no debugger attached and is paused at "
+          "exit. Connect to Observatory to debug.\n", name);
+      break;
+    case ServiceEvent::kPauseException:
+      OS::PrintErr(
+          "vm-service: isolate '%s' has no debugger attached and is paused due "
+          "to exception. Connect to Observatory to debug.\n", name);
+      break;
+    case ServiceEvent::kPauseInterrupted:
+      OS::PrintErr(
+          "vm-service: isolate '%s' has no debugger attached and is paused due "
+          "to interrupt. Connect to Observatory to debug.\n", name);
+      break;
+    case ServiceEvent::kPauseBreakpoint:
+      OS::PrintErr(
+          "vm-service: isolate '%s' has no debugger attached and is paused. "
+          "Connect to Observatory to debug.\n", name);
+      break;
+    default:
+      UNREACHABLE();
+      break;
+  }
+}
+
+
 void Service::HandleEvent(ServiceEvent* event) {
   if (event->isolate() != NULL &&
       ServiceIsolate::IsServiceIsolateDescendant(event->isolate())) {
     return;
   }
+  if (FLAG_warn_on_pause_with_no_debugger &&
+      event->IsPause() && !Service::debug_stream.enabled()) {
+    // If we are about to pause a running program which has no
+    // debugger connected, tell the user about it.
+    ReportPauseOnConsole(event);
+  }
   if (!ServiceIsolate::IsRunning()) {
     return;
   }
   JSONStream js;
   const char* stream_id = event->stream_id();
   ASSERT(stream_id != NULL);
   {
     JSONObject jsobj(&js);
     jsobj.AddProperty("jsonrpc", "2.0");
     jsobj.AddProperty("method", "streamNotify");
@@ skipping 2926 matching lines @@
     const ServiceMethodDescriptor& method = service_methods_[i];
     if (strcmp(method_name, method.name) == 0) {
       return &method;
     }
   }
   return NULL;
 }
 
 
 }  // namespace dart