Add an explicit way of making IDs local or global

base/trace_event/trace_event_impl.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/trace_event/trace_event_impl.h"
 
 #include <stddef.h>
 
 #include "base/format_macros.h"
 #include "base/json/string_escape.h"
@@ skipping 340 matching lines @@
     StringAppendF(out, ",\"tts\":%" PRId64, thread_time_int64);
   }
 
   // Output async tts marker field if flag is set.
   if (flags_ & TRACE_EVENT_FLAG_ASYNC_TTS) {
     StringAppendF(out, ", \"use_async_tts\":1");
   }
 
   // If id_ is set, print it out as a hex string so we don't loose any
   // bits (it might be a 64-bit pointer).
-  if (flags_ & TRACE_EVENT_FLAG_HAS_ID) {
+  unsigned int id_flags = flags_ & TRACE_EVENT_FLAG_ID_MASK;
+  if (id_flags != TRACE_EVENT_FLAG_NONE) {
     if (scope_ != trace_event_internal::kGlobalScope)
       StringAppendF(out, ",\"scope\":\"%s\"", scope_);
     StringAppendF(out, ",\"id\":\"0x%" PRIx64 "\"", static_cast<uint64_t>(id_));
+    if (id_flags == TRACE_EVENT_FLAG_HAS_LOCAL_ID)
+      *out += ",\"id_is_global\":false";

[caseq, 2016/08/22 19:48:52]

rather than emitting it as a bool flag, can we vary the field name instead? I.e.
only emit "id" if the id is global and call it "localId" otherwise for async
events?

[chiniforooshan, 2016/08/23 19:54:53]

Is there anything wrong with using a boolean here? Without the boolean, wouldn't
the "id" field be ambiguous for importers? It can mean "global ID" if the trace
is new or have a type-related scope if the trace is old (and there is no way of
finding out which one it is).

+    if (id_flags == TRACE_EVENT_FLAG_HAS_GLOBAL_ID)
+      *out += ",\"id_is_global\":true";
   }
 
   if (flags_ & TRACE_EVENT_FLAG_BIND_TO_ENCLOSING)
     StringAppendF(out, ",\"bp\":\"e\"");
 
   if ((flags_ & TRACE_EVENT_FLAG_FLOW_OUT) ||
       (flags_ & TRACE_EVENT_FLAG_FLOW_IN) ||
       phase_ == TRACE_EVENT_PHASE_BIND_IDS) {
     StringAppendF(out, ",\"bind_id\":\"0x%" PRIx64 "\"",
                   static_cast<uint64_t>(bind_id_));
@@ skipping 43 matching lines @@
         AppendValueAsJSON(arg_types_[i], arg_values_[i], &value_as_text);
 
       *out << value_as_text;
     }
     *out << "}";
   }
 }
 
 }  // namespace trace_event
 }  // namespace base