tracing: Introduce API for composite IDs

base/trace_event/trace_event_impl.cc

Index: base/trace_event/trace_event_impl.cc
diff --git a/base/trace_event/trace_event_impl.cc b/base/trace_event/trace_event_impl.cc
index f9792d0d6df6aea004447b3bf2a293f50d08e646..0da94758dcc0d40df01660d277fb7506356a54fd 100644
--- a/base/trace_event/trace_event_impl.cc
+++ b/base/trace_event/trace_event_impl.cc
@@ -458,30 +458,34 @@ TraceID::AsConvertableToTraceFormat() const {
 
   if (scope_ != kGlobalScope)
     value->SetString("scope", scope_);
-  switch (id_flags_) {
-    case TRACE_EVENT_FLAG_HAS_ID:
-      value->SetString(
-          "id",
-          base::StringPrintf("0x%" PRIx64, static_cast<uint64_t>(raw_id_)));
-      break;
-    case TRACE_EVENT_FLAG_HAS_GLOBAL_ID:
-      value->BeginDictionary("id2");
-      value->SetString(
-          "global",
-          base::StringPrintf("0x%" PRIx64, static_cast<uint64_t>(raw_id_)));
-      value->EndDictionary();
-      break;
-    case TRACE_EVENT_FLAG_HAS_LOCAL_ID:
-      value->BeginDictionary("id2");
-      value->SetString(
-          "local",
-          base::StringPrintf("0x%" PRIx64, static_cast<uint64_t>(raw_id_)));
-      value->EndDictionary();
-      break;
-    default:
-      NOTREACHED() << "Unrecognized ID flag";
+
+  const char* id_field_name = "id";
+  if (id_flags_ == TRACE_EVENT_FLAG_HAS_GLOBAL_ID) {
+    id_field_name = "global";
+    value->BeginDictionary("id2");
+  } else if (id_flags_ == TRACE_EVENT_FLAG_HAS_LOCAL_ID) {
+    id_field_name = "local";
+    value->BeginDictionary("id2");
+  } else if (id_flags_ != TRACE_EVENT_FLAG_HAS_ID) {
+    NOTREACHED() << "Unrecognized ID flag";
   }
 
+  if (has_prefix_) {
+    value->BeginArray(id_field_name);

[Primiano Tucci (use gerrit), 2017/01/17 12:25:47]

by looking at the else case below this value can be either a string or an array,
which might be confusing as a the trace format.
wouldn't be easier for everybody if you just concatenate these into a string,
instead of creating an array, e.g.:
"$prefix/$raw_id" ?

[chiniforooshan1, 2017/01/17 16:44:09]

Done.

+    value->AppendString(
+        base::StringPrintf("0x%" PRIx64, static_cast<uint64_t>(prefix_)));
+    value->AppendString(
+        base::StringPrintf("0x%" PRIx64, static_cast<uint64_t>(raw_id_)));
+    value->EndArray();
+  } else {
+    value->SetString(
+        id_field_name,
+        base::StringPrintf("0x%" PRIx64, static_cast<uint64_t>(raw_id_)));
+  }
+
+  if (id_flags_ != TRACE_EVENT_FLAG_HAS_ID)
+    value->EndDictionary();
+
   return std::move(value);
 }