[Tracing Clock Sync] Add TracingAgent interface in Chrome

base/trace_event/tracing_agent.h

+// Copyright 2015 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.
+
+#ifndef BASE_TRACE_EVENT_TRACING_AGENT_H_
+#define BASE_TRACE_EVENT_TRACING_AGENT_H_
+
+#include "base/base_export.h"
+#include "base/callback.h"
+#include "base/memory/ref_counted_memory.h"
+#include "base/trace_event/trace_config.h"

[Primiano Tucci (use gerrit), 2015/12/01 11:42:43]

this header is not needed, just forward declare when possible (like in the case
of TraceConfig)

[Zhen Wang, 2015/12/01 18:02:52]

Done.

+#include "base/values.h"
+
+namespace base {
+namespace trace_event {
+
+// A tracing agent is the entity that records its own sort of trace. Each
+// tracing method that produces its own trace log should implement this
+// interface. And all tracing agents are and should only be controlled by the
+// TracingController.
+//
+// Some existing examples include TracingControllerImpl for Chrome trace events,
+// DebugDaemonClient for CrOs system trace, EtwSystemEventConsumer for Windows
+// system trace and PowerTracingAgent for BattOr power trace.
+class BASE_EXPORT TracingAgent {
+ public:
+  typedef base::Callback<void(

[Primiano Tucci (use gerrit), 2015/12/01 11:42:43]

these days "using" is the thing :)
using StopAgentTracingCallback = base::Callback ...

[Zhen Wang, 2015/12/01 18:02:52]

Done.

+      const std::string& agent_name,
+      const std::string& events_label,
+      const scoped_refptr<base::RefCountedString>& events_str_ptr)>
+      StopAgentTracingCallback;
+
+  // Get the name of the tracing agent. The name should be one of the possible
+  // tracing agent names declared below.
+  virtual std::string GetTracingAgentName() = 0;
+
+  // Get the trace event label of this tracing agent. The label should be one of
+  // the possible trace event labels declared below.
+  virtual std::string GetTraceEventLabel() = 0;
+
+  // Start tracing on the tracing agent with the trace configuration.
+  virtual bool StartAgentTracing(const TraceConfig& trace_config) = 0;
+
+  // Stop tracing on the tracing agent. The trace data will be passed back to
+  // the TracingController via the callback.
+  virtual void StopAgentTracing(const StopAgentTracingCallback& callback) = 0;
+
+  // Check if the tracing agent supports explicit clock synchronization.
+  virtual bool SupportsExplicitClockSync();
+
+  // Record a clock sync marker issued by another tracing agent. This is only
+  // used if the tracing agent supports explicit clock synchronization.
+  virtual void RecordClockSyncMarker(
+      scoped_ptr<base::DictionaryValue> marker);

[Primiano Tucci (use gerrit), 2015/12/01 11:42:43]

Not related with this CL but out curiosity why this is not a TracedValue?

[Zhen Wang, 2015/12/01 18:02:52]

Because this marker will be recorded by the agent, which is not necessarily
Chrome (e.g., could be BattOr power monitor).

+
+  // Issue clock sync markers to other tracing agents if possible. This is only
+  // used if the tracing agent supports explicit clock synchronization.
+  virtual void IssueClockSyncMarker();
+
+  virtual ~TracingAgent() {}
+
+  // Declare all possible tracing agent names here. All new tracing agents

[Primiano Tucci (use gerrit), 2015/12/01 11:42:43]

See https://google.github.io/styleguide/cppguide.html#Declaration_Order

- Move these constants up
- Then Ctors and dtor
- then the rest

[Zhen Wang, 2015/12/01 18:02:52]

Done.

+  // should declare their name here.
+  static const char kChromeTracingAgentName[];
+  static const char kCrOSTracingAgentName[];
+  static const char kETWTracingAgentName[];
+  static const char kPowerTracingAgentName[];
+
+  // Declare all possible trace event labels here.
+  static const char kChromeTraceLabel[];
+  static const char kMetadataTraceLabel[];
+  static const char kPowerTraceLabel[];
+  static const char kSystemTraceLabel[];
+};
+
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_TRACING_AGENT_H_