[Tracing Clock Sync] Add TracingAgent interface in Chrome

base/trace_event/tracing_agent.h

Index: base/trace_event/tracing_agent.h
diff --git a/base/trace_event/tracing_agent.h b/base/trace_event/tracing_agent.h
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+++ b/base/trace_event/tracing_agent.h
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+// 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/values.h"
+
+namespace base {
+
+class TimeTicks;
+
+namespace trace_event {
+
+class TraceConfig;
+
+// A tracing agent is an entity that records its own sort of trace. Each
+// tracing method that produces its own trace log should implement this
+// interface. All tracing agents must only be controlled by 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:
+  using StopAgentTracingCallback = base::Callback<void(
+      const std::string& agent_name,
+      const std::string& events_label,
+      const scoped_refptr<base::RefCountedString>& events_str_ptr)>;
+  using RecordClockSyncMarkerCallback = base::Callback<void(
+      int sync_id,
+      const TimeTicks& sync_ts,
+      const TimeTicks& sync_end_ts)>;
+
+  virtual ~TracingAgent();
+
+  // Gets the name of the tracing agent. Each tracing agent's name should be
+  // unique.
+  virtual std::string GetTracingAgentName() = 0;
+
+  // Gets the trace event label of this tracing agent. The label will be used to
+  // label this agent's trace when all traces from different tracing agents are
+  // combined. Multiple tracing agents could have the same label. The tracing
+  // agents using the same label should not be able to run at the same time. For
+  // example, ETW on Windows and CrOS system tracing both use
+  // "systemTraceEvents" as the label. Those two agents never run at the same
+  // time because they are for different platforms.
+  virtual std::string GetTraceEventLabel() = 0;
+
+  // Starts tracing on the tracing agent with the trace configuration.
+  virtual bool StartAgentTracing(const TraceConfig& trace_config) = 0;
+
+  // Stops 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;
+
+  // Checks if the tracing agent supports explicit clock synchronization.
+  virtual bool SupportsExplicitClockSync();
+
+  // Records a clock sync marker issued by another tracing agent. This is only
+  // used if the tracing agent supports explicit clock synchronization.
+  //
+  // Two things need to be done:
+  // 1. The issuer asks the receiver to record the clock sync marker.
+  // 2. The issuer records how long the receiver takes to do the recording.
+  //
+  // In Chrome, the receiver thread also runs in Chrome and it will talk to the
+  // real receiver entity, e.g., power monitor or Android device system, via
+  // different communication methods, e.g., through USB or file reading/writing.
+  // The 2nd task measures that communication latency.
+  //
+  // Having a reliable timing measurement for the 2nd task requires synchronous
+  // function call without any cross-thread or cross-process activity. However,
+  // tracing agents in Chrome run in their own threads. Therefore, the issuer
+  // needs to dedicate the 2nd task to the receiver to take time measurements
+  // in the receiver thread, and the receiver thread needs to pass them back to
+  // the issuer in the callback.
+  //
+  // The assumption is that the receiver thread knows the issuer's clock, which
+  // is true in Chrome because all agent threads' clocks are Chrome clock.
+  virtual void RecordClockSyncMarker(
+      int sync_id,
+      const RecordClockSyncMarkerCallback& callback);
+};
+
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_TRACING_AGENT_H_