tracing: Add macros for generating context trace events

base/trace_event/trace_event.h

 // 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.
 
 #ifndef BASE_TRACE_EVENT_TRACE_EVENT_H_
 #define BASE_TRACE_EVENT_TRACE_EVENT_H_
 
 // This header file defines implementation details of how the trace macros in
 // trace_event_common.h collect and store trace events. Anything not
 // implementation-specific should go in trace_macros_common.h instead of here.
 
 #include <string>
 
 #include "base/atomicops.h"
+#include "base/macros.h"
 #include "base/time/time.h"
 #include "base/trace_event/common/trace_event_common.h"
 #include "base/trace_event/trace_event_system_stats_monitor.h"
 #include "base/trace_event/trace_log.h"
 #include "build/build_config.h"
 
 // By default, const char* argument values are assumed to have long-lived scope
 // and will not be copied. Use this macro to force a const char* to be copied.
 #define TRACE_STR_COPY(str) \
     trace_event_internal::TraceStringWithCopy(str)
@@ skipping 303 matching lines @@
             id, &trace_event_flags); \
         trace_event_internal::AddTraceEventWithThreadIdAndTimestamp( \
             phase, INTERNAL_TRACE_EVENT_UID(category_group_enabled), \
             name, trace_event_trace_id.data(), trace_event_internal::kNoId, \
             thread_id, base::TimeTicks::FromInternalValue(timestamp), \
             trace_event_flags | TRACE_EVENT_FLAG_EXPLICIT_TIMESTAMP, \
             trace_event_internal::kNoId, ##__VA_ARGS__); \
       } \
     } while (0)
 
+// Implementation detail: internal macro to enter and leave a context based on
+// the current scope.
+#define INTERNAL_TRACE_EVENT_SCOPED_CONTEXT(category_group, name, context) \
+  trace_event_internal::TraceScopedContext INTERNAL_TRACE_EVENT_UID(       \
+      scoped_trace)(category_group, name, context)

[fmeawad, 2016/01/19 18:38:10]

Can you move this to common as well, I do not see anything platform specific in
the TraceScopedContext and it would make it easier to start using everywhere?

[Sami, 2016/01/20 10:34:08]

Yeah, I wasn't sure where we draw the line. I didn't see any classes or
namespace declarations in common -- does it make sense to start declaring these
kind of things there?

TraceScopeContext uses DISALLOW_COPY_AND_ASSIGN which is kinda sorta platform
specific.

 
 namespace trace_event_internal {
 
 // Specify these values when the corresponding argument of AddTraceEvent is not
 // used.
 const int kZeroNumArgs = 0;
 const unsigned long long kNoId = 0;
 
 // TraceID encapsulates an ID that can either be an integer or pointer. Pointers
 // are by default mangled with the Process ID so that they are unlikely to
@@ skipping 542 matching lines @@
     TRACE_EVENT_API_ATOMIC_STORE(
       g_trace_state[BucketNumber],
       reinterpret_cast<TRACE_EVENT_API_ATOMIC_WORD>(
         const_cast<char*>(category_and_name)));
   }
 
  private:
   const char* previous_state_;
 };
 
+using TraceContext = const void*;
+
+class TraceScopedContext {
+ public:
+  TraceScopedContext(const char* category_group,
+                     const char* name,
+                     TraceContext context)
+      : category_group_(category_group), name_(name), context_(context) {
+    TRACE_EVENT_ENTER_CONTEXT(category_group_, name_, context_);
+  }
+
+  ~TraceScopedContext() {
+    TRACE_EVENT_LEAVE_CONTEXT(category_group_, name_, context_);
+  }
+
+ private:
+  const char* category_group_;
+  const char* name_;
+  TraceContext context_;
+  DISALLOW_COPY_AND_ASSIGN(TraceScopedContext);
+};
+
 }  // namespace trace_event_internal
 
 namespace base {
 namespace trace_event {
 
+using trace_event_internal::TraceContext;
+
 template<typename IDType> class TraceScopedTrackableObject {
  public:
   TraceScopedTrackableObject(const char* category_group, const char* name,
       IDType id)
     : category_group_(category_group),
       name_(name),
       id_(id) {
     TRACE_EVENT_OBJECT_CREATED_WITH_ID(category_group_, name_, id_);
   }
 
@@ skipping 10 matching lines @@
   const char* name_;
   IDType id_;
 
   DISALLOW_COPY_AND_ASSIGN(TraceScopedTrackableObject);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_TRACE_EVENT_H_