tracing: split out the CategoryRegistry from the TraceLog

base/trace_event/trace_category.h

+// Copyright 2016 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_CATEGORY_H_
+#define BASE_TRACE_EVENT_TRACE_CATEGORY_H_
+
+#include <stdint.h>
+
+namespace base {
+namespace trace_event {
+
+// Captures the state of an invidivual trace category. Nothing except tracing
+// internals (e.g., TraceLog) is supposed to have non-const Category pointers.
+class TraceCategory {
+ public:
+  // The TRACE_EVENT macros should only use this value as a bool.
+  // These enum values are effectively a public API and third_party projects
+  // depend on their value. Hence, never remove or recycle existing bits, unless
+  // you are sure that all the third-party projects that depend on this have
+  // been updated.
+  enum StateFlags : uint8_t {
+    ENABLED_FOR_RECORDING = 1 << 0,
+
+    // Not used anymore.
+    DEPRECATED_ENABLED_FOR_MONITORING = 1 << 1,
+    DEPRECATED_ENABLED_FOR_EVENT_CALLBACK = 1 << 2,
+
+    ENABLED_FOR_ETW_EXPORT = 1 << 3,
+    ENABLED_FOR_FILTERING = 1 << 4
+  };
+
+  TraceCategory() : TraceCategory(nullptr) {}
+  TraceCategory(const char* name)
+      : state_(0), enabled_filters_(0), name_(name) {
+    static_assert(
+        offsetof(TraceCategory, state_) == 0,
+        "|state_| must be the first field of the TraceCategory class.");
+  }
+
+  bool is_valid() const { return name_ != nullptr; }
+  void set_name(const char* name) { name_ = name; }
+  const char* name() const {
+    DCHECK(is_valid());
+    return name_;
+  }
+
+  // TODO(primiano): This is an intermediate solution to deal with the fact that
+  // today TRACE_EVENT* macros cache the state ptr. They should just cache the
+  // full TraceCategory ptr, which is immutable, and use these helper function
+  // here. This will get rid of the need of this awkward ptr getter completely.
+  const uint8_t* state_ptr() const {
+    return const_cast<const uint8_t*>(&state_);
+  }
+
+  bool is_enabled() const { return state_ != 0; }
+  bool is(StateFlags flag) const { return (state_ & flag) != 0; }
+  void set_state(uint8_t state) { state_ = state; }
+  void clear_state_flag(StateFlags flag) { state_ &= ~flag; }
+  void set_state_flag(StateFlags flag) { state_ |= flag; }
+
+  bool is_filter_enabled(size_t index) const {
+    DCHECK(index < sizeof(enabled_filters_) * 8);
+    return (enabled_filters_ & (1 << index)) != 0;
+  }
+  uint16_t enabled_filters() const { return enabled_filters_; }
+  void set_enabled_filters(uint16_t enabled_filters) {
+    enabled_filters_ = enabled_filters;
+  }
+
+  void reset_for_testing() {
+    state_ = 0;
+    enabled_filters_ = 0;
+  }
+
+ private:
+  // The enabled state. TRACE_EVENT* macros should capture the event if any of
+  // the flags here are set. This state is racy by design: this is polled by
+  // TRACE_EVENTx macros in a lot of fast-paths all over the codebase. We don't
+  // want to slow them down with any kind of barrier. This state is effectively
+  // mutated when starting/stopping tracing, in which case don't care if we miss
+  // some events.
+  volatile uint8_t state_;
+
+  // When ENABLED_FOR_FILTERING is set, this contains a bitmap to the
+  // coressponding filter (see event_filters.h).
+  uint16_t enabled_filters_;
+
+  // TraceCategory group names are long lived static strings.
+  const char* name_;
+
+  DISALLOW_COPY_AND_ASSIGN(TraceCategory);
+};
+
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_TRACE_CATEGORY_H_