tracing: split out the CategoryRegistry from the TraceLog

base/trace_event/category_registry.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_CATEGORY_H_
+#define BASE_TRACE_EVENT_CATEGORY_H_
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "base/base_export.h"
+
+namespace base {
+namespace trace_event {
+
+class TraceCategory;
+class TraceLog;
+
+// Keeps track of the state of all tracing categories. The reason why this
+// is a fully static class with global state is to allow to statically define
+// known categories as global linker-initialized structs, without requiring
+// static initializers.
+class BASE_EXPORT CategoryRegistry {
+ public:
+  // Allows for-each iterations over a slice of the categories array.
+  class Range {
+   public:
+    Range(TraceCategory* begin, TraceCategory* end)
+        : begin_(begin), end_(end) {}

[oystein (OOO til 10th of July), 2016/10/28 06:28:00]

Can we DCHECK(begin_ <= end_)?

[Primiano Tucci (use gerrit), 2016/10/28 15:28:39]

Done.

+    TraceCategory* begin() const { return begin_; }
+    TraceCategory* end() const { return end_; }
+
+   private:
+    TraceCategory* const begin_;
+    TraceCategory* const end_;
+  };
+
+  // Known categories.
+  static TraceCategory* const kCategoryExhausted;
+  static TraceCategory* const kCategoryMetadata;
+  static TraceCategory* const kCategoryAlreadyShutdown;
+
+  // Returns a category entry from the Category.state_ptr() pointer.
+  // TODO(primiano): trace macros should just keep a pointer to the entire
+  // TraceCategory, not just the enabled state pointer. That would remove the
+  // need for this function and make everything cleaner at no extra cost (as
+  // long as the |state_| is the first field of the struct, which can be
+  // guaranteed via static_assert, see TraceCategory ctor).
+  static const TraceCategory* GetCategoryByStatePtr(
+      const uint8_t* category_state);
+
+  static bool IsBuiltinCategory(const TraceCategory*);
+
+ private:
+  friend class TraceLog;
+
+  // Only for debugging/testing purposes, is a no-op on release builds.
+  static void Initialize();
+
+  // Resets the state of all categories, to clear up the state between tests.
+  static void ResetForTesting();
+
+  // The output |category| argument is an undefinitely lived pointer to the
+  // TraceCategory owned by the registry. TRACE_EVENTx macros will cache this
+  // pointer and use it for checks in their fast-paths.
+  // Returns false if the category was already present, true if the category
+  // has just been added and hence requires initialization.
+  static bool GetOrCreateCategoryByName(const char* category_name,
+                                        TraceCategory** category);
+
+  // Allows to iterate over the valid categories in a for-each loop.
+  // This includes builtin categories such as __metadata.
+  static Range GetAllCategories();
+};
+
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_CATEGORY_H_