tracing: split trace event filter classes out of TraceLog

base/trace_event/trace_event_filter.h

Index: base/trace_event/trace_event_filter.h
diff --git a/base/trace_event/trace_event_filter.h b/base/trace_event/trace_event_filter.h
new file mode 100644
index 0000000000000000000000000000000000000000..ba407801a029774e74db72a69f98eb448183b382
--- /dev/null
+++ b/base/trace_event/trace_event_filter.h
@@ -0,0 +1,51 @@
+// 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_EVENT_FILTER_H_
+#define BASE_TRACE_EVENT_TRACE_EVENT_FILTER_H_
+
+#include <memory>
+
+#include "base/base_export.h"
+#include "base/macros.h"
+
+namespace base {
+namespace trace_event {
+
+class TraceEvent;
+
+// TraceEventFilter is like iptables for TRACE_EVENT macros. Filters can be
+// enabled on a per-category basis, hence a single filter instance can serve
+// more than a TraceCategory. There are two use cases for filters:
+// 1. Snooping TRACE_EVENT macros without adding them to the TraceLog. This is
+//    possible by setting the ENABLED_FOR_FILTERING flag on a category w/o
+//    ENABLED_FOR_RECORDING (see TraceConfig for user-facing configuration).
+// 2. Filtering TRACE_EVENT macros before they are added to the TraceLog. This
+//    requires both the ENABLED_FOR_FILTERING and ENABLED_FOR_RECORDING flags
+//    on the category.
+// More importantly, filters must be thread-safe. The FilterTraceEvent and
+// EndEvent methods can be called concurrently as trace macros are hit on
+// different threads.
+class BASE_EXPORT TraceEventFilter {
+ public:
+  TraceEventFilter();
+  virtual ~TraceEventFilter();
+
+  // If the category is ENABLED_FOR_RECORDING, the event is added iff all the
+  // filters enabled for the category return true. false causes the event to be
+  // dropped on the floor.

[kraynov, 2016/12/06 18:24:23]

Maybe 'discarded'? For me, 'dropped on the floor' sounds like we're forced to
discard silently and we don't like it.

[Primiano Tucci (use gerrit), 2016/12/07 11:17:47]

Done.

+  virtual bool FilterTraceEvent(const TraceEvent& trace_event) const = 0;
+
+  // Notifies the end of a duration event when the RAII macro goes out of scope.
+  virtual void EndEvent(const char* category_name,
+                        const char* event_name) const;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(TraceEventFilter);
+};
+
+}  // namespace trace_event
+}  // namespace base
+
+#endif  // BASE_TRACE_EVENT_TRACE_EVENT_FILTER_H_