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// 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.
#include <stdint.h>
#include <atomic>
#include <map>
#include <string>
#include <vector>
#include "base/gtest_prod_util.h"
#include "base/macros.h"
#include "base/metrics/histogram_base.h"
namespace base {
class HistogramSamples;
class HistogramFlattener;
// HistogramSnapshotManager handles the logistics of gathering up available
// histograms for recording either to disk or for transmission (such as from
// renderer to browser, or from browser to UMA upload). Since histograms can sit
// in memory for an extended period of time, and are vulnerable to memory
// corruption, this class also validates as much redundancy as it can before
// calling for the marginal change (a.k.a., delta) in a histogram to be
// recorded.
class BASE_EXPORT HistogramSnapshotManager final {
explicit HistogramSnapshotManager(HistogramFlattener* histogram_flattener);
// Snapshot all histograms, and ask |histogram_flattener_| to record the
// delta. |flags_to_set| is used to set flags for each histogram.
// |required_flags| is used to select histograms to be recorded.
// Only histograms that have all the flags specified by the argument will be
// chosen. If all histograms should be recorded, set it to
// |Histogram::kNoFlags|.
void PrepareDeltas(const std::vector<HistogramBase*>& histograms,
HistogramBase::Flags flags_to_set,
HistogramBase::Flags required_flags) {
for (HistogramBase* const histogram : histograms) {
if ((histogram->flags() & required_flags) == required_flags)
// When the collection is not so simple as can be done using a single
// iterator, the steps can be performed separately. Call PerpareDelta()
// as many times as necessary. PrepareFinalDelta() works like PrepareDelta()
// except that it does not update the previous logged values and can thus
// be used with read-only files.
void PrepareDelta(HistogramBase* histogram);
void PrepareFinalDelta(const HistogramBase* histogram);
FRIEND_TEST_ALL_PREFIXES(HistogramSnapshotManagerTest, CheckMerge);
// During a snapshot, samples are acquired and aggregated. This structure
// contains all the information for a given histogram that persists between
// collections.
struct SampleInfo {
// The set of inconsistencies (flags) already seen for the histogram.
// See HistogramBase::Inconsistency for values.
uint32_t inconsistencies = 0;
// Capture and hold samples from a histogram. This does all the heavy
// lifting for PrepareDelta() and PrepareAbsolute().
void PrepareSamples(const HistogramBase* histogram,
std::unique_ptr<HistogramSamples> samples);
// |histogram_flattener_| handles the logistics of recording the histogram
// deltas.
HistogramFlattener* const histogram_flattener_; // Weak.
// For histograms, track what has been previously seen, indexed
// by the hash of the histogram name.
std::map<uint64_t, SampleInfo> known_histograms_;
// A flag indicating if a thread is currently doing an operation. This is
// used to check against concurrent access which is not supported. A Thread-
// Checker is not sufficient because it may be guarded by at outside lock
// (as is the case with cronet).
std::atomic<bool> is_active_;
} // namespace base