Add UMA for how long service workers run for.

content/browser/service_worker/service_worker_lifetime_tracker.cc

+// Copyright 2017 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 "content/browser/service_worker/service_worker_lifetime_tracker.h"
+
+#include "base/bind.h"
+#include "base/stl_util.h"
+#include "base/time/default_tick_clock.h"
+#include "content/browser/service_worker/service_worker_metrics.h"
+
+namespace content {
+
+// Five minutes is somewhat ad-hoc. We want to capture workers that are
+// running for a long time in UMA even if they never stop. A timer running too
+// frequently would be wasteful and too infrequently could miss long-running
+// workers before browser shutdown. Five minutes seems about right; it's also
+// the default keep-alive time limit for a service worker event so workers
+// running longer than this are a bit special: they must have had multiple
+// events in their lifetime.
+constexpr base::TimeDelta kTimerDuration = base::TimeDelta::FromMinutes(5);
+
+ServiceWorkerLifetimeTracker::ServiceWorkerLifetimeTracker()
+    : ServiceWorkerLifetimeTracker(base::MakeUnique<base::DefaultTickClock>()) {
+}
+
+ServiceWorkerLifetimeTracker::ServiceWorkerLifetimeTracker(
+    std::unique_ptr<base::TickClock> tick_clock)
+    : tick_clock_(std::move(tick_clock)), timer_(tick_clock_.get()) {}
+
+ServiceWorkerLifetimeTracker::~ServiceWorkerLifetimeTracker() = default;
+
+void ServiceWorkerLifetimeTracker::StartTiming(int64_t version_id) {
+  DCHECK(!base::ContainsKey(running_workers_, version_id));

[falken, 2017/02/24 08:32:26]

I'm hitting this DCHECK in unit tests, but more generally there is no strong
guarantee that StopTiming is called for every StartTiming. For example in
SWVersion dtor, we do a last gasp StopWorker() on EWInstance but the SWVersion
doesn't stick around to listen for the OnStopped event.

So this still needs refinement. If we don't get a StopTiming, the UMA gets
unreliable.

+
+  running_workers_[version_id] = tick_clock_->NowTicks();
+  if (!timer_.IsRunning()) {
+    timer_.Start(FROM_HERE, kTimerDuration, this,
+                 &ServiceWorkerLifetimeTracker::RecordHistograms);
+  }
+}
+
+void ServiceWorkerLifetimeTracker::StopTiming(int64_t version_id) {
+  auto it = running_workers_.find(version_id);
+  // The entry may have been removed by AbortTiming.
+  if (it == running_workers_.end())
+    return;
+  ServiceWorkerMetrics::RecordRuntime(tick_clock_->NowTicks() - it->second);
+  running_workers_.erase(it);
+}
+
+void ServiceWorkerLifetimeTracker::AbortTiming(int64_t version_id) {
+  auto it = running_workers_.find(version_id);
+  // The entry may have been removed by AbortTiming.
+  if (it == running_workers_.end())
+    return;
+  running_workers_.erase(it);
+}
+
+void ServiceWorkerLifetimeTracker::RecordHistograms() {
+  base::TimeTicks now = tick_clock_->NowTicks();
+
+  for (auto& item : running_workers_) {
+    base::TimeDelta runtime = now - item.second;
+    if (runtime > kTimerDuration)
+      ServiceWorkerMetrics::RecordStillRunning(runtime);
+  }
+
+  if (running_workers_.empty())
+    timer_.Stop();
+}
+
+}  // namespace content