Use a monotonic clock (TimeTicks) to report network times to WebCore.

content/common/inter_process_time_ticks_converter.h

+// Copyright (c) 2011 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 CONTENT_COMMON_INTER_PROCESS_TIME_TICKS_CONVERTER_H_
+#define CONTENT_COMMON_INTER_PROCESS_TIME_TICKS_CONVERTER_H_
+
+#include "base/time.h"
+
+namespace content {
+
+class LocalTimeDelta;
+class LocalTimeTicks;
+class RemoteTimeDelta;
+class RemoteTimeTicks;
+
+// On Windows, TimeTicks are not consistent between processes. Often, the values
+// on one process have a static offset relative to another. Occasionally, these
+// offsets shift while running.
+//
+// To combat this, any TimeTicks values sent from the remote process to the
+// local process must be tweaked in order to appear monotonic.
+//
+// In order to properly tweak ticks, we need 4 reference points:
+//
+// - |local_lower_bound|:  A known point, recorded on the local process, that
+//                         occurs before any remote values that will be
+//                         converted.
+// - |remote_lower_bound|: The equivalent point on the remote process. This
+//                         should be recorded immediately after
+//                         |local_lower_bound|.
+// - |local_upper_bound|:  A known point, recorded on the local process, that
+//                         occurs after any remote values that will be
+//                         converted.
+// - |remote_upper_bound|: The equivalent point on the remote process. This
+//                         should be recorded immediately before
+//                         |local_upper_bound|.
+//
+// Once these bounds are determined, values within the remote process's range
+// can be converted to the local process's range. The values are converted as
+// follows:
+//
+// 1. If the remote's range exceeds the local's range, it is scaled to fit.
+//    Any values converted will have the same scale factor applied.
+//
+// 2. The remote's range is shifted so that it is centered within the
+//    local's range. Any values converted will be shifted the same amount.
+class InterProcessTimeTicksConverter {
+ public:
+  InterProcessTimeTicksConverter(const LocalTimeTicks& local_lower_bound,
+                                 const LocalTimeTicks& local_upper_bound,
+                                 const RemoteTimeTicks& remote_lower_bound,
+                                 const RemoteTimeTicks& remote_upper_bound);
+
+  // Returns the value within the local's bounds that correlates to
+  // |remote_ms|.
+  LocalTimeTicks ToLocalTimeTicks(const RemoteTimeTicks& remote_ms);
+
+  // Returns the equivalent delta after applying remote-to-local scaling to
+  // |remote_delta|.
+  LocalTimeDelta ToLocalTimeDelta(const RemoteTimeDelta& remote_delta);
+
+ private:
+  int64 Convert(int64 value);
+
+  int64 offset_;
+  int64 numerator_;
+  int64 denominator_;
+
+  int64 remote_lower_bound_;
+  int64 remote_upper_bound_;
+};
+
+class LocalTimeDelta {
+ public:
+  int ToInt32() const { return value_; }
+
+ private:
+  friend class InterProcessTimeTicksConverter;
+  friend class LocalTimeTicks;
+
+  LocalTimeDelta(int value) : value_(value) {}
+
+  int value_;
+};
+
+class LocalTimeTicks {
+ public:
+  static LocalTimeTicks FromTimeTicks(const base::TimeTicks& value) {
+    return LocalTimeTicks(value.ToInternalValue());
+  }
+
+  base::TimeTicks ToTimeTicks() {
+    return base::TimeTicks::FromInternalValue(value_);
+  }
+
+  LocalTimeTicks operator+(const LocalTimeDelta& delta) {
+    return LocalTimeTicks(value_ + delta.value_);
+  }
+
+ private:
+  friend class InterProcessTimeTicksConverter;
+
+  LocalTimeTicks(int64 value) : value_(value) {}
+
+  int64 value_;
+};
+
+class RemoteTimeDelta {
+ public:
+  static RemoteTimeDelta FromRawDelta(int delta) {
+    return RemoteTimeDelta(delta);
+  }
+
+ private:
+  friend class InterProcessTimeTicksConverter;
+  friend class RemoteTimeTicks;
+
+  RemoteTimeDelta(int value) : value_(value) {}
+
+  int value_;
+};
+
+class RemoteTimeTicks {
+ public:
+  static RemoteTimeTicks FromTimeTicks(const base::TimeTicks& ticks) {
+    return RemoteTimeTicks(ticks.ToInternalValue());
+  }
+
+  RemoteTimeDelta operator-(const RemoteTimeTicks& rhs) const {
+    return RemoteTimeDelta(value_ - rhs.value_);
+  }
+
+ private:
+  friend class InterProcessTimeTicksConverter;
+
+  RemoteTimeTicks(int64 value) : value_(value) {}
+
+  int64 value_;
+};
+
+}  // namespace content
+
+#endif  // CONTENT_COMMON_INTER_PROCESS_TIME_TICKS_CONVERTER_H_