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 ConvertMilliseconds(const RemoteTimeTicks& remote_ms);

[darin (slow to review), 2011/12/12 22:25:46]

nit: What do you think about renaming this ToLocalTimeTicks?  That way you are
emphasizing the type names involved, and it should make the conversion a little
more apparent at the callsite.

[James Simonsen, 2011/12/12 23:35:34]

Agreed. Done.

+
+  // Returns the equivalent delta after applying remote-to-local scaling to
+  // |remote_delta|.
+  LocalTimeDelta ConvertDelta(const RemoteTimeDelta& remote_delta);

[darin (slow to review), 2011/12/12 22:25:46]

nit: ToLocalTimeDelta?

[James Simonsen, 2011/12/12 23:35:34]

Done.

+
+  // Returns |remote_value| after it has been clamped within the range
+  // [lower_bound, upper_bound]. This function is useful when we lack tight
+  // enough bounds to use ConvertMilliseconds().
+  static LocalTimeTicks ClampTimeInRange(const RemoteTimeTicks& remote_value,

[darin (slow to review), 2011/12/12 22:25:46]

This function feels like a small, orthogonal appendage.  Maybe it could / should
just be a static helper function in the file where it is used or the code should
be inlined where needed?  However, perhaps the downside to doing so would be
that you could no longer unit test it.  Hmm...

If it stays here, then it might be nice to also give it a name like
ToLocalTimeTicks, but add some modifier to convey the fact that it is a "less
good" conversion.

[James Simonsen, 2011/12/12 23:35:34]

It's too simple to worry about unit testing and I want to discourage others from
using it. So, I moved it into ResourceDispatcher::ToRendererCompletionTime.

+                                         const LocalTimeTicks& lower_bound,
+                                         const LocalTimeTicks& upper_bound);
+
+ 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_