PSA: Better Time types available in Blink

[Majid Valipour]

We have now access to base time types* in Blink. These types can replace most, if not all, existing usage of double/uint64_t to represent time values and duration in Blink. I strongly recommend doing so and leverage type system to help prevent easy to make but hard to detect bugs due to incorrect conversion and information loss issues. This also improves documentation of clock type and resolution for time values everywhere. 

I have written a document to help you with such a conversion. Feel free to reach out if you have any question or concerns.

* There are in WTF namespace with a slightly more limited API compared to base/time. 

Thanks

Majid

[TAMURA, Kent]

base::Time can't represent 1601-01-01 00:00:00 UTC because it's the null value, right?  I confirmed Time::ToJsTime() didn't work for this specific time.

Date/time input types need to handle it correctly.

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TAMURA Kent
Software Engineer, Google

[Elliott Sprehn]

How did we represent that when using doubles? Can we add a bit to the blink time type for it or add it to base?

[TAMURA, Kent]

It's just -11,644,473,600,000.0 in double.  It is converted to 0 when it's stored in base::TimeBase::us_, and TimeBase::is_null() returns true.

Only conversion functions use TimeBase::is_null() in base/time/*.  It might be possible to add Time::ToEcmaTime() and FromEcmaTime() so that they don't check is_null(), and add comments that "Do not use is_null() for this!"

Note: ECMAScript asks to support year -271,821 to year 275,760 in Date object, and Date also has invalid state. V8 represents a Date object by a single double, and invalid state is NaN in double.  Date/time input types use the same representation, however the lower limit is 0001-01-01.

[Majid Valipour]

Only conversion functions use TimeBase::is_null() in base/time/*.  It might be possible to add Time::ToEcmaTime() and FromEcmaTime() so that they don't check is_null(), and add comments that "Do not use is_null() for this!"

 

The |is_null()| check in that conversion function seems sketchy. I will send a patch to fix it in base. Note that To/FromJsTime are not yet exposed in WTF interface anyways.

Majid

[Yutaka Hirano]

I found that I could use base::Time in platform/ but I couldn't use it in public/platform. Is it OK to add +base/time in public/platform/DEPS?

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[Jeremy Roman]

Personally, I'd be okay with adding base/time/time.h to public/platform/DEPS. It's rather self-contained. We'd probably want to also extend wtf/Time.h to allow the WTF types to be more easily converted to/from the base types for passing between Chromium and Blink.

Other people might disagree; I know there has been reticence to pass base types across the Blink API.