RFR: 8357539: TimeSource.now() is not monotonic

[Jaikiran Pai]

On Thu, 22 May 2025 09:59:58 GMT, Daniel Fuchs <dfuchs at openjdk.org> wrote:

>> We observed a case where the instants returned by `TimeSource.now()` were returned in non-monotonic order. The reason was that sometimes we were using a delay calculated with one `localSource` as an input to a different (updated on another thread) `localSource`. This was confirmed by putting `assert firstNanos + delay == nanos;` under `instant(long, long)`.
>> 
>> The fix ensures that we won't accidentally use the incorrect delay by removing the `instant(long, long)` overload, and calculating the delay in the method where it is used.
>> 
>> No new test; instrumenting this class for testing would likely double its size.
>
> src/java.net.http/share/classes/jdk/internal/net/http/common/TimeSource.java line 97:
> 
>> 95:         // use localSource if possible to avoid a volatile read
>> 96:         if (source.isInWindow(delay)) {
>> 97:             return source.instant(nanos);
> 
> I would keep the two args intant() method which avoids computing the delay twice. Using a local variable `source` in this method should be enough to solve the bug.

Hello Daniel, with this proposed change, is there any reason to have the `localSource` field in this `TimeSource` anymore?

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PR Review Comment: https://git.openjdk.org/jdk/pull/25390#discussion_r2102234265