RFR: 8357539: TimeSource.now() is not monotonic [v2]

[Daniel Fuchs]

On Thu, 22 May 2025 18:56:29 GMT, Daniel Jeliński <djelinski 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. Tier2 tests continue to pass.
>
> Daniel Jeliński has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Restore instant(long,long)

Marked as reviewed by dfuchs (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/25390#pullrequestreview-2862461160