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

[Daniel Fuchs]

On Thu, 22 May 2025 09:53:26 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.

src/java.net.http/share/classes/jdk/internal/net/http/common/TimeSource.java line 85:

> 83: 
> 84:         boolean isInWindow(long delay) {
> 85:             return delay >= -TIME_WINDOW && delay <= TIME_WINDOW;

OK

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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25390#discussion_r2102158627
PR Review Comment: https://git.openjdk.org/jdk/pull/25390#discussion_r2102153039