fix: Fix invalid time comparison

[vivek807]

Fixes #19602.

Description

forceOrWaitOngoingDatabasePoll in SqlSegmentsMetadataManager checks whether a concurrent OnDemandDatabasePoll that completed while the calling thread waited for the write lock makes a new poll unnecessary. This check was always false due to a clock mismatch, causing an extra database poll on every contended call.

Fixed invalid clock comparison in forceOrWaitOngoingDatabasePoll

OnDemandDatabasePoll.initiationTimeNanos is assigned via System.nanoTime(), which is a monotonic clock with an arbitrary JVM-internal reference point.
The comparison value was constructed as:

long checkStartTimeNanos = TimeUnit.MILLISECONDS.toNanos(checkStartTime);
// checkStartTime = System.currentTimeMillis()

System.currentTimeMillis() returns milliseconds since the Unix epoch (~1.7 × 10¹² ms). Converting to nanoseconds yields ~1.7 × 10²¹ ns, which is orders of magnitude larger than any System.nanoTime() value. The condition initiationTimeNanos > checkStartTimeNanos was therefore always false, so an in-progress on-demand poll was never recognised as sufficient.

This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[FrankChen021]

Severity	Findings
P0	0
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P2	1
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Total	1

Severity	Findings
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P2	1
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Total	1

Reviewed 1 of 1 changed files.

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This is an automated review by Codex GPT-5.5

[FrankChen021]

Severity	Findings
P0	0
P1	0
P2	0
P3	1
Total	1

Reviewed 2 of 2 changed files.

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This is an automated review by Codex GPT-5.5

[maytasm]

@vivek807
Thanks for flagging this issue. You are right, the values are not comparable.
I'm thinking that maybe instead we should standardize the timestamps in OnDemandDatabasePoll and PeriodicDatabasePoll to use the same format (i.e. both use System.nanoTime() or both use System.currentTimeMillis()). This can avoid similar confusion / bug like this in the future. Wdyt? Thanks!

[vivek807]

@vivek807 Thanks for flagging this issue. You are right, the values are not comparable. I'm thinking that maybe instead we should standardize the timestamps in OnDemandDatabasePoll and PeriodicDatabasePoll to use the same format (i.e. both use System.nanoTime() or both use System.currentTimeMillis()). This can avoid similar confusion / bug like this in the future. Wdyt? Thanks!

Yes, this will avoid confusion in future. Let me refactor it to use same type of clock here

[FrankChen021]

Severity	Findings
P0	0
P1	0
P2	1
P3	0
Total	1

Reviewed 2 of 2 changed files.

---

This is an automated review by Codex GPT-5.5

[FrankChen021]

[P2] Use monotonic time for freshness elapsed checks

millisElapsedFromInitiation() now computes elapsed time from System.currentTimeMillis(). If the wall clock moves backward after an on-demand poll starts, this value can become negative or too small, so useLatestSnapshotIfWithinDelay() can keep accepting a stale snapshot and the periodic poll task can sleep longer than periodicPollDelay. Keep a monotonic nanoTime() value for elapsed/freshness calculations and use a separate wall-clock timestamp only where this code compares against wall-clock poll start times.

[kfaraz]

Thanks for catching this @vivek807 !
I believe we should stick to using nanos, and just update the couple of places which use System.currentTimeMillis() to use System.nanoTime().

In the future, we may just use druid.java.util.common.Stopwatch for simplicity but that change may not be worth the effort right now since this class will soon be deprecated in favor of using SqlSegmentsMetadataManagerV2 which uses SegmentMetadataCache.