[Java]Fix NPE in UnboundedSolaceReader.getWatermark during disconnection (fixes #32660)

[Suvrat1629]

Please add a meaningful description for your change here

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Fixes #32660
Fixed NPE in UnboundedSolaceReader.getWatermark during Solace broker disconnections by adding explicit null check for watermarkPolicy.getWatermark() and returning BoundedWindow.TIMESTAMP_MIN_VALUE as a safe fallback.

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[gemini-code-assist]

Summary of Changes

Hello @Suvrat1629, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses and resolves a NullPointerException (NPE) that could occur in the UnboundedSolaceReader's getWatermark() method. The NPE was triggered specifically during disconnections from a Solace broker when the watermarkPolicy.getWatermark() call returned null. The fix ensures the pipeline's stability by introducing a null check and providing a safe fallback watermark, preventing crashes and allowing the system to gracefully handle temporary disconnections.

Highlights

• NullPointerException Fix: Implemented a null check for the watermarkPolicy.getWatermark() method call within UnboundedSolaceReader.getWatermark().
• Robustness Improvement: When watermarkPolicy.getWatermark() returns null (indicating a likely disconnection), the method now returns BoundedWindow.TIMESTAMP_MIN_VALUE to prevent pipeline failure and provide a safe fallback.
• Logging: Added a warning log message when watermarkPolicy returns null to provide visibility into the fallback mechanism.

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[kennknowles]

This value is forbidden from being null, so we shouldn't need the check. It is not marked @Nullable in the class WatermarkPolicy and null checking is enabled for that class so the type should be accurate.

Where is the null coming from?

[Suvrat1629]

Oh well my pr was a follow up fix for my older pr that got closed due to inactivity(#34296), this is the fix I thought @Abacn had asked for and since I had gotten back to contributing I thought about putting some work into this :)

Also as you said that:

null checking is enabled for that class so the type should be accurate.

since I am trying to learn about the codebase could you tell me how this works.

Thank you.

[kennknowles]

Oh well my pr was a follow up fix for my older pr that got closed due to inactivity(#34296), this is the fix I thought @Abacn had asked for and since I had gotten back to contributing I thought about putting some work into this :)

Thank you! Yes, I still think we should try to fix it. There is a null getting in somewhere that should be impossible, and we can track it down.

Also as you said that:

null checking is enabled for that class so the type should be accurate.

since I am trying to learn about the codebase could you tell me how this works.

Yes! We use checkerframework to modernize Java's type system. Any time you see a type Foo anywhere in the Beam codebase this means it is a non-null Foo. Only @Nullable Foo can be null.

But because we added this after the project was somewhat large, many classes have @SuppressWarnings("nullness") and that turns it off. But this is forbidden for new code. The SolaceIO connector is in good shape; it does not turn off the null checking.

So this null that violates the type system is probqably coming from one of the following places:

• an external library that returns null but is not annotated that it may return null,
• some part of the Beam codebase where null checking is disabled,

So the solution here is to figure out how we have a forbidden null. I read the javadoc on WatermarkPolicy.getWatermark() and it seems pretty clear that it should never return null. I clicked through some of the other methods that could end up leaking a null into it and they all were autovalue classes that also do not allow null, so I'm not sure!

[Suvrat1629]

Oh thanks for the explanation I had suspected it to be checkerframework but I just wanted to be sure.

So this null that violates the type system is probqably coming from one of the following places:

an external library that returns null but is not annotated that it may return null,
some part of the Beam codebase where null checking is disabled,

Yeah probably, I'll see what I can come up with when I go through the code. Until then if you expect me to make any improvements to the pr then please tell me so.

Thank you.

[kennknowles]

Responding to your top level comment here, since it is where there is work to do: this null check should be removed. It is forbidden by the type system. If you have a reproduction where a null shows up here, then that is the bug to address.

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[kennknowles]

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[Suvrat1629]

@kennknowles Based on the comment tree can you tell me what I can do here?

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[tvalentyn]

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[stankiewicz]

npe was happening when code was as follows:

  @Override
  public Instant getWatermark() {
    // should be only used by a test receiver
    if (checkNotNull(sessionService).getReceiver().isEOF()) {
      return BoundedWindow.TIMESTAMP_MAX_VALUE;
    }
    return watermarkPolicy.getWatermark();

Current implementation is different so I'm not sure if this PR and issue is relevant anymore.