[BUG] seekTo throws NPE when no offset found for a timestamp

[authorjapps]

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reported by @a1shadows :

(1)
If there are partitions for which there is no valid offset corresponding to the timestamp to seek to, the offsetForTimestamp method still includes it in the Map returned, but with a NULL value. We need to skip all partitions that have offset as NULL when subscribing to avoid an error when we try to seek to the offset at a later step.

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(2)
There was a bug in the seekTo feature . In situations where there are multiple partitions on the topic and some of the partitions do not have a valid offset corresponding the timestamp to seek to, the seek operation will give an NPE.

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More discussions are here:
#630 (comment)