JoshRosen opened a new pull request #26076: [SPARK-29419][SQL] Fix Encoder 
thread-safety issue in createDataset(Seq)
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   This PR fixes a thread-safety bug in `SparkSession.createDataset(Seq)`: if 
the caller-supplied `Encoder` is used in multiple threads then createDataset's 
usage of the encoder may lead to incorrect / corrupt results because the 
Encoder's internal mutable state will be updated from multiple threads.
   Here is an example demonstrating the problem:
   import org.apache.spark.sql._
   val enc = implicitly[Encoder[(Int, Int)]]
   val datasets = (1 to 100) { _ =>
     val pairs = (1 to 100).map(x => (x, x))
   datasets.reduce(_ union _).collect().foreach {
     pair => require(pair._1 == pair._2, s"Pair elements are mismatched: $pair")
   Before this PR's change, the above example fails because Spark produces 
corrupted records where different input records' fields have been co-mingled.
   This bug is similar to SPARK-22355 / #19577, a similar problem in 
   The fix implemented here is based on #24735's updated version of the 
`Datataset.collect()` bugfix: use `.copy()`. For consistency, I used same [code 
 / explanation as that PR.
   ### Does this PR introduce any user-facing change?
   If yes, please clarify the previous behavior and the change this PR proposes 
- provide the console output, description and/or an example to show the 
behavior difference if possible.
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   ### How was this patch tested?
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test cases that check the changes thoroughly including negative and positive 
cases if possible.
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how you tested step by step, ideally copy and paste-able, so that other 
reviewers can test and check, and descendants can verify in the future.
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it was difficult to add.
   Tested manually using the example listed above.
   Thanks to @smcnamara-stripe for identifying this bug.

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