[I] MR: IcebergSplit.getLocations discards cached block locations on repeated calls [iceberg]

["thswlsqls (via GitHub)" <gi...@apache.org>]

thswlsqls opened a new issue, #18032:
URL: https://github.com/apache/iceberg/issues/18032

   **Apache Iceberg version**
   main @ 9d7b2c52f
   
   **Query engine**
   None — the bug is in `iceberg-mr` and is engine-agnostic.
   
   **Please describe the bug**
   `IcebergSplit.getLocations()` (`mr/src/main/java/org/apache/iceberg/mr/mapreduce/IcebergSplit.java` line 70-82) returns the computed block locations on the first call and `["*"]` on every call after that, even though nothing about the split changed.
   
   The `transient locations` field and the `locations == null` check say the value is computed once and cached. The `else` branch defeats that: it runs whenever `locations` is already set and overwrites it with `ANYWHERE`. Hadoop's `InputSplit.getLocations()` contract expects repeated calls on an unchanged split to return the same value.
   
   The `else` branch came from #1582, which kept a deserialized split on a worker node (where `conf` is null) from hitting an NPE. Before that, #1192 had `if (locations == null)` alone.
   
   **Steps to reproduce**
   Build splits with `iceberg.mr.locality` enabled (`InputFormatConfig.ConfigBuilder.preferLocality()`) and call `getLocations()` twice on the same split.
   
   - Expected: both calls return the computed hosts, e.g. `["localhost"]`.
   - Actual: the first returns `["localhost"]`, the second returns `["*"]`.
   
   `TestIcebergInputFormats#testLocality` reproduces this without Docker.
   
   **Additional context**
   N/A
   


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