Avoid reusing source providers for script based block loaders. #117792
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This change attempts to solve the following issue: ValueSourceOperator can read values in row striding or columnar fashion. When values are read in columnar fashion and multiple runtime fields synthetize source then this can cause the same `SourceProvider` evaluation the same range of docs ids multiple times. This can then result in unexpected io errors at the codec level. This is because the same doc value instances are used by `SourceProvider`. Re-evaluating the same docids is in violation of the contract of the DocIdSetIterator#advance(...) / DocIdSetIterator#advanceExact(...) methods, which documents that unexpected behaviour can occur if target docid is lower than current docid position. Note that this is only an issue for synthetic source loader and not for stored source loader. And not when executing in row stride fashion which sometimes happen in compute engine and always happen in _search api. Closes elastic#117644
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This change attempts to solve the following issue:
ValueSourceOperator can read values in row striding or columnar fashion. When values are read in columnar fashion and multiple runtime fields synthetize source then this can cause the same
SourceProviderevaluation the same range of docs ids multiple times. This can then result in unexpected io errors at the codec level. This is because the same doc value instances are used bySourceProvider.Re-evaluating the same docids is in violation of the contract of the DocIdSetIterator#advance(...) / DocIdSetIterator#advanceExact(...) methods, which documents that unexpected behaviour can occur if target docid is lower than current docid position.
Note that this is only an issue for synthetic source loader and not for stored source loader. And not when executing in row stride fashion which sometimes happen in compute engine and always happen in _search api.
Closes #117644