ARROW-7390: [C++][Dataset] Fix RecordBatchProjector race #6661
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I could not come up with an deterministic unit test for this, it also had to be run in release mode. I tested the issue with scanning the nyc dataset locally. |
pitrou
reviewed
Mar 19, 2020
| @@ -44,7 +44,10 @@ static inline RecordBatchIterator ProjectRecordBatch(RecordBatchIterator it, | |||
| RecordBatchProjector* projector, | |||
| MemoryPool* pool) { | |||
| return MakeMaybeMapIterator( | |||
| [=](std::shared_ptr<RecordBatch> in) { return projector->Project(*in, pool); }, | |||
| [=](std::shared_ptr<RecordBatch> in) { | |||
| RecordBatchProjector local_projector{*projector}; | |||
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Hmm... perhaps add a comment explaining why this is needed?
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wesm
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Mar 20, 2020
cpp/src/arrow/dataset/projector.cc
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| @@ -63,7 +63,7 @@ Status RecordBatchProjector::SetDefaultValue(FieldRef ref, | |||
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| Result<std::shared_ptr<RecordBatch>> RecordBatchProjector::Project( | |||
| const RecordBatch& batch, MemoryPool* pool) { | |||
| if (from_ == nullptr || !batch.schema()->Equals(*from_)) { | |||
| if (from_ == nullptr || !batch.schema()->Equals(*from_, false /*check_metadata*/)) { | |||
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Nit: we should try to use /*param=*/ consistently (this is used elsewhere)
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+1 At some point we should examine Fragment::splitable() to determine whether ScanTasks from a single Fragment should be kept in a single thread. For such fragments local_projector will be unnecessary |
The RecordBatchProjector is shared accross ScanTasks of the same Fragment. The resize operation of missing columns is not thread safe. This change ensure that each ScanTask gets his own projector. The copy should not be costly since it's copying empty vectors and one shared pointer.
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The RecordBatchProjector is shared accross ScanTasks of the same Fragment. The resize operation of missing columns is not thread safe. This change ensure that each ScanTask gets his own projector. The copy should not be costly since it's copying empty vectors and one shared pointer.