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GH-36503: [C++] Make DictionaryArray::dictionary() thread-safe #48905

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GH-36503: [C++] Make DictionaryArray::dictionary() thread-safe #48905

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6 changes: 2 additions & 4 deletions cpp/src/arrow/array/array_dict.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -108,10 +108,8 @@ DictionaryArray::DictionaryArray(const std::shared_ptr<DataType>& type,
}

const std::shared_ptr<Array>& DictionaryArray::dictionary() const {
if (!dictionary_) {
// TODO(GH-36503) this isn't thread safe
dictionary_ = MakeArray(data_->dictionary);
}
std::call_once(dictionary_init_flag_,
[this]() { dictionary_ = MakeArray(data_->dictionary); });
return dictionary_;
}

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5 changes: 4 additions & 1 deletion cpp/src/arrow/array/array_dict.h
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Expand Up @@ -19,6 +19,7 @@

#include <cstdint>
#include <memory>
#include <mutex>

#include "arrow/array/array_base.h"
#include "arrow/array/data.h"
Expand Down Expand Up @@ -118,8 +119,10 @@ class ARROW_EXPORT DictionaryArray : public Array {
const DictionaryType* dict_type_;
std::shared_ptr<Array> indices_;

// Lazily initialized when invoking dictionary()
// Lazily initialized when invoking dictionary().
// Thread-safe initialization is ensured by dictionary_init_flag_.
mutable std::shared_ptr<Array> dictionary_;
mutable std::once_flag dictionary_init_flag_;
};

/// \brief Helper class for incremental dictionary unification
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