Make DictionaryArray::dictionary() thread-safe

Author: HyukjinKwon

cpp/src/arrow/array/array_dict.cc

@@ -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_;
 }
 

cpp/src/arrow/array/array_dict.h

@@ -19,6 +19,7 @@
 
 #include <cstdint>
 #include <memory>
+#include <mutex>
 
 #include "arrow/array/array_base.h"
 #include "arrow/array/data.h"
@@ -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