[GitHub] [arrow] bkietz commented on a change in pull request #8050: ARROW-9852: [C++] Validate dictionaries fully when combining deltas

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bkietz commented on a change in pull request #8050:
URL: https://github.com/apache/arrow/pull/8050#discussion_r482179398



##########
File path: cpp/src/arrow/ipc/dictionary.cc
##########
@@ -143,10 +143,43 @@ int DictionaryFieldMapper::num_fields() const { return impl_->num_fields(); }
 // DictionaryMemo implementation
 
 struct DictionaryMemo::Impl {
-  // Map of dictionary id to dictionary array
-  std::unordered_map<int64_t, std::shared_ptr<ArrayData>> id_to_dictionary_;
+  // Map of dictionary id to dictionary array(s) (several in case of deltas)
+  std::unordered_map<int64_t, ArrayDataVector> id_to_dictionary_;
   std::unordered_map<int64_t, std::shared_ptr<DataType>> id_to_type_;
   DictionaryFieldMapper mapper_;
+
+  Result<decltype(id_to_dictionary_)::iterator> FindDictionary(int64_t id) {
+    auto it = id_to_dictionary_.find(id);
+    if (it == id_to_dictionary_.end()) {
+      return Status::KeyError("Dictionary with id ", id, " not found");
+    }
+    return it;
+  }
+
+  Result<std::shared_ptr<ArrayData>> ReifyDictionary(int64_t id, MemoryPool* pool) {
+    ARROW_ASSIGN_OR_RAISE(auto it, FindDictionary(id));
+    ArrayDataVector* data_vector = &it->second;
+
+    DCHECK(!data_vector->empty());
+    if (data_vector->size() > 1) {
+      // There are deltas, we need to concatenate them to the first dictionary.
+      ArrayVector to_combine;
+      to_combine.reserve(data_vector->size());
+      // IMPORTANT: At this point, the dictionary data may be untrusted.
+      // We need to validate it, as concatenation can crash on invalid or
+      // corrupted data.  Full validation is necessary for certain types
+      // (for example nested dictionaries).

Review comment:
       IIUC it's also necessary for `string`. If not, it would probably be worthwhile to add a fast branch for dictionaries of primitives

##########
File path: cpp/src/arrow/ipc/dictionary.cc
##########
@@ -143,10 +143,43 @@ int DictionaryFieldMapper::num_fields() const { return impl_->num_fields(); }
 // DictionaryMemo implementation
 
 struct DictionaryMemo::Impl {
-  // Map of dictionary id to dictionary array
-  std::unordered_map<int64_t, std::shared_ptr<ArrayData>> id_to_dictionary_;
+  // Map of dictionary id to dictionary array(s) (several in case of deltas)
+  std::unordered_map<int64_t, ArrayDataVector> id_to_dictionary_;
   std::unordered_map<int64_t, std::shared_ptr<DataType>> id_to_type_;
   DictionaryFieldMapper mapper_;
+
+  Result<decltype(id_to_dictionary_)::iterator> FindDictionary(int64_t id) {
+    auto it = id_to_dictionary_.find(id);
+    if (it == id_to_dictionary_.end()) {
+      return Status::KeyError("Dictionary with id ", id, " not found");
+    }
+    return it;
+  }
+
+  Result<std::shared_ptr<ArrayData>> ReifyDictionary(int64_t id, MemoryPool* pool) {
+    ARROW_ASSIGN_OR_RAISE(auto it, FindDictionary(id));
+    ArrayDataVector* data_vector = &it->second;
+
+    DCHECK(!data_vector->empty());
+    if (data_vector->size() > 1) {
+      // There are deltas, we need to concatenate them to the first dictionary.
+      ArrayVector to_combine;
+      to_combine.reserve(data_vector->size());
+      // IMPORTANT: At this point, the dictionary data may be untrusted.
+      // We need to validate it, as concatenation can crash on invalid or
+      // corrupted data.  Full validation is necessary for certain types
+      // (for example nested dictionaries).
+      // XXX: this won't work if there are unresolved nested dictionaries.

Review comment:
       Would it be worthwhile to add a test verifying that an unresolved nested dictionary will emit an error instead of crashing?




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