[jira] [Created] (ARROW-5935) [C++] ArrayBuilders with mutable type are not robustly supported

["Benjamin Kietzman (JIRA)" <ji...@apache.org>]

Benjamin Kietzman created ARROW-5935:
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             Summary: [C++] ArrayBuilders with mutable type are not robustly supported
                 Key: ARROW-5935
                 URL: https://issues.apache.org/jira/browse/ARROW-5935
             Project: Apache Arrow
          Issue Type: Bug
          Components: C++
            Reporter: Benjamin Kietzman
            Assignee: Benjamin Kietzman


(Dense|Sparse)UnionBuilder, DictionaryBuilder, Addaptive(U)IntBuilders and any nested builder which contains one of those may Finish to an array whose type disagrees with what was passed to MakeBuilder. This is not well documented or supported; ListBuilder checks if its child has changed type but StructBuilder does not. Furthermore ListBuilder's check does not catch modifications to a DictionaryBuidler's type and results in an invalid array on Finish: https://github.com/apache/arrow/blob/1bcfbe1/cpp/src/arrow/array-dict-test.cc#L951-L994

Let's add to the ArrayBuilder contract: the type property is null iff that builder's type is indeterminate until Finish() is called. Then all nested builders can check this on their children at construction and bubble type mutability correclty



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