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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