Daniel Becker created IMPALA-12775:
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             Summary: Check nullness in 
SlotDescriptor::CodegenWriteCollectionStructChild
                 Key: IMPALA-12775
                 URL: https://issues.apache.org/jira/browse/IMPALA-12775
             Project: IMPALA
          Issue Type: Bug
          Components: Backend
            Reporter: Daniel Becker


In SlotDescriptor::CodegenWriteCollectionStructChild() we do not check whether 
the struct (which is a child of a collection) is NULL. The code still works, 
probably because if the struct is NULL then the NULL bits for its members are 
also set, so we don't
try to read and copy invalid memory (for example the variable length part of a 
STRING). But we still check all children for NULL-ness, which is extra work.

Note that this does not affect whether the struct is NULL in the result, this 
function is called to make sure that the variable length data belonging to 
children of arrays is copied correctly (recursively).

See 
https://github.com/apache/impala/blob/00247c26e2a73d4ece4962be6fd5078843d938ae/be/src/runtime/descriptors.cc#L1475.



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