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Posted to issues@arrow.apache.org by "goel-skd (via GitHub)" <gi...@apache.org> on 2026/07/29 03:27:08 UTC

[I] [C++] ComputeLogicalNullCount crashes for dictionary arrays with null value type [arrow]

goel-skd opened a new issue, #50699:
URL: https://github.com/apache/arrow/issues/50699

   ### Describe the bug, including details regarding any error messages, version, and platform.
   
   ### Describe the bug, including details regarding any error messages, version, and platform
   
   `Array::ComputeLogicalNullCount` segfaults for dictionary arrays whose value type is `null()`, even though the array passes full validation:
   
   ```c++
   auto type = dictionary(int32(), null());
   auto array = DictArrayFromJSON(type, R"([0, 0, null])", "[null]");
   ASSERT_OK(array->ValidateFull());
   array->ComputeLogicalNullCount();  // segfault
   ```
   
   The values array of such a dictionary is a `NullArray`, which reports `null_count == length` without having a validity bitmap (`buffers[0]` is null). The early-out in `dict_util::LogicalNullCount` only triggers when the dictionary's null count is 0, so the per-index loop unconditionally reads the null validity-bitmap pointer:
   
   https://github.com/apache/arrow/blob/main/cpp/src/arrow/util/dict_util.cc#L56-L59
   
   Note that element-wise `Array::IsValid` handles the same data correctly (it falls back to `null_count == length` when there is no bitmap), so the aggregate and per-element views of logical nullness disagree.
   
   Found while working on GH-50338 (#50347).
   
   The crash is also reachable from the bindings through the C++ `count` kernel, which uses `ComputeLogicalNullCount` when counting nulls (e.g. `pyarrow.compute.count(arr, mode="only_null")` on such an array).
   
   ### Component(s)
   
   C++
   
   ### Component(s)
   
   C++


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Re: [I] [C++] ComputeLogicalNullCount crashes for dictionary arrays with null value type [arrow]

Posted by "goel-skd (via GitHub)" <gi...@apache.org>.
goel-skd commented on issue #50699:
URL: https://github.com/apache/arrow/issues/50699#issuecomment-5112445054

   take


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Re: [I] [C++] ComputeLogicalNullCount crashes for dictionary arrays with null value type [arrow]

Posted by "goel-skd (via GitHub)" <gi...@apache.org>.
goel-skd commented on issue #50699:
URL: https://github.com/apache/arrow/issues/50699#issuecomment-6429143686

   Will create a patch soon


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Re: [I] [C++] ComputeLogicalNullCount crashes for dictionary arrays with null value type [arrow]

Posted by "thisisnic (via GitHub)" <gi...@apache.org>.
thisisnic commented on issue #50699:
URL: https://github.com/apache/arrow/issues/50699#issuecomment-6428659758

   Confirmed can reproduce on 26.0.0


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Re: [I] [C++] ComputeLogicalNullCount crashes for dictionary arrays with null value type [arrow]

Posted by "thisisnic (via GitHub)" <gi...@apache.org>.
thisisnic commented on issue #50699:
URL: https://github.com/apache/arrow/issues/50699#issuecomment-6429432050

   Thanks!


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