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https://issues.apache.org/jira/browse/ARROW-18274?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17630055#comment-17630055
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Laurent Querel commented on ARROW-18274:
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[~zeroshade] The same kind of problem occurs if I try to add data to the
created constructor. I simplified the initial code by removing the different
variants and fields to focus the report on what highlights the problem.
> [Go] Sparse union of structs is buggy
> -------------------------------------
>
> Key: ARROW-18274
> URL: https://issues.apache.org/jira/browse/ARROW-18274
> Project: Apache Arrow
> Issue Type: Bug
> Components: Go
> Affects Versions: 10.0.0
> Reporter: Laurent Querel
> Priority: Major
>
> There is a bug with union of structs in V10.
> The first unit test crash with a panic (i.e. invalid memory address or nil
> pointer dereference). The second test works as expected.
>
> {code:go}
> func TestDoesNotWork(t *testing.T) {
> dt1 := arrow.SparseUnionOf([]arrow.Field{
> {Name: "c", Type: arrow2.DictU16String},
> }, []arrow.UnionTypeCode{0})
> dt2 := arrow.StructOf(
> arrow.Field{Name: "b", Type: dt1},
> )
> dt3 := arrow.SparseUnionOf([]arrow.Field{
> {Name: "a", Type: dt2},
> }, []arrow.UnionTypeCode{0})
> pool := memory.NewGoAllocator()
> builder := array.NewSparseUnionBuilder(pool, dt3)
> arr := builder.NewArray()
> assert.Equal(t, 0, arr.Len())
> }
> func TestWorksAsExpected(t *testing.T) {
> dt1 := arrow.SparseUnionOf([]arrow.Field{
> {Name: "c", Type: &arrow.DictionaryType{
> IndexType: arrow.PrimitiveTypes.Uint16,
> ValueType: arrow.BinaryTypes.String,
> Ordered: false,
> }},
> }, []arrow.UnionTypeCode{0})
> dt2 := arrow.SparseUnionOf([]arrow.Field{
> {Name: "a", Type: dt1},
> }, []arrow.UnionTypeCode{0})
> pool := memory.NewGoAllocator()
> builder := array.NewSparseUnionBuilder(pool, dt2)
> arr := builder.NewArray()
> assert.Equal(t, 0, arr.Len())
> }
> {code}
>
> *Analysis:*
> - The `NewSparseUnionBuilder` calls the builders for each variant and also
> calls defer builder.Release.
> - The Struct Release method calls the Release methods of every field even if
> the refCount is not 0, so the Release method of the second union is called
> followed by the Release method of the dictionary.
> - Although, the union builder is returned without error, the builder is not
> usable.
> - This bug doesn't happen with 2 nested unions. As the internal counter is
> properly tested.
>
> First, I don't understand why the Release method of each variant is called
> right after the Union constructor is created. I also don't understand why the
> Release method of the structure calls the Release method of each field
> regardless of the value of the internal refCount. This looks like a bug to
> me, but I'm not quite sure yet what the right way to fix it will be.
>
> Any idea?
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