hcrosse opened a new issue, #1463:
URL: https://github.com/apache/arrow-go/issues/1463
### Describe the bug, including details regarding any error messages,
version, and platform.
Passing a null dictionary scalar to `is_null` crashes the process instead of
returning `true`. The panic happens in an executor goroutine, so the caller
cannot recover it.
I reproduced this on v18.8.0 and on main at `c620eaa3`, on macOS with Go
1.27.3.
**Reproduction**
Save this as `main.go` in a module that requires
`github.com/apache/arrow-go/v18 v18.8.0`, or in a new subdirectory of an
arrow-go checkout at `c620eaa363c1b68ed93a810da7a8d46bd436f122`, then run `go
run .` from that directory:
```go
package main
import (
"context"
"fmt"
"github.com/apache/arrow-go/v18/arrow"
"github.com/apache/arrow-go/v18/arrow/compute"
"github.com/apache/arrow-go/v18/arrow/scalar"
)
func main() {
dt := &arrow.DictionaryType{IndexType: arrow.PrimitiveTypes.Int8,
ValueType: arrow.BinaryTypes.String}
s := scalar.MakeNullScalar(dt)
out, err := compute.CallFunction(context.Background(), "is_null", nil,
compute.NewDatum(s))
if err != nil {
panic(err)
}
defer out.Release()
fmt.Println(out)
}
```
It exits with:
```text
panic: runtime error: invalid memory address or nil pointer dereference
```
**Expected behavior**
`is_null` should return `true` without panicking. In pyarrow `26.0.0`, the
equivalent null dictionary scalar has an empty string dictionary, passes full
validation, and returns `True` from `is_null`.
**Cause**
`NewNullDictScalar`, which `MakeNullScalar` calls for dictionary types,
leaves `Value.Dict` nil:
https://github.com/apache/arrow-go/blob/c620eaa363c1b68ed93a810da7a8d46bd436f122/arrow/scalar/nested.go#L352-L357
During promotion of the scalar to an array, `exec.ArraySpan.FillFromScalar`
dereferences that dictionary:
https://github.com/apache/arrow-go/blob/c620eaa363c1b68ed93a810da7a8d46bd436f122/arrow/compute/exec/span.go#L300-L306
An empty dictionary of the value type avoids this panic, as in Arrow C++'s
[`DictionaryScalar(type)`
constructor](https://github.com/apache/arrow/blob/0ab78e2df489055b71a7d26a666a52823f87f01c/cpp/src/arrow/scalar.cc#L768-L773).
I have a patch with tests and can open a PR.
### Component(s)
Other
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