GH-39519: [Swift] Fix null count when using reader

[abandy]

Currently the reader is not properly setting the null count when building an array from a stream. This PR adds a fix for this.

• Closes: [Swift] Null count is not being set when reading from stream #39519

[github-actions]

⚠️ GitHub issue #39519 has been automatically assigned in GitHub to PR creator.

[kou]

This approach passes nullCount from Apache Arrow data (.arrow/.arrows data) to ArrowArray via ArrowNullBuffer, right?
The C++ implementation passes it to arrow::Array (arrow::ArrayData) directly:

arrow/cpp/src/arrow/ipc/reader.cc

Line 275 in 1e74ace

out->null_count = node->null_count();

If we use this approach in Swift too, we will use API something like the following:

diff --git a/swift/Arrow/Sources/Arrow/ArrowReader.swift b/swift/Arrow/Sources/Arrow/ArrowReader.swift
index d9dc1bdb47..c985b37ca6 100644
--- a/swift/Arrow/Sources/Arrow/ArrowReader.swift
+++ b/swift/Arrow/Sources/Arrow/ArrowReader.swift
@@ -88,7 +88,7 @@ public class ArrowReader {
             let valueBuffer = loadInfo.recordBatch.buffers(at: loadInfo.bufferIndex + 2)!
             let arrowValueBuffer = makeBuffer(valueBuffer, fileData: loadInfo.fileData,
                                               length: UInt(node.length), messageOffset: loadInfo.messageOffset)
-            return makeArrayHolder(loadInfo.field, buffers: [arrowNullBuffer, arrowOffsetBuffer, arrowValueBuffer])
+            return makeArrayHolder(loadInfo.field, buffers: [arrowNullBuffer, arrowOffsetBuffer, arrowValueBuffer], UInt(node.nullCount))
         } catch let error as ArrowError {
             return .failure(error)
         } catch {
diff --git a/swift/Arrow/Sources/Arrow/ArrowReaderHelper.swift b/swift/Arrow/Sources/Arrow/ArrowReaderHelper.swift
index fa52160478..13d6209dee 100644
--- a/swift/Arrow/Sources/Arrow/ArrowReaderHelper.swift
+++ b/swift/Arrow/Sources/Arrow/ArrowReaderHelper.swift
@@ -126,7 +126,8 @@ private func makeFixedHolder<T>(
 
 func makeArrayHolder( // swiftlint:disable:this cyclomatic_complexity
     _ field: org_apache_arrow_flatbuf_Field,
-    buffers: [ArrowBuffer]
+    buffers: [ArrowBuffer],
+    nullCount: UInt
 ) -> Result<ArrowArrayHolder, ArrowError> {
     let type = field.typeType
     switch type {

What do you think this approach?

[abandy]

Hi @kou, I have made the requested updates. Please review when you get a chance. Thank you!

[kou]

+1

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 5 benchmarking runs that have been run so far on merge-commit e632364.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 2 possible false positives for unstable benchmarks that are known to sometimes produce them.