apacheGH-41697: [Go][Parquet] Release BufferWriter when BufferedPageW…

…riter is closed (apache#41698)

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### Rationale for this change

A small buffer gets reallocated after calling `Finish()`, causing that
memory to leak.

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### What changes are included in this PR?
Release the buffer when the pagewriter is closed.

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### Are these changes tested?
Yes

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### Are there any user-facing changes?
Memory will not leak on this code path.

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<!-- **This PR contains a "Critical Fix".** -->
* GitHub Issue: apache#41697

go/parquet/internal/encoding/types.go

@@ -361,11 +361,16 @@ func (b *BufferWriter) Truncate() {
 func (b *BufferWriter) Reset(initial int) {
 	if b.buffer != nil {
 		b.buffer.Release()
+	} else {
+		b.buffer = memory.NewResizableBuffer(b.mem)
 	}
 
 	b.pos = 0
 	b.offset = 0
-	b.Reserve(initial)
+
+	if initial > 0 {
+		b.Reserve(initial)
+	}
 }
 
 // Reserve ensures that there is at least enough capacity to write nbytes

go/parquet/pqarrow/file_writer_test.go

@@ -18,6 +18,7 @@ package pqarrow_test
 
 import (
 	"bytes"
+	"math"
 	"strings"
 	"testing"
 
@@ -87,3 +88,44 @@ func TestFileWriterNumRows(t *testing.T) {
 	require.NoError(t, writer.Close())
 	assert.Equal(t, 4, writer.NumRows())
 }
+
+func TestFileWriterBuffered(t *testing.T) {
+	schema := arrow.NewSchema([]arrow.Field{
+		{Name: "one", Nullable: true, Type: arrow.PrimitiveTypes.Float64},
+		{Name: "two", Nullable: true, Type: arrow.PrimitiveTypes.Float64},
+	}, nil)
+
+	data := `[
+		{"one": 1, "two": 2},
+		{"one": 1, "two": null},
+		{"one": null, "two": 2},
+		{"one": null, "two": null}
+	]`
+
+	alloc := memory.NewCheckedAllocator(memory.DefaultAllocator)
+	defer alloc.AssertSize(t, 0)
+
+	record, _, err := array.RecordFromJSON(alloc, schema, strings.NewReader(data))
+	require.NoError(t, err)
+	defer record.Release()
+
+	output := &bytes.Buffer{}
+	writer, err := pqarrow.NewFileWriter(
+		schema,
+		output,
+		parquet.NewWriterProperties(
+			parquet.WithAllocator(alloc),
+			// Ensure enough space so we can close the writer with rows still buffered
+			parquet.WithMaxRowGroupLength(math.MaxInt64),
+		),
+		pqarrow.NewArrowWriterProperties(
+			pqarrow.WithAllocator(alloc),
+		),
+	)
+	require.NoError(t, err)
+
+	require.NoError(t, writer.WriteBuffered(record))
+
+	require.NoError(t, writer.Close())
+	assert.Equal(t, 4, writer.NumRows())
+}