fix: guard multiBufferBuilder UnsafeAppend against short copy (#896)

Author: fallintoplace

arrow/array/bufferbuilder.go

@@ -243,6 +243,9 @@ func (b *multiBufferBuilder) UnsafeAppend(hdr *arrow.ViewHeader, val []byte) {
 	hdr.SetIndexOffset(int32(idx), int32(offset))
 
 	n := copy(buf.Buf()[offset:], val)
+	if n != len(val) {
+		panic("arrow/array: multibufferbuilder could not append full value")
+	}
 	buf.ResizeNoShrink(offset + n)
 }
 

arrow/array/bufferbuilder_test.go

@@ -0,0 +1,41 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License");  you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package array
+
+import (
+	"testing"
+
+	"github.com/apache/arrow-go/v18/arrow"
+	"github.com/apache/arrow-go/v18/arrow/memory"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMultiBufferBuilderUnsafeAppendPanicsOnTruncatedCopy(t *testing.T) {
+	builder := multiBufferBuilder{mem: memory.NewGoAllocator(), blockSize: 8}
+	builder.Retain()
+	defer builder.Release()
+
+	buf := memory.NewResizableBuffer(builder.mem)
+	buf.ResizeNoShrink(1)
+	builder.blocks = []*memory.Buffer{buf}
+	builder.currentOutBuffer = 0
+
+	var hdr arrow.ViewHeader
+	assert.Panics(t, func() {
+		builder.UnsafeAppend(&hdr, make([]byte, 64))
+	})
+}