GH-47447: [C++] Fix replace_with_mask for null type arrays

cpp/src/arrow/compute/kernels/vector_replace.cc

@@ -217,15 +217,15 @@ struct ReplaceMaskImpl<Type, enable_if_null<Type>> {
   static Result<int64_t> ExecScalarMask(KernelContext* ctx, const ArraySpan& array,
                                         const BooleanScalar& mask, ExecValue replacements,
                                         int64_t replacements_offset, ExecResult* out) {
-    out->value = array;
-    return Status::OK();
+    out->value = array.ToArrayData();
+    return replacements_offset;
   }
   static Result<int64_t> ExecArrayMask(KernelContext* ctx, const ArraySpan& array,
                                        const ArraySpan& mask, int64_t mask_offset,
                                        ExecValue replacements,
                                        int64_t replacements_offset, ExecResult* out) {
-    out->value = array;
-    return Status::OK();
+    out->value = array.ToArrayData();
+    return replacements_offset;
   }
 };
 

cpp/src/arrow/compute/kernels/vector_replace_test.cc

@@ -199,6 +199,13 @@ class TestReplaceBoolean : public TestReplaceKernel<BooleanType> {
   }
 };
 
+class TestReplaceNull : public TestReplaceKernel<NullType> {
+ protected:
+  std::shared_ptr<DataType> type() override {
+    return TypeTraits<NullType>::type_singleton();
+  }
+};
+
 class TestReplaceFixedSizeBinary : public TestReplaceKernel<FixedSizeBinaryType> {
  protected:
   std::shared_ptr<DataType> type() override { return fixed_size_binary(3); }
@@ -538,6 +545,35 @@ TEST_F(TestReplaceBoolean, ReplaceWithMask) {
   }
 }
 
+TEST_F(TestReplaceNull, ReplaceWithMask) {
+  std::vector<ReplaceWithMaskCase> cases = {
+      {this->array("[]"), this->mask_scalar(false), this->array("[]"), this->array("[]")},
+      {this->array("[]"), this->mask_scalar(true), this->array("[]"), this->array("[]")},
+      {this->array("[]"), this->null_mask_scalar(), this->array("[]"), this->array("[]")},
+
+      {this->array("[null]"), this->mask_scalar(false), this->array("[]"),
+       this->array("[null]")},
+
+      {this->array("[null]"), this->mask_scalar(true), this->array("[null]"),
+       this->array("[null]")},
+
+      {this->array("[null]"), this->null_mask_scalar(), this->array("[]"),
+       this->array("[null]")},
+
+      {this->array("[null, null]"), this->mask("[false, false]"), this->array("[]"),
+       this->array("[null, null]")},
+      {this->array("[null, null]"), this->mask("[true, true]"),
+       this->array("[null, null]"), this->array("[null, null]")},
+      {this->array("[null, null]"), this->mask("[null, null]"), this->array("[]"),
+       this->array("[null, null]")},
+  };
+
+  for (auto test_case : cases) {
+    this->Assert(ReplaceWithMask, test_case.input, test_case.mask, test_case.replacements,
+                 test_case.expected);
+  }
+}
+
 TEST_F(TestReplaceBoolean, ReplaceWithMaskErrors) {
   EXPECT_RAISES_WITH_MESSAGE_THAT(
       Invalid,

python/pyarrow/tests/test_compute.py

@@ -1269,6 +1269,34 @@ def test_extract_regex_span():
     assert struct.tolist() == expected
 
 
+def test_replace_with_mask_null_type():
+    # GH-47447: replace_with_mask crashed for null type arrays
+    input = pa.array([None], pa.null())
+    replacements = pa.array([None], pa.null())
+
+    result = pc.replace_with_mask(input, True, replacements)
+    assert result.type == pa.null()
+    result.validate(full=True)
+    assert result.to_pylist() == [None]
+
+    result = pc.replace_with_mask(input, False, replacements)
+    assert result.type == pa.null()
+    result.validate(full=True)
+    assert result.to_pylist() == [None]
+
+    mask = pa.array([True])
+    result = pc.replace_with_mask(input, mask, replacements)
+    assert result.type == pa.null()
+    result.validate(full=True)
+    assert result.to_pylist() == [None]
+
+    mask = pa.array([False])
+    result = pc.replace_with_mask(input, mask, replacements)
+    assert result.type == pa.null()
+    result.validate(full=True)
+    assert result.to_pylist() == [None]
+
+
 def test_binary_join():
     ar_list = pa.array([['foo', 'bar'], None, []])
     expected = pa.array(['foo-bar', None, ''])