[BugFix] [Enhancement] Fix nullptr and support Iceberg null padding

PR #48151 introduced a regression, where what would previously return an
Error now caused a nullptr dereference and crashed the entire CN.
This change removes the crashing and, for Iceberg, also adds support for
padding evolved fields with null values, as per Iceberg spec.

Signed-off-by: Samrose Ahmed <[email protected]>

be/src/formats/CMakeLists.txt

@@ -58,6 +58,7 @@ add_library(Formats STATIC
         parquet/column_reader.cpp
         parquet/scalar_column_reader.cpp
         parquet/complex_column_reader.cpp
+        parquet/default_column_reader.h
         parquet/encoding.cpp
         parquet/level_codec.cpp
         parquet/page_reader.cpp

be/src/formats/parquet/column_reader.cpp

@@ -200,7 +200,6 @@ void ColumnReader::get_subfield_pos_with_pruned_type(const ParquetField& field,
         if (parquet_field_it == field_id_2_pos.end()) {
             // Means newly added struct subfield not existed in original parquet file, we put nullptr
             // column reader in children_reader, we will append default value for this subfield later.
-            LOG(INFO) << "Struct subfield name: " + format_subfield_name + " not found in ParquetField.";
             pos[i] = -1;
             iceberg_schema_subfield[i] = nullptr;
             continue;

be/src/formats/parquet/default_column_reader.h

@@ -0,0 +1,40 @@
+// Copyright 2021-present StarRocks, Inc. All rights reserved.
+//
+// Licensed 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
+//
+//     https://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.
+
+#pragma once
+
+#include "formats/parquet/column_reader.h"
+
+namespace starrocks::parquet {
+
+// Used to pad with nulls for schema evolved struct columns, as per Iceberg spec.
+class DefaultColumnReader final : public ColumnReader {
+public:
+    explicit DefaultColumnReader() {}
+    ~DefaultColumnReader() override = default;
+
+    Status read_range(const Range<uint64_t>& range, const Filter* filter, ColumnPtr& dst) override {
+        dst->append_default(range.span_size());
+        return Status::OK();
+    }
+
+    void get_levels(level_t** def_levels, level_t** rep_levels, size_t* num_levels) override {}
+    void set_need_parse_levels(bool need_parse_levels) override { }
+    void select_offset_index(const SparseRange<uint64_t>& range, const uint64_t rg_first_row) override {}
+    void collect_column_io_range(std::vector<io::SharedBufferedInputStream::IORange>* ranges, int64_t* end_offset,
+                                 ColumnIOType type, bool active) override {}
+
+};
+
+} // namespace starrocks::parquet

be/src/formats/parquet/group_reader.cpp

@@ -32,6 +32,7 @@
 #include "formats/parquet/page_index_reader.h"
 #include "formats/parquet/schema.h"
 #include "gutil/strings/substitute.h"
+#include "default_column_reader.h"
 #include "runtime/types.h"
 #include "simd/simd.h"
 #include "storage/chunk_helper.h"
@@ -274,6 +275,14 @@ Status GroupReader::_create_column_reader(const GroupReaderParam::Column& column
             RETURN_IF_ERROR(ColumnReader::create(_column_reader_opts, schema_node, column.slot_type(),
                                                  column.t_iceberg_schema_field, &column_reader));
         }
+        if (column_reader == nullptr) {
+            if (column.t_iceberg_schema_field == nullptr) {
+                return Status::InternalError("No valid column reader.");
+            }
+            // pad with nulls for Iceberg to support schema evolution
+            std::unique_ptr<DefaultColumnReader> reader(new DefaultColumnReader());
+            column_reader = std::move(reader);
+        }
 
         if (column.slot_type().is_complex_type()) {
             // For complex type columns, we need parse def & rep levels.