pitrou commented on code in PR #42174:
URL: https://github.com/apache/arrow/pull/42174#discussion_r1682928278


##########
cpp/tools/parquet/parquet_dump_footer.cc:
##########
@@ -0,0 +1,248 @@
+// 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.
+
+#include <cstdint>
+#include <cstring>
+#include <exception>
+#include <fstream>
+#include <iostream>
+#include <limits>
+#include <optional>
+#include <random>
+
+#include <thrift/protocol/TCompactProtocol.h>
+#include <thrift/transport/TBufferTransports.h>
+
+#include "arrow/filesystem/filesystem.h"
+#include "arrow/util/endian.h"
+#include "arrow/util/ubsan.h"
+#include "generated/parquet_types.h"
+#include "parquet/thrift_internal.h"
+
+using apache::thrift::protocol::TCompactProtocol;
+using apache::thrift::transport::TMemoryBuffer;
+using apache::thrift::transport::TTransport;
+
+namespace {

Review Comment:
   I would recommend a  `namespace parquet` around all this (except the `main` 
obviously), both to more easily access Parquet APIs and avoid potential 
conflicts in the top-level namespace.



##########
cpp/tools/parquet/parquet_dump_footer.cc:
##########
@@ -0,0 +1,248 @@
+// 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.
+
+#include <cstdint>
+#include <cstring>
+#include <exception>
+#include <fstream>
+#include <iostream>
+#include <limits>
+#include <optional>
+#include <random>
+
+#include <thrift/protocol/TCompactProtocol.h>
+#include <thrift/transport/TBufferTransports.h>
+
+#include "arrow/filesystem/filesystem.h"
+#include "arrow/util/endian.h"
+#include "arrow/util/ubsan.h"
+#include "generated/parquet_types.h"
+#include "parquet/thrift_internal.h"
+
+using apache::thrift::protocol::TCompactProtocol;
+using apache::thrift::transport::TMemoryBuffer;
+using apache::thrift::transport::TTransport;
+
+namespace {
+int PrintHelp() {
+  std::cerr << R"(
+Usage: parquet-dump-footer
+  -h|--help    Print help and exit
+  --no-scrub   Do not scrub potentially PII metadata
+  --json       Output JSON instead of binary
+  --in         Input file: required
+  --out        Output file: defaults to stdout
+
+  Dumps the footer of a Parquet file to stdout or a file, optionally with
+  potentially user specific metadata scrubbed.
+)";
+  return 1;
+}
+
+uint32_t ReadLE32(const void* p) {
+  uint32_t x = arrow::util::SafeLoadAs<uint32_t>(static_cast<const 
uint8_t*>(p));
+  return arrow::bit_util::FromLittleEndian(x);
+}
+
+void AppendLE32(uint32_t v, std::string* out) {
+  v = arrow::bit_util::ToLittleEndian(v);
+  out->append(reinterpret_cast<const char*>(&v), sizeof(v));
+}
+
+template <typename T>
+bool Deserialize(const char* data, uint32_t len, T* obj) {
+  parquet::ThriftDeserializer des(/*string_size_limit=*/1 << 30,
+                                  /*container_size_limit=*/1 << 30);
+  try {
+    des.DeserializeMessage(reinterpret_cast<const uint8_t*>(data), &len, obj);
+    return true;
+  } catch (const std::exception& e) {
+    std::cerr << "Failed to deserialize: " << e.what() << "\n";
+    return false;
+  }
+}
+
+template <typename T>
+bool Serialize(const T& obj, std::string* out) {
+  parquet::ThriftSerializer ser(/*initial_buffer_size=*/10 << 20);
+  try {
+    ser.SerializeToString(&obj, out);
+    return true;
+  } catch (const std::exception& e) {
+    std::cerr << "Failed to serialize: " << e.what() << "\n";
+    return false;
+  }
+}
+
+// Replace the contents of s with random data of the same length.
+void Scrub(std::string* s) {
+  static char pool[4096];
+  static std::mt19937 rng(std::random_device{}());
+  static const bool kPoolInit = [] {
+    std::uniform_int_distribution<> caps(65, 90);
+    for (size_t i = 0; i < sizeof(pool); i++) pool[i] = caps(rng);
+    return true;
+  }();
+  (void)kPoolInit;
+
+  const size_t n = s->size();
+  s->clear();
+  while (s->size() < n) {
+    // To avoid repeating patterns we start somewhere up to halfway through 
the pool and
+    // append up to half the pool chars.
+    std::uniform_int_distribution<size_t> half(0, sizeof(pool) / 2 - 1);
+    size_t m = std::min(n, half(rng) + 1 /* at least one */);
+    s->append(&pool[half(rng)], m);
+  }

Review Comment:
   This seems very complicated. Why not simply overwrite `s` with random chars?
   Something like (untested):
   ```suggestion
     std::uniform_int_distribution<unsigned int> char_dist(0, 255);
     for (char& c : *s) {
       c = static_cast<char>(char_dist(rng));
     }
   ```



##########
cpp/tools/parquet/parquet_dump_footer.cc:
##########
@@ -0,0 +1,248 @@
+// 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.
+
+#include <cstdint>
+#include <cstring>
+#include <exception>
+#include <fstream>
+#include <iostream>
+#include <limits>
+#include <optional>
+#include <random>
+
+#include <thrift/protocol/TCompactProtocol.h>
+#include <thrift/transport/TBufferTransports.h>
+
+#include "arrow/filesystem/filesystem.h"
+#include "arrow/util/endian.h"
+#include "arrow/util/ubsan.h"
+#include "generated/parquet_types.h"
+#include "parquet/thrift_internal.h"
+
+using apache::thrift::protocol::TCompactProtocol;
+using apache::thrift::transport::TMemoryBuffer;
+using apache::thrift::transport::TTransport;
+
+namespace {
+int PrintHelp() {
+  std::cerr << R"(
+Usage: parquet-dump-footer
+  -h|--help    Print help and exit
+  --no-scrub   Do not scrub potentially PII metadata
+  --json       Output JSON instead of binary
+  --in         Input file: required
+  --out        Output file: defaults to stdout
+
+  Dumps the footer of a Parquet file to stdout or a file, optionally with
+  potentially user specific metadata scrubbed.
+)";
+  return 1;
+}
+
+uint32_t ReadLE32(const void* p) {
+  uint32_t x = arrow::util::SafeLoadAs<uint32_t>(static_cast<const 
uint8_t*>(p));
+  return arrow::bit_util::FromLittleEndian(x);
+}
+
+void AppendLE32(uint32_t v, std::string* out) {
+  v = arrow::bit_util::ToLittleEndian(v);
+  out->append(reinterpret_cast<const char*>(&v), sizeof(v));
+}
+
+template <typename T>
+bool Deserialize(const char* data, uint32_t len, T* obj) {

Review Comment:
   I think we should either keep throwing an exception here, like in Parquet 
C++, or return a Status or Result<T>, like in Arrow C++.



##########
cpp/tools/parquet/parquet_dump_footer.cc:
##########
@@ -0,0 +1,248 @@
+// 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.
+
+#include <cstdint>
+#include <cstring>
+#include <exception>
+#include <fstream>
+#include <iostream>
+#include <limits>
+#include <optional>
+#include <random>
+
+#include <thrift/protocol/TCompactProtocol.h>
+#include <thrift/transport/TBufferTransports.h>
+
+#include "arrow/filesystem/filesystem.h"
+#include "arrow/util/endian.h"
+#include "arrow/util/ubsan.h"
+#include "generated/parquet_types.h"
+#include "parquet/thrift_internal.h"
+
+using apache::thrift::protocol::TCompactProtocol;
+using apache::thrift::transport::TMemoryBuffer;
+using apache::thrift::transport::TTransport;
+
+namespace {
+int PrintHelp() {
+  std::cerr << R"(
+Usage: parquet-dump-footer
+  -h|--help    Print help and exit
+  --no-scrub   Do not scrub potentially PII metadata
+  --json       Output JSON instead of binary
+  --in         Input file: required
+  --out        Output file: defaults to stdout
+
+  Dumps the footer of a Parquet file to stdout or a file, optionally with
+  potentially user specific metadata scrubbed.
+)";
+  return 1;
+}
+
+uint32_t ReadLE32(const void* p) {
+  uint32_t x = arrow::util::SafeLoadAs<uint32_t>(static_cast<const 
uint8_t*>(p));
+  return arrow::bit_util::FromLittleEndian(x);
+}
+
+void AppendLE32(uint32_t v, std::string* out) {
+  v = arrow::bit_util::ToLittleEndian(v);
+  out->append(reinterpret_cast<const char*>(&v), sizeof(v));
+}
+
+template <typename T>
+bool Deserialize(const char* data, uint32_t len, T* obj) {
+  parquet::ThriftDeserializer des(/*string_size_limit=*/1 << 30,
+                                  /*container_size_limit=*/1 << 30);
+  try {
+    des.DeserializeMessage(reinterpret_cast<const uint8_t*>(data), &len, obj);
+    return true;
+  } catch (const std::exception& e) {
+    std::cerr << "Failed to deserialize: " << e.what() << "\n";
+    return false;
+  }
+}
+
+template <typename T>
+bool Serialize(const T& obj, std::string* out) {
+  parquet::ThriftSerializer ser(/*initial_buffer_size=*/10 << 20);
+  try {
+    ser.SerializeToString(&obj, out);
+    return true;
+  } catch (const std::exception& e) {
+    std::cerr << "Failed to serialize: " << e.what() << "\n";
+    return false;
+  }
+}
+
+// Replace the contents of s with random data of the same length.
+void Scrub(std::string* s) {
+  static char pool[4096];
+  static std::mt19937 rng(std::random_device{}());
+  static const bool kPoolInit = [] {
+    std::uniform_int_distribution<> caps(65, 90);
+    for (size_t i = 0; i < sizeof(pool); i++) pool[i] = caps(rng);
+    return true;
+  }();
+  (void)kPoolInit;
+
+  const size_t n = s->size();
+  s->clear();
+  while (s->size() < n) {
+    // To avoid repeating patterns we start somewhere up to halfway through 
the pool and
+    // append up to half the pool chars.
+    std::uniform_int_distribution<size_t> half(0, sizeof(pool) / 2 - 1);
+    size_t m = std::min(n, half(rng) + 1 /* at least one */);
+    s->append(&pool[half(rng)], m);
+  }
+}
+
+void Scrub(parquet::format::FileMetaData* md) {
+  for (auto& s : md->schema) {
+    Scrub(&s.name);
+  }
+  for (auto& r : md->row_groups) {
+    for (auto& c : r.columns) {
+      Scrub(&c.file_path);
+      if (c.__isset.meta_data) {
+        auto& m = c.meta_data;
+        for (auto& p : m.path_in_schema) Scrub(&p);
+        for (auto& kv : m.key_value_metadata) {
+          Scrub(&kv.key);
+          Scrub(&kv.value);
+        }
+        Scrub(&m.statistics.max_value);
+        Scrub(&m.statistics.min_value);
+        Scrub(&m.statistics.min);
+        Scrub(&m.statistics.max);
+      }
+
+      if (c.crypto_metadata.__isset.ENCRYPTION_WITH_COLUMN_KEY) {
+        auto& m = c.crypto_metadata.ENCRYPTION_WITH_COLUMN_KEY;
+        for (auto& p : m.path_in_schema) Scrub(&p);
+        Scrub(&m.key_metadata);
+      }
+      Scrub(&c.encrypted_column_metadata);
+    }
+  }
+  for (auto& kv : md->key_value_metadata) {
+    Scrub(&kv.key);
+    Scrub(&kv.value);
+  }
+  Scrub(&md->footer_signing_key_metadata);
+}
+
+// Returns:
+// - 0 on success
+// - -1 on error
+// - the size of the footer if tail is too small
+int64_t ParseFooter(const std::string& tail, parquet::format::FileMetaData* 
md) {
+  if (tail.size() > std::numeric_limits<int32_t>::max()) return -1;
+
+  const char* p = tail.data();
+  const int32_t n = static_cast<int32_t>(tail.size());
+  int32_t len = ReadLE32(p + n - 8);
+  if (len > n - 8) return len;
+
+  if (!Deserialize(tail.data() + n - 8 - len, len, md)) return -1;
+  return 0;
+}
+}  // namespace
+
+int main(int argc, char** argv) {
+  bool help = false;
+  bool scrub = true;
+  bool json = false;
+  std::string in;
+  std::string out;
+  for (int i = 1; i < argc; i++) {
+    char* arg = argv[i];
+    help |= !std::strcmp(arg, "-h") || !std::strcmp(arg, "--help");
+    scrub &= !!std::strcmp(arg, "--no-scrub");
+    json |= !std::strcmp(arg, "--json");
+    if (!std::strcmp(arg, "--in")) {
+      if (i + 1 >= argc) return PrintHelp();
+      in = argv[++i];
+    }
+    if (!std::strcmp(arg, "--out")) {
+      if (i + 1 >= argc) return PrintHelp();
+      out = argv[++i];
+    }
+  }
+  if (help || in.empty()) return PrintHelp();
+  std::string path;

Review Comment:
   I would recommend to keep `main` to the minimum (for example only 
command-line option parsing), then delegate actual functionality to helper 
function(s).



##########
cpp/tools/parquet/CMakeLists.txt:
##########
@@ -32,5 +32,16 @@ if(PARQUET_BUILD_EXECUTABLES)
             RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
   endforeach(TOOL)
 
+  # Only build parquet-dump-footer when statically linking.

Review Comment:
   Hmm, why is that required? Because we're accessing Thrift internals perhaps? 
In that case, I would recommend exposing the necessary Thrift-accessing 
functions as internal APIs from libparquet, and remove the static linking 
requirement.



##########
cpp/tools/parquet/parquet_dump_footer.cc:
##########
@@ -0,0 +1,248 @@
+// 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.
+
+#include <cstdint>
+#include <cstring>
+#include <exception>
+#include <fstream>
+#include <iostream>
+#include <limits>
+#include <optional>
+#include <random>
+
+#include <thrift/protocol/TCompactProtocol.h>
+#include <thrift/transport/TBufferTransports.h>
+
+#include "arrow/filesystem/filesystem.h"
+#include "arrow/util/endian.h"
+#include "arrow/util/ubsan.h"
+#include "generated/parquet_types.h"
+#include "parquet/thrift_internal.h"
+
+using apache::thrift::protocol::TCompactProtocol;
+using apache::thrift::transport::TMemoryBuffer;
+using apache::thrift::transport::TTransport;
+
+namespace {
+int PrintHelp() {
+  std::cerr << R"(
+Usage: parquet-dump-footer
+  -h|--help    Print help and exit
+  --no-scrub   Do not scrub potentially PII metadata
+  --json       Output JSON instead of binary
+  --in         Input file: required
+  --out        Output file: defaults to stdout
+
+  Dumps the footer of a Parquet file to stdout or a file, optionally with
+  potentially user specific metadata scrubbed.
+)";
+  return 1;
+}
+
+uint32_t ReadLE32(const void* p) {
+  uint32_t x = arrow::util::SafeLoadAs<uint32_t>(static_cast<const 
uint8_t*>(p));
+  return arrow::bit_util::FromLittleEndian(x);
+}
+
+void AppendLE32(uint32_t v, std::string* out) {
+  v = arrow::bit_util::ToLittleEndian(v);
+  out->append(reinterpret_cast<const char*>(&v), sizeof(v));
+}
+
+template <typename T>
+bool Deserialize(const char* data, uint32_t len, T* obj) {
+  parquet::ThriftDeserializer des(/*string_size_limit=*/1 << 30,
+                                  /*container_size_limit=*/1 << 30);
+  try {
+    des.DeserializeMessage(reinterpret_cast<const uint8_t*>(data), &len, obj);
+    return true;
+  } catch (const std::exception& e) {
+    std::cerr << "Failed to deserialize: " << e.what() << "\n";
+    return false;
+  }
+}
+
+template <typename T>
+bool Serialize(const T& obj, std::string* out) {
+  parquet::ThriftSerializer ser(/*initial_buffer_size=*/10 << 20);
+  try {
+    ser.SerializeToString(&obj, out);
+    return true;
+  } catch (const std::exception& e) {
+    std::cerr << "Failed to serialize: " << e.what() << "\n";
+    return false;
+  }
+}
+
+// Replace the contents of s with random data of the same length.
+void Scrub(std::string* s) {
+  static char pool[4096];
+  static std::mt19937 rng(std::random_device{}());
+  static const bool kPoolInit = [] {
+    std::uniform_int_distribution<> caps(65, 90);
+    for (size_t i = 0; i < sizeof(pool); i++) pool[i] = caps(rng);
+    return true;
+  }();
+  (void)kPoolInit;
+
+  const size_t n = s->size();
+  s->clear();
+  while (s->size() < n) {
+    // To avoid repeating patterns we start somewhere up to halfway through 
the pool and
+    // append up to half the pool chars.
+    std::uniform_int_distribution<size_t> half(0, sizeof(pool) / 2 - 1);
+    size_t m = std::min(n, half(rng) + 1 /* at least one */);
+    s->append(&pool[half(rng)], m);
+  }
+}
+
+void Scrub(parquet::format::FileMetaData* md) {
+  for (auto& s : md->schema) {
+    Scrub(&s.name);
+  }
+  for (auto& r : md->row_groups) {
+    for (auto& c : r.columns) {
+      Scrub(&c.file_path);
+      if (c.__isset.meta_data) {
+        auto& m = c.meta_data;
+        for (auto& p : m.path_in_schema) Scrub(&p);
+        for (auto& kv : m.key_value_metadata) {
+          Scrub(&kv.key);
+          Scrub(&kv.value);
+        }
+        Scrub(&m.statistics.max_value);
+        Scrub(&m.statistics.min_value);
+        Scrub(&m.statistics.min);
+        Scrub(&m.statistics.max);
+      }
+
+      if (c.crypto_metadata.__isset.ENCRYPTION_WITH_COLUMN_KEY) {
+        auto& m = c.crypto_metadata.ENCRYPTION_WITH_COLUMN_KEY;
+        for (auto& p : m.path_in_schema) Scrub(&p);
+        Scrub(&m.key_metadata);
+      }
+      Scrub(&c.encrypted_column_metadata);
+    }
+  }
+  for (auto& kv : md->key_value_metadata) {
+    Scrub(&kv.key);
+    Scrub(&kv.value);
+  }
+  Scrub(&md->footer_signing_key_metadata);
+}
+
+// Returns:
+// - 0 on success
+// - -1 on error
+// - the size of the footer if tail is too small

Review Comment:
   Same here: why not either throw an exception on error, or return a 
Result<int64_t>`?



##########
cpp/tools/parquet/parquet_dump_footer.cc:
##########
@@ -0,0 +1,248 @@
+// 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.
+
+#include <cstdint>
+#include <cstring>
+#include <exception>
+#include <fstream>
+#include <iostream>
+#include <limits>
+#include <optional>
+#include <random>
+
+#include <thrift/protocol/TCompactProtocol.h>
+#include <thrift/transport/TBufferTransports.h>
+
+#include "arrow/filesystem/filesystem.h"
+#include "arrow/util/endian.h"
+#include "arrow/util/ubsan.h"
+#include "generated/parquet_types.h"
+#include "parquet/thrift_internal.h"
+
+using apache::thrift::protocol::TCompactProtocol;
+using apache::thrift::transport::TMemoryBuffer;
+using apache::thrift::transport::TTransport;
+
+namespace {
+int PrintHelp() {
+  std::cerr << R"(
+Usage: parquet-dump-footer
+  -h|--help    Print help and exit
+  --no-scrub   Do not scrub potentially PII metadata
+  --json       Output JSON instead of binary
+  --in         Input file: required
+  --out        Output file: defaults to stdout
+
+  Dumps the footer of a Parquet file to stdout or a file, optionally with
+  potentially user specific metadata scrubbed.
+)";
+  return 1;
+}
+
+uint32_t ReadLE32(const void* p) {
+  uint32_t x = arrow::util::SafeLoadAs<uint32_t>(static_cast<const 
uint8_t*>(p));
+  return arrow::bit_util::FromLittleEndian(x);
+}
+
+void AppendLE32(uint32_t v, std::string* out) {
+  v = arrow::bit_util::ToLittleEndian(v);
+  out->append(reinterpret_cast<const char*>(&v), sizeof(v));
+}
+
+template <typename T>
+bool Deserialize(const char* data, uint32_t len, T* obj) {
+  parquet::ThriftDeserializer des(/*string_size_limit=*/1 << 30,
+                                  /*container_size_limit=*/1 << 30);
+  try {
+    des.DeserializeMessage(reinterpret_cast<const uint8_t*>(data), &len, obj);
+    return true;
+  } catch (const std::exception& e) {
+    std::cerr << "Failed to deserialize: " << e.what() << "\n";
+    return false;
+  }
+}
+
+template <typename T>
+bool Serialize(const T& obj, std::string* out) {
+  parquet::ThriftSerializer ser(/*initial_buffer_size=*/10 << 20);
+  try {
+    ser.SerializeToString(&obj, out);
+    return true;
+  } catch (const std::exception& e) {
+    std::cerr << "Failed to serialize: " << e.what() << "\n";
+    return false;
+  }
+}
+
+// Replace the contents of s with random data of the same length.
+void Scrub(std::string* s) {
+  static char pool[4096];
+  static std::mt19937 rng(std::random_device{}());
+  static const bool kPoolInit = [] {
+    std::uniform_int_distribution<> caps(65, 90);
+    for (size_t i = 0; i < sizeof(pool); i++) pool[i] = caps(rng);
+    return true;
+  }();
+  (void)kPoolInit;
+
+  const size_t n = s->size();
+  s->clear();
+  while (s->size() < n) {
+    // To avoid repeating patterns we start somewhere up to halfway through 
the pool and
+    // append up to half the pool chars.
+    std::uniform_int_distribution<size_t> half(0, sizeof(pool) / 2 - 1);
+    size_t m = std::min(n, half(rng) + 1 /* at least one */);
+    s->append(&pool[half(rng)], m);
+  }

Review Comment:
   (if you're worried about RNG speed, you can use one of the PCG engines from 
`arrow/util/pcg_random.h`)



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