[GitHub] [arrow] bkietz commented on a change in pull request #8023: ARROW-9318: [C++] Parquet encryption key management

[GitBox <gi...@apache.org>]

bkietz commented on a change in pull request #8023:
URL: https://github.com/apache/arrow/pull/8023#discussion_r497661719



##########
File path: cpp/src/arrow/util/concurrent_map.h
##########
@@ -0,0 +1,81 @@
+// 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.
+
+#pragma once
+
+#include <functional>
+#include <unordered_map>
+#include <utility>
+
+#include "arrow/util/mutex.h"
+
+namespace arrow {
+namespace util {
+
+template <typename V>
+class ConcurrentMap {

Review comment:
       In all, I think this doesn't add sufficient value over an inlined `container, mutex` pair. I think this class should be removed, maybe extracting a helper for single lookup insertion:
   ```c++
   template <typename K, typename V, typename Hash, typename Eq, typename Gen>
   auto GetOrInsert(std::unordered_map<K, V, Hash, Eq>* map, const K& key, Gen&& gen, V placeholder = V{})
     -> decltype(map->begin()) {
     auto it_success = map->emplace(key, placeholder);
     if (!it_success.second) {
       // insertion of placeholder was blocked by an existing entry, return that
       return it_success.first;
     }
     // overwrite placeholder with computed value
     it_success.first->second = gen();
     return it_success.first;
   }
   ```




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