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

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

pitrou commented on a change in pull request #8023:
URL: https://github.com/apache/arrow/pull/8023#discussion_r498073329



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File path: cpp/src/arrow/util/concurrent_map.h
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@@ -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:
       That depends if _only_ the map is accessed by multiple threads. Generally, the map is only one field inside a class and other fields must be synchronized as well, so the class has its own mutex.
   




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