SERVER-38249 Implement stdx unordered_map and unordered_set as absl node hash map/set.

Remove stdx::unordered_multimap and multiset.

Custom hashers to stdx::unordered_map are not trusted by default, we
will rehash the produced hash with absl again to ensure we have a good
hash function.

1 files changed, 66 insertions, 0 deletions

diff --git a/src/mongo/stdx/trusted_hasher.h b/src/mongo/stdx/trusted_hasher.h
new file mode 100644
index 00000000000..8e3df6f07e0
--- /dev/null
+++ b/src/mongo/stdx/trusted_hasher.h
@@ -0,0 +1,66 @@
+/**
+ *    Copyright (C) 2018-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+// This file depends on internal implementation details in abseil. In a library upgrade you may have
+// to re-write this file significantly.
+#include <absl/container/internal/hash_function_defaults.h>
+
+/**
+ * To be safe we let abseil hash the produced hash one more time to protect ourselves against bad
+ * hash functions. If you know your hash function is good and can be trusted you get mark it as
+ * trusted by specializing a template like this:
+ *
+ * namespace mongo {
+ *     template <>
+ *     struct IsTrustedHasher<YourHash, YourKey> : std::true_type {};
+ * }
+ */
+
+namespace mongo {
+template <typename Key>
+using DefaultHasher = absl::container_internal::hash_default_hash<Key>;
+
+template <typename Hasher, typename Key>
+struct IsTrustedHasher : std::is_same<Hasher, DefaultHasher<Key>> {};
+
+template <typename Hasher, typename Key>
+struct HashImprover : private Hasher {
+    HashImprover(const Hasher& hasher = Hasher()) : Hasher(hasher) {}
+    std::size_t operator()(const Key& k) const {
+        return absl::Hash<std::size_t>{}(Hasher::operator()(k));
+    }
+};
+
+template <typename Hasher, typename Key>
+using EnsureTrustedHasher =
+    std::conditional_t<IsTrustedHasher<Hasher, Key>::value, Hasher, HashImprover<Hasher, Key>>;
+
+}  // namespace mongo