SERVER-41944 Validate the given shard key for refineCollectionShardKey

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diff --git a/src/mongo/db/s/shard_key_util.h b/src/mongo/db/s/shard_key_util.h
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+/**
+ *    Copyright (C) 2019-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
+
+#include "mongo/s/request_types/shard_collection_gen.h"
+#include "mongo/s/shard_util.h"
+
+namespace mongo {
+namespace shardkeyutil {
+
+/**
+ * Constructs the BSON specification document for the create indexes command using the given
+ * namespace, index key and options.
+ */
+BSONObj makeCreateIndexesCmd(const NamespaceString& nss,
+                             const BSONObj& keys,
+                             const BSONObj& collation,
+                             bool unique);
+
+/**
+ * Compares the proposed shard key with the collection's existing indexes on the primary shard to
+ * ensure they are a legal combination.
+ *
+ * Creates the required index if and only if (i) the collection is empty, (ii) no index on the shard
+ * key exists, and (iii) createIndexIfPossible is true.
+ *
+ * The proposed shard key must be validated against the set of existing indexes.
+ * In particular, we must ensure the following constraints:
+ *
+ * 1. All existing unique indexes, except those which start with the _id index,
+ *    must contain the proposed key as a prefix (uniqueness of the _id index is
+ *    ensured by the _id generation process or guaranteed by the user).
+ *
+ * 2. If the collection is not empty, there must exist at least one index that
+ *    is "useful" for the proposed key.  A "useful" index is defined as adhering to
+ *    all of the following properties:
+ *         i. contains proposedKey as a prefix
+ *         ii. is not a sparse index, partial index, or index with a non-simple collation
+ *         iii. contains no null values
+ *         iv. is not multikey (maybe lift this restriction later)
+ *         v. if a hashed index, has default seed (lift this restriction later)
+ *
+ * 3. If the proposed shard key is specified as unique, there must exist a useful,
+ *    unique index exactly equal to the proposedKey (not just a prefix).
+ *
+ * After validating these constraints:
+ *
+ * 4. If there is no useful index, and the collection is non-empty or createIndexIfPossible
+ *    is false, we must fail.
+ *
+ * 5. If the collection is empty and createIndexIfPossible is true, and it's still possible
+ *    to create an index on the proposed key, we go ahead and do so.
+ */
+void validateShardKeyAgainstExistingIndexes(OperationContext* opCtx,
+                                            const NamespaceString& nss,
+                                            const BSONObj& proposedKey,
+                                            const ShardKeyPattern& shardKeyPattern,
+                                            const std::shared_ptr<Shard>& primaryShard,
+                                            const boost::optional<BSONObj>& defaultCollation,
+                                            const bool unique,
+                                            const bool createIndexIfPossible);
+
+}  // namespace shardkeyutil
+}  // namespace mongo