SERVER-56669 Refactor change streams stage building logic

1 files changed, 111 insertions, 0 deletions

diff --git a/src/mongo/db/pipeline/document_source_change_stream_handle_topology_change.h b/src/mongo/db/pipeline/document_source_change_stream_handle_topology_change.h
new file mode 100644
index 00000000000..1d240be5e3c
--- /dev/null
+++ b/src/mongo/db/pipeline/document_source_change_stream_handle_topology_change.h
@@ -0,0 +1,111 @@
+/**
+ *    Copyright (C) 2021-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 <memory>
+
+#include "mongo/db/pipeline/change_stream_constants.h"
+#include "mongo/db/pipeline/document_source.h"
+#include "mongo/s/query/document_source_merge_cursors.h"
+#include "mongo/s/shard_id.h"
+
+namespace mongo {
+
+/**
+ * An internal stage used as part of the change streams infrastructure to listen for an event
+ * signaling that a new shard now has potentially matching data. For example, this stage will
+ * detect if a collection is being watched and a chunk for that collection migrates to a shard for
+ * the first time. When this event is detected, this stage will establish a new cursor on that
+ * shard and add it to the cursors being merged.
+ */
+class DocumentSourceChangeStreamHandleTopologyChange final : public DocumentSource {
+public:
+    static constexpr StringData kStageName = "$_internalChangeStreamHandleTopologyChange"_sd;
+
+    /**
+     * Creates a new stage which will establish a new cursor and add it to the cursors being merged
+     * by 'mergeCursorsStage' whenever a new shard is detected by a change stream.
+     */
+    static boost::intrusive_ptr<DocumentSourceChangeStreamHandleTopologyChange> create(
+        const boost::intrusive_ptr<ExpressionContext>&);
+
+    Value serialize(boost::optional<ExplainOptions::Verbosity> explain) const final {
+        return (explain ? Value(Document{{kStageName, Value()}}) : Value());
+    }
+
+    virtual StageConstraints constraints(Pipeline::SplitState) const {
+        return {StreamType::kStreaming,
+                PositionRequirement::kNone,
+                HostTypeRequirement::kMongoS,
+                DiskUseRequirement::kNoDiskUse,
+                FacetRequirement::kNotAllowed,
+                TransactionRequirement::kNotAllowed,
+                LookupRequirement::kNotAllowed,
+                UnionRequirement::kNotAllowed,
+                ChangeStreamRequirement::kChangeStreamStage};
+    }
+
+    boost::optional<DistributedPlanLogic> distributedPlanLogic() final {
+        return DistributedPlanLogic{nullptr, this, change_stream_constants::kSortSpec};
+    }
+
+private:
+    DocumentSourceChangeStreamHandleTopologyChange(const boost::intrusive_ptr<ExpressionContext>&);
+
+    GetNextResult doGetNext() final;
+
+    /**
+     * Establish the new cursors and tell the RouterStageMerge about them.
+     */
+    void addNewShardCursors(const Document& newShardDetectedObj);
+
+    /**
+     * Open the cursors on the new shards.
+     */
+    std::vector<RemoteCursor> establishShardCursorsOnNewShards(const Document& newShardDetectedObj);
+
+    /**
+     * Updates the $changeStream stage in the '_originalAggregateCommand' to reflect the start time
+     * for the newly-added shard(s), then generates the final command object to be run on those
+     * shards.
+     */
+    BSONObj createUpdatedCommandForNewShard(Timestamp shardAddedTime);
+
+    /**
+     * Given the '_originalAggregateCommand' and a resume token, returns a new BSON object with the
+     * same command except with the addition of a resumeAfter option containing the resume token.
+     * If there was a previous resumeAfter option, it will be removed.
+     */
+    BSONObj replaceResumeTokenInCommand(Document resumeToken);
+
+    boost::intrusive_ptr<DocumentSourceMergeCursors> _mergeCursors;
+    BSONObj _originalAggregateCommand;
+};
+}  // namespace mongo