/*
 * 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.
 */
package org.apache.cassandra.service;

import java.net.InetAddress;
import java.util.*;
import java.util.concurrent.TimeoutException;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Joiner;
import com.google.common.collect.Iterables;

import org.apache.cassandra.concurrent.Stage;
import org.apache.cassandra.concurrent.StageManager;
import org.apache.cassandra.config.*;
import org.apache.cassandra.db.*;
import org.apache.cassandra.db.filter.*;
import org.apache.cassandra.db.filter.DataLimits.Counter;
import org.apache.cassandra.db.partitions.*;
import org.apache.cassandra.db.rows.*;
import org.apache.cassandra.db.transform.*;
import org.apache.cassandra.dht.AbstractBounds;
import org.apache.cassandra.dht.ExcludingBounds;
import org.apache.cassandra.dht.Range;
import org.apache.cassandra.exceptions.ReadTimeoutException;
import org.apache.cassandra.net.*;
import org.apache.cassandra.tracing.Tracing;
import org.apache.cassandra.utils.FBUtilities;

public class DataResolver extends ResponseResolver
{
    private static final boolean DROP_OVERSIZED_READ_REPAIR_MUTATIONS =
        Boolean.getBoolean("cassandra.drop_oversized_readrepair_mutations");

    @VisibleForTesting
    final List<AsyncOneResponse> repairResults = Collections.synchronizedList(new ArrayList<>());
    private final long queryStartNanoTime;
    private final boolean enforceStrictLiveness;

    DataResolver(Keyspace keyspace, ReadCommand command, ConsistencyLevel consistency, int maxResponseCount, long queryStartNanoTime)
    {
        super(keyspace, command, consistency, maxResponseCount);
        this.queryStartNanoTime = queryStartNanoTime;
        this.enforceStrictLiveness = command.metadata().enforceStrictLiveness();
    }

    public PartitionIterator getData()
    {
        ReadResponse response = responses.iterator().next().payload;
        return UnfilteredPartitionIterators.filter(response.makeIterator(command), command.nowInSec());
    }

    public boolean isDataPresent()
    {
        return !responses.isEmpty();
    }

    public void compareResponses()
    {
        // We need to fully consume the results to trigger read repairs if appropriate
        try (PartitionIterator iterator = resolve())
        {
            PartitionIterators.consume(iterator);
        }
    }

    public PartitionIterator resolve()
    {
        // We could get more responses while this method runs, which is ok (we're happy to ignore any response not here
        // at the beginning of this method), so grab the response count once and use that through the method.
        int count = responses.size();
        List<UnfilteredPartitionIterator> iters = new ArrayList<>(count);
        InetAddress[] sources = new InetAddress[count];
        for (int i = 0; i < count; i++)
        {
            MessageIn<ReadResponse> msg = responses.get(i);
            iters.add(msg.payload.makeIterator(command));
            sources[i] = msg.from;
        }

        /*
         * Even though every response, individually, will honor the limit, it is possible that we will, after the merge,
         * have more rows than the client requested. To make sure that we still conform to the original limit,
         * we apply a top-level post-reconciliation counter to the merged partition iterator.
         *
         * Short read protection logic (ShortReadRowsProtection.moreContents()) relies on this counter to be applied
         * to the current partition to work. For this reason we have to apply the counter transformation before
         * empty partition discard logic kicks in - for it will eagerly consume the iterator.
         *
         * That's why the order here is: 1) merge; 2) filter rows; 3) count; 4) discard empty partitions
         *
         * See CASSANDRA-13747 for more details.
         */

        DataLimits.Counter mergedResultCounter =
            command.limits().newCounter(command.nowInSec(), true, command.selectsFullPartition(), enforceStrictLiveness);

        UnfilteredPartitionIterator merged = mergeWithShortReadProtection(iters, sources, mergedResultCounter);
        FilteredPartitions filtered =
            FilteredPartitions.filter(merged, new Filter(command.nowInSec(), command.metadata().enforceStrictLiveness()));
        PartitionIterator counted = Transformation.apply(filtered, mergedResultCounter);

        return command.isForThrift()
             ? counted
             : Transformation.apply(counted, new EmptyPartitionsDiscarder());
    }

    private UnfilteredPartitionIterator mergeWithShortReadProtection(List<UnfilteredPartitionIterator> results,
                                                                     InetAddress[] sources,
                                                                     DataLimits.Counter mergedResultCounter)
    {
        // If we have only one results, there is no read repair to do and we can't get short reads
        if (results.size() == 1)
            return results.get(0);

        /*
         * So-called short reads stems from nodes returning only a subset of the results they have due to the limit,
         * but that subset not being enough post-reconciliation. So if we don't have a limit, don't bother.
         */
        if (!command.limits().isUnlimited())
            for (int i = 0; i < results.size(); i++)
                results.set(i, extendWithShortReadProtection(results.get(i), sources[i], mergedResultCounter));

        return UnfilteredPartitionIterators.merge(results, command.nowInSec(), new RepairMergeListener(sources));
    }

    private class RepairMergeListener implements UnfilteredPartitionIterators.MergeListener
    {
        private final InetAddress[] sources;

        private RepairMergeListener(InetAddress[] sources)
        {
            this.sources = sources;
        }

        public UnfilteredRowIterators.MergeListener getRowMergeListener(DecoratedKey partitionKey, List<UnfilteredRowIterator> versions)
        {
            return new MergeListener(partitionKey, columns(versions), isReversed(versions));
        }

        private PartitionColumns columns(List<UnfilteredRowIterator> versions)
        {
            Columns statics = Columns.NONE;
            Columns regulars = Columns.NONE;
            for (UnfilteredRowIterator iter : versions)
            {
                if (iter == null)
                    continue;

                PartitionColumns cols = iter.columns();
                statics = statics.mergeTo(cols.statics);
                regulars = regulars.mergeTo(cols.regulars);
            }
            return new PartitionColumns(statics, regulars);
        }

        private boolean isReversed(List<UnfilteredRowIterator> versions)
        {
            for (UnfilteredRowIterator iter : versions)
            {
                if (iter == null)
                    continue;

                // Everything will be in the same order
                return iter.isReverseOrder();
            }

            assert false : "Expected at least one iterator";
            return false;
        }

        public void close()
        {
            try
            {
                FBUtilities.waitOnFutures(repairResults, DatabaseDescriptor.getWriteRpcTimeout());
            }
            catch (TimeoutException ex)
            {
                // We got all responses, but timed out while repairing
                int blockFor = consistency.blockFor(keyspace);
                if (Tracing.isTracing())
                    Tracing.trace("Timed out while read-repairing after receiving all {} data and digest responses", blockFor);
                else
                    logger.debug("Timeout while read-repairing after receiving all {} data and digest responses", blockFor);

                throw new ReadTimeoutException(consistency, blockFor-1, blockFor, true);
            }
        }

        private class MergeListener implements UnfilteredRowIterators.MergeListener
        {
            private final DecoratedKey partitionKey;
            private final PartitionColumns columns;
            private final boolean isReversed;
            private final PartitionUpdate[] repairs = new PartitionUpdate[sources.length];

            private final Row.Builder[] currentRows = new Row.Builder[sources.length];
            private final RowDiffListener diffListener;

            // The partition level deletion for the merge row.
            private DeletionTime partitionLevelDeletion;
            // When merged has a currently open marker, its time. null otherwise.
            private DeletionTime mergedDeletionTime;
            // For each source, the time of the current deletion as known by the source.
            private final DeletionTime[] sourceDeletionTime = new DeletionTime[sources.length];
            // For each source, record if there is an open range to send as repair, and from where.
            private final ClusteringBound[] markerToRepair = new ClusteringBound[sources.length];

            private MergeListener(DecoratedKey partitionKey, PartitionColumns columns, boolean isReversed)
            {
                this.partitionKey = partitionKey;
                this.columns = columns;
                this.isReversed = isReversed;

                this.diffListener = new RowDiffListener()
                {
                    public void onPrimaryKeyLivenessInfo(int i, Clustering clustering, LivenessInfo merged, LivenessInfo original)
                    {
                        if (merged != null && !merged.equals(original))
                            currentRow(i, clustering).addPrimaryKeyLivenessInfo(merged);
                    }

                    public void onDeletion(int i, Clustering clustering, Row.Deletion merged, Row.Deletion original)
                    {
                        if (merged != null && !merged.equals(original))
                            currentRow(i, clustering).addRowDeletion(merged);
                    }

                    public void onComplexDeletion(int i, Clustering clustering, ColumnDefinition column, DeletionTime merged, DeletionTime original)
                    {
                        if (merged != null && !merged.equals(original))
                            currentRow(i, clustering).addComplexDeletion(column, merged);
                    }

                    public void onCell(int i, Clustering clustering, Cell merged, Cell original)
                    {
                        if (merged != null && !merged.equals(original) && isQueried(merged))
                            currentRow(i, clustering).addCell(merged);
                    }

                    private boolean isQueried(Cell cell)
                    {
                        // When we read, we may have some cell that have been fetched but are not selected by the user. Those cells may
                        // have empty values as optimization (see CASSANDRA-10655) and hence they should not be included in the read-repair.
                        // This is fine since those columns are not actually requested by the user and are only present for the sake of CQL
                        // semantic (making sure we can always distinguish between a row that doesn't exist from one that do exist but has
                        /// no value for the column requested by the user) and so it won't be unexpected by the user that those columns are
                        // not repaired.
                        ColumnDefinition column = cell.column();
                        ColumnFilter filter = command.columnFilter();
                        return column.isComplex() ? filter.fetchedCellIsQueried(column, cell.path()) : filter.fetchedColumnIsQueried(column);
                    }
                };
            }

            private PartitionUpdate update(int i)
            {
                if (repairs[i] == null)
                    repairs[i] = new PartitionUpdate(command.metadata(), partitionKey, columns, 1);
                return repairs[i];
            }

            
The partition level deletion with with which source i is currently repaired, or DeletionTime.LIVE if the source is not repaired on the partition level deletion (meaning it was up to date on it). The output* of this method is only valid after the call to onMergedPartitionLevelDeletion. /**
             * The partition level deletion with with which source {@code i} is currently repaired, or
             * {@code DeletionTime.LIVE} if the source is not repaired on the partition level deletion (meaning it was
             * up to date on it). The output* of this method is only valid after the call to
             * {@link #onMergedPartitionLevelDeletion}.
             */
            private DeletionTime partitionLevelRepairDeletion(int i)
            {
                return repairs[i] == null ? DeletionTime.LIVE : repairs[i].partitionLevelDeletion();
            }

            private Row.Builder currentRow(int i, Clustering clustering)
            {
                if (currentRows[i] == null)
                {
                    currentRows[i] = BTreeRow.sortedBuilder();
                    currentRows[i].newRow(clustering);
                }
                return currentRows[i];
            }

            public void onMergedPartitionLevelDeletion(DeletionTime mergedDeletion, DeletionTime[] versions)
            {
                this.partitionLevelDeletion = mergedDeletion;
                for (int i = 0; i < versions.length; i++)
                {
                    if (mergedDeletion.supersedes(versions[i]))
                        update(i).addPartitionDeletion(mergedDeletion);
                }
            }

            public void onMergedRows(Row merged, Row[] versions)
            {
                // If a row was shadowed post merged, it must be by a partition level or range tombstone, and we handle
                // those case directly in their respective methods (in other words, it would be inefficient to send a row
                // deletion as repair when we know we've already send a partition level or range tombstone that covers it).
                if (merged.isEmpty())
                    return;

                Rows.diff(diffListener, merged, versions);
                for (int i = 0; i < currentRows.length; i++)
                {
                    if (currentRows[i] != null)
                        update(i).add(currentRows[i].build());
                }
                Arrays.fill(currentRows, null);
            }

            private DeletionTime currentDeletion()
            {
                return mergedDeletionTime == null ? partitionLevelDeletion : mergedDeletionTime;
            }

            public void onMergedRangeTombstoneMarkers(RangeTombstoneMarker merged, RangeTombstoneMarker[] versions)
            {
                try
                {
                    // The code for merging range tombstones is a tad complex and we had the assertions there triggered
                    // unexpectedly in a few occasions (CASSANDRA-13237, CASSANDRA-13719). It's hard to get insights
                    // when that happen without more context that what the assertion errors give us however, hence the
                    // catch here that basically gather as much as context as reasonable.
                    internalOnMergedRangeTombstoneMarkers(merged, versions);
                }
                catch (AssertionError e)
                {
                    // The following can be pretty verbose, but it's really only triggered if a bug happen, so we'd
                    // rather get more info to debug than not.
                    CFMetaData table = command.metadata();
                    String details = String.format("Error merging RTs on %s.%s: command=%s, reversed=%b, merged=%s, versions=%s, sources={%s}, responses:%n %s",
                                                   table.ksName, table.cfName,
                                                   command.toCQLString(),
                                                   isReversed,
                                                   merged == null ? "null" : merged.toString(table),
                                                   '[' + Joiner.on(", ").join(Iterables.transform(Arrays.asList(versions), rt -> rt == null ? "null" : rt.toString(table))) + ']',
                                                   Arrays.toString(sources),
                                                   makeResponsesDebugString());
                    throw new AssertionError(details, e);
                }
            }

            private String makeResponsesDebugString()
            {
                return Joiner.on(",\n")
                             .join(Iterables.transform(getMessages(), m -> m.from + " => " + m.payload.toDebugString(command, partitionKey)));
            }

            private void internalOnMergedRangeTombstoneMarkers(RangeTombstoneMarker merged, RangeTombstoneMarker[] versions)
            {
                // The current deletion as of dealing with this marker.
                DeletionTime currentDeletion = currentDeletion();

                for (int i = 0; i < versions.length; i++)
                {
                    RangeTombstoneMarker marker = versions[i];

                    // Update what the source now thinks is the current deletion
                    if (marker != null)
                        sourceDeletionTime[i] = marker.isOpen(isReversed) ? marker.openDeletionTime(isReversed) : null;

                    // If merged == null, some of the source is opening or closing a marker
                    if (merged == null)
                    {
                        // but if it's not this source, move to the next one
                        if (marker == null)
                            continue;

                        // We have a close and/or open marker for a source, with nothing corresponding in merged.
                        // Because merged is a superset, this imply that we have a current deletion (being it due to an
                        // early opening in merged or a partition level deletion) and that this deletion will still be
                        // active after that point. Further whatever deletion was open or is open by this marker on the
                        // source, that deletion cannot supersedes the current one.
                        //
                        // But while the marker deletion (before and/or after this point) cannot supersede the current
                        // deletion, we want to know if it's equal to it (both before and after), because in that case
                        // the source is up to date and we don't want to include repair.
                        //
                        // So in practice we have 2 possible case:
                        //  1) the source was up-to-date on deletion up to that point: then it won't be from that point
                        //     on unless it's a boundary and the new opened deletion time is also equal to the current
                        //     deletion (note that this implies the boundary has the same closing and opening deletion
                        //     time, which should generally not happen, but can due to legacy reading code not avoiding
                        //     this for a while, see CASSANDRA-13237).
                        //  2) the source wasn't up-to-date on deletion up to that point and it may now be (if it isn't
                        //     we just have nothing to do for that marker).
                        assert !currentDeletion.isLive() : currentDeletion.toString();

                        // Is the source up to date on deletion? It's up to date if it doesn't have an open RT repair
                        // nor an "active" partition level deletion (where "active" means that it's greater or equal
                        // to the current deletion: if the source has a repaired partition deletion lower than the
                        // current deletion, this means the current deletion is due to a previously open range tombstone,
                        // and if the source isn't currently repaired for that RT, then it means it's up to date on it).
                        DeletionTime partitionRepairDeletion = partitionLevelRepairDeletion(i);
                        if (markerToRepair[i] == null && currentDeletion.supersedes(partitionRepairDeletion))
                        {
                            /*
                             * Since there is an ongoing merged deletion, the only way we don't have an open repair for
                             * this source is that it had a range open with the same deletion as current marker,
                             * and the marker is closing it.
                             */
                            assert marker.isClose(isReversed) && currentDeletion.equals(marker.closeDeletionTime(isReversed))
                                 : String.format("currentDeletion=%s, marker=%s", currentDeletion, marker.toString(command.metadata()));

                            // and so unless it's a boundary whose opening deletion time is still equal to the current
                            // deletion (see comment above for why this can actually happen), we have to repair the source
                            // from that point on.
                            if (!(marker.isOpen(isReversed) && currentDeletion.equals(marker.openDeletionTime(isReversed))))
                                markerToRepair[i] = marker.closeBound(isReversed).invert();
                        }
                        // In case 2) above, we only have something to do if the source is up-to-date after that point
                        // (which, since the source isn't up-to-date before that point, means we're opening a new deletion
                        // that is equal to the current one).
                        else if (marker.isOpen(isReversed) && currentDeletion.equals(marker.openDeletionTime(isReversed)))
                        {
                            closeOpenMarker(i, marker.openBound(isReversed).invert());
                        }
                    }
                    else
                    {
                        // We have a change of current deletion in merged (potentially to/from no deletion at all).

                        if (merged.isClose(isReversed))
                        {
                            // We're closing the merged range. If we're recorded that this should be repaird for the
                            // source, close and add said range to the repair to send.
                            if (markerToRepair[i] != null)
                                closeOpenMarker(i, merged.closeBound(isReversed));

                        }

                        if (merged.isOpen(isReversed))
                        {
                            // If we're opening a new merged range (or just switching deletion), then unless the source
                            // is up to date on that deletion (note that we've updated what the source deleteion is
                            // above), we'll have to sent the range to the source.
                            DeletionTime newDeletion = merged.openDeletionTime(isReversed);
                            DeletionTime sourceDeletion = sourceDeletionTime[i];
                            if (!newDeletion.equals(sourceDeletion))
                                markerToRepair[i] = merged.openBound(isReversed);
                        }
                    }
                }

                if (merged != null)
                    mergedDeletionTime = merged.isOpen(isReversed) ? merged.openDeletionTime(isReversed) : null;
            }

            private void closeOpenMarker(int i, ClusteringBound close)
            {
                ClusteringBound open = markerToRepair[i];
                update(i).add(new RangeTombstone(Slice.make(isReversed ? close : open, isReversed ? open : close), currentDeletion()));
                markerToRepair[i] = null;
            }

            public void close()
            {
                for (int i = 0; i < repairs.length; i++)
                    if (null != repairs[i])
                        sendRepairMutation(repairs[i], sources[i]);
            }

            private void sendRepairMutation(PartitionUpdate partition, InetAddress destination)
            {
                Mutation mutation = new Mutation(partition);
                int messagingVersion = MessagingService.instance().getVersion(destination);

                int    mutationSize = (int) Mutation.serializer.serializedSize(mutation, messagingVersion);
                int maxMutationSize = DatabaseDescriptor.getMaxMutationSize();

                if (mutationSize <= maxMutationSize)
                {
                    Tracing.trace("Sending read-repair-mutation to {}", destination);
                    // use a separate verb here to avoid writing hints on timeouts
                    MessageOut<Mutation> message = mutation.createMessage(MessagingService.Verb.READ_REPAIR);
                    repairResults.add(MessagingService.instance().sendRR(message, destination));
                    ColumnFamilyStore.metricsFor(command.metadata().cfId).readRepairRequests.mark();
                }
                else if (DROP_OVERSIZED_READ_REPAIR_MUTATIONS)
                {
                    logger.debug("Encountered an oversized ({}/{}) read repair mutation for table {}.{}, key {}, node {}",
                                 mutationSize,
                                 maxMutationSize,
                                 command.metadata().ksName,
                                 command.metadata().cfName,
                                 command.metadata().getKeyValidator().getString(partitionKey.getKey()),
                                 destination);
                }
                else
                {
                    logger.warn("Encountered an oversized ({}/{}) read repair mutation for table {}.{}, key {}, node {}",
                                mutationSize,
                                maxMutationSize,
                                command.metadata().ksName,
                                command.metadata().cfName,
                                command.metadata().getKeyValidator().getString(partitionKey.getKey()),
                                destination);

                    int blockFor = consistency.blockFor(keyspace);
                    Tracing.trace("Timed out while read-repairing after receiving all {} data and digest responses", blockFor);
                    throw new ReadTimeoutException(consistency, blockFor - 1, blockFor, true);
                }
            }
        }
    }

    private UnfilteredPartitionIterator extendWithShortReadProtection(UnfilteredPartitionIterator partitions,
                                                                      InetAddress source,
                                                                      DataLimits.Counter mergedResultCounter)
    {
        DataLimits.Counter singleResultCounter =
            command.limits().newCounter(command.nowInSec(), false, command.selectsFullPartition(), enforceStrictLiveness).onlyCount();

        ShortReadPartitionsProtection protection =
            new ShortReadPartitionsProtection(source, singleResultCounter, mergedResultCounter, queryStartNanoTime);

        /*
         * The order of extention and transformations is important here. Extending with more partitions has to happen
         * first due to the way BaseIterator.hasMoreContents() works: only transformations applied after extension will
         * be called on the first partition of the extended iterator.
         *
         * Additionally, we want singleResultCounter to be applied after SRPP, so that its applyToPartition() method will
         * be called last, after the extension done by SRRP.applyToPartition() call. That way we preserve the same order
         * when it comes to calling SRRP.moreContents() and applyToRow() callbacks.
         *
         * See ShortReadPartitionsProtection.applyToPartition() for more details.
         */

        // extend with moreContents() only if it's a range read command with no partition key specified
        if (!command.isLimitedToOnePartition())
            partitions = MorePartitions.extend(partitions, protection);     // register SRPP.moreContents()

        partitions = Transformation.apply(partitions, protection);          // register SRPP.applyToPartition()
        partitions = Transformation.apply(partitions, singleResultCounter); // register the per-source counter

        return partitions;
    }

    /*
     * We have a potential short read if the result from a given node contains the requested number of rows
     * (i.e. it has stopped returning results due to the limit), but some of them haven't
     * made it into the final post-reconciliation result due to other nodes' row, range, and/or partition tombstones.
     *
     * If that is the case, then that node may have more rows that we should fetch, as otherwise we could
     * ultimately return fewer rows than required. Also, those additional rows may contain tombstones which
     * which we also need to fetch as they may shadow rows or partitions from other replicas' results, which we would
     * otherwise return incorrectly.
     */
    private class ShortReadPartitionsProtection extends Transformation<UnfilteredRowIterator> implements MorePartitions<UnfilteredPartitionIterator>
    {
        private final InetAddress source;

        private final DataLimits.Counter singleResultCounter; // unmerged per-source counter
        private final DataLimits.Counter mergedResultCounter; // merged end-result counter

        private DecoratedKey lastPartitionKey; // key of the last observed partition

        private boolean partitionsFetched; // whether we've seen any new partitions since iteration start or last moreContents() call

        private final long queryStartNanoTime;

        private ShortReadPartitionsProtection(InetAddress source,
                                              DataLimits.Counter singleResultCounter,
                                              DataLimits.Counter mergedResultCounter,
                                              long queryStartNanoTime)
        {
            this.source = source;
            this.singleResultCounter = singleResultCounter;
            this.mergedResultCounter = mergedResultCounter;
            this.queryStartNanoTime = queryStartNanoTime;
        }

        @Override
        public UnfilteredRowIterator applyToPartition(UnfilteredRowIterator partition)
        {
            partitionsFetched = true;

            lastPartitionKey = partition.partitionKey();

            /*
             * Extend for moreContents() then apply protection to track lastClustering by applyToRow().
             *
             * If we don't apply the transformation *after* extending the partition with MoreRows,
             * applyToRow() method of protection will not be called on the first row of the new extension iterator.
             */
            ShortReadRowsProtection protection = new ShortReadRowsProtection(partition.metadata(), partition.partitionKey());
            return Transformation.apply(MoreRows.extend(partition, protection), protection);
        }

        /*
         * We only get here once all the rows and partitions in this iterator have been iterated over, and so
         * if the node had returned the requested number of rows but we still get here, then some results were
         * skipped during reconciliation.
         */
        public UnfilteredPartitionIterator moreContents()
        {
            // never try to request additional partitions from replicas if our reconciled partitions are already filled to the limit
            assert !mergedResultCounter.isDone();

            // we do not apply short read protection when we have no limits at all
            assert !command.limits().isUnlimited();

            /*
             * If this is a single partition read command or an (indexed) partition range read command with
             * a partition key specified, then we can't and shouldn't try fetch more partitions.
             */
            assert !command.isLimitedToOnePartition();

            /*
             * If the returned result doesn't have enough rows/partitions to satisfy even the original limit, don't ask for more.
             *
             * Can only take the short cut if there is no per partition limit set. Otherwise it's possible to hit false
             * positives due to some rows being uncounted for in certain scenarios (see CASSANDRA-13911).
             */
            if (!singleResultCounter.isDone() && command.limits().perPartitionCount() == DataLimits.NO_LIMIT)
                return null;

            /*
             * Either we had an empty iterator as the initial response, or our moreContents() call got us an empty iterator.
             * There is no point to ask the replica for more rows - it has no more in the requested range.
             */
            if (!partitionsFetched)
                return null;
            partitionsFetched = false;

            /*
             * We are going to fetch one partition at a time for thrift and potentially more for CQL.
             * The row limit will either be set to the per partition limit - if the command has no total row limit set, or
             * the total # of rows remaining - if it has some. If we don't grab enough rows in some of the partitions,
             * then future ShortReadRowsProtection.moreContents() calls will fetch the missing ones.
             */
            int toQuery = command.limits().count() != DataLimits.NO_LIMIT
                        ? command.limits().count() - counted(mergedResultCounter)
                        : command.limits().perPartitionCount();

            ColumnFamilyStore.metricsFor(command.metadata().cfId).shortReadProtectionRequests.mark();
            Tracing.trace("Requesting {} extra rows from {} for short read protection", toQuery, source);

            PartitionRangeReadCommand cmd = makeFetchAdditionalPartitionReadCommand(toQuery);
            return executeReadCommand(cmd);
        }

        // Counts the number of rows for regular queries and the number of groups for GROUP BY queries
        private int counted(Counter counter)
        {
            return command.limits().isGroupByLimit()
                 ? counter.rowCounted()
                 : counter.counted();
        }

        private PartitionRangeReadCommand makeFetchAdditionalPartitionReadCommand(int toQuery)
        {
            PartitionRangeReadCommand cmd = (PartitionRangeReadCommand) command;

            DataLimits newLimits = cmd.limits().forShortReadRetry(toQuery);

            AbstractBounds<PartitionPosition> bounds = cmd.dataRange().keyRange();
            AbstractBounds<PartitionPosition> newBounds = bounds.inclusiveRight()
                                                        ? new Range<>(lastPartitionKey, bounds.right)
                                                        : new ExcludingBounds<>(lastPartitionKey, bounds.right);
            DataRange newDataRange = cmd.dataRange().forSubRange(newBounds);

            return cmd.withUpdatedLimitsAndDataRange(newLimits, newDataRange);
        }

        private class ShortReadRowsProtection extends Transformation implements MoreRows<UnfilteredRowIterator>
        {
            private final CFMetaData metadata;
            private final DecoratedKey partitionKey;

            private Clustering lastClustering; // clustering of the last observed row

            private int lastCounted = 0; // last seen recorded # before attempting to fetch more rows
            private int lastFetched = 0; // # rows returned by last attempt to get more (or by the original read command)
            private int lastQueried = 0; // # extra rows requested from the replica last time

            private ShortReadRowsProtection(CFMetaData metadata, DecoratedKey partitionKey)
            {
                this.metadata = metadata;
                this.partitionKey = partitionKey;
            }

            @Override
            public Row applyToRow(Row row)
            {
                lastClustering = row.clustering();
                return row;
            }

            /*
             * We only get here once all the rows in this iterator have been iterated over, and so if the node
             * had returned the requested number of rows but we still get here, then some results were skipped
             * during reconciliation.
             */
            public UnfilteredRowIterator moreContents()
            {
                // never try to request additional rows from replicas if our reconciled partition is already filled to the limit
                assert !mergedResultCounter.isDoneForPartition();

                // we do not apply short read protection when we have no limits at all
                assert !command.limits().isUnlimited();

                /*
                 * If the returned partition doesn't have enough rows to satisfy even the original limit, don't ask for more.
                 *
                 * Can only take the short cut if there is no per partition limit set. Otherwise it's possible to hit false
                 * positives due to some rows being uncounted for in certain scenarios (see CASSANDRA-13911).
                 */
                if (!singleResultCounter.isDoneForPartition() && command.limits().perPartitionCount() == DataLimits.NO_LIMIT)
                    return null;

                /*
                 * If the replica has no live rows in the partition, don't try to fetch more.
                 *
                 * Note that the previous branch [if (!singleResultCounter.isDoneForPartition()) return null] doesn't
                 * always cover this scenario:
                 * isDoneForPartition() is defined as [isDone() || rowInCurrentPartition >= perPartitionLimit],
                 * and will return true if isDone() returns true, even if there are 0 rows counted in the current partition.
                 *
                 * This can happen with a range read if after 1+ rounds of short read protection requests we managed to fetch
                 * enough extra rows for other partitions to satisfy the singleResultCounter's total row limit, but only
                 * have tombstones in the current partition.
                 *
                 * One other way we can hit this condition is when the partition only has a live static row and no regular
                 * rows. In that scenario the counter will remain at 0 until the partition is closed - which happens after
                 * the moreContents() call.
                 */
                if (countedInCurrentPartition(singleResultCounter) == 0)
                    return null;

                /*
                 * This is a table with no clustering columns, and has at most one row per partition - with EMPTY clustering.
                 * We already have the row, so there is no point in asking for more from the partition.
                 */
                if (Clustering.EMPTY == lastClustering)
                    return null;

                lastFetched = countedInCurrentPartition(singleResultCounter) - lastCounted;
                lastCounted = countedInCurrentPartition(singleResultCounter);

                // getting back fewer rows than we asked for means the partition on the replica has been fully consumed
                if (lastQueried > 0 && lastFetched < lastQueried)
                    return null;

                /*
                 * At this point we know that:
                 *     1. the replica returned [repeatedly?] as many rows as we asked for and potentially has more
                 *        rows in the partition
                 *     2. at least one of those returned rows was shadowed by a tombstone returned from another
                 *        replica
                 *     3. we haven't satisfied the client's limits yet, and should attempt to query for more rows to
                 *        avoid a short read
                 *
                 * In the ideal scenario, we would get exactly min(a, b) or fewer rows from the next request, where a and b
                 * are defined as follows:
                 *     [a] limits.count() - mergedResultCounter.counted()
                 *     [b] limits.perPartitionCount() - mergedResultCounter.countedInCurrentPartition()
                 *
                 * It would be naive to query for exactly that many rows, as it's possible and not unlikely
                 * that some of the returned rows would also be shadowed by tombstones from other hosts.
                 *
                 * Note: we don't know, nor do we care, how many rows from the replica made it into the reconciled result;
                 * we can only tell how many in total we queried for, and that [0, mrc.countedInCurrentPartition()) made it.
                 *
                 * In general, our goal should be to minimise the number of extra requests - *not* to minimise the number
                 * of rows fetched: there is a high transactional cost for every individual request, but a relatively low
                 * marginal cost for each extra row requested.
                 *
                 * As such it's better to overfetch than to underfetch extra rows from a host; but at the same
                 * time we want to respect paging limits and not blow up spectacularly.
                 *
                 * Note: it's ok to retrieve more rows that necessary since singleResultCounter is not stopping and only
                 * counts.
                 *
                 * With that in mind, we'll just request the minimum of (count(), perPartitionCount()) limits.
                 *
                 * See CASSANDRA-13794 for more details.
                 */
                lastQueried = Math.min(command.limits().count(), command.limits().perPartitionCount());

                ColumnFamilyStore.metricsFor(metadata.cfId).shortReadProtectionRequests.mark();
                Tracing.trace("Requesting {} extra rows from {} for short read protection", lastQueried, source);

                SinglePartitionReadCommand cmd = makeFetchAdditionalRowsReadCommand(lastQueried);
                return UnfilteredPartitionIterators.getOnlyElement(executeReadCommand(cmd), cmd);
            }

            // Counts the number of rows for regular queries and the number of groups for GROUP BY queries
            private int countedInCurrentPartition(Counter counter)
            {
                return command.limits().isGroupByLimit()
                     ? counter.rowCountedInCurrentPartition()
                     : counter.countedInCurrentPartition();
            }

            private SinglePartitionReadCommand makeFetchAdditionalRowsReadCommand(int toQuery)
            {
                ClusteringIndexFilter filter = command.clusteringIndexFilter(partitionKey);
                if (null != lastClustering)
                    filter = filter.forPaging(metadata.comparator, lastClustering, false);

                return SinglePartitionReadCommand.create(command.isForThrift(),
                                                         command.metadata(),
                                                         command.nowInSec(),
                                                         command.columnFilter(),
                                                         command.rowFilter(),
                                                         command.limits().forShortReadRetry(toQuery),
                                                         partitionKey,
                                                         filter,
                                                         command.indexMetadata());
            }
        }

        private UnfilteredPartitionIterator executeReadCommand(ReadCommand cmd)
        {
            DataResolver resolver = new DataResolver(keyspace, cmd, ConsistencyLevel.ONE, 1, queryStartNanoTime);
            ReadCallback handler = new ReadCallback(resolver, ConsistencyLevel.ONE, cmd, Collections.singletonList(source), queryStartNanoTime);

            if (StorageProxy.canDoLocalRequest(source))
                StageManager.getStage(Stage.READ).maybeExecuteImmediately(new StorageProxy.LocalReadRunnable(cmd, handler));
            else
                MessagingService.instance().sendRRWithFailure(cmd.createMessage(MessagingService.current_version), source, handler);

            // We don't call handler.get() because we want to preserve tombstones since we're still in the middle of merging node results.
            handler.awaitResults();
            assert resolver.responses.size() == 1;
            return resolver.responses.get(0).payload.makeIterator(command);
        }
    }
}