FileOperationHandler.java

/*
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package org.dcache.resilience.handlers;

import com.google.common.collect.ImmutableList;
import com.google.common.collect.Sets;
import com.google.common.util.concurrent.RateLimiter;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.net.InetSocketAddress;
import java.net.URI;
import java.net.URISyntaxException;
import java.time.Instant;
import java.time.temporal.ChronoUnit;
import java.util.Collection;
import java.util.Collections;
import java.util.NoSuchElementException;
import java.util.List;
import java.util.Set;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.stream.Collectors;

import diskCacheV111.util.CacheException;
import diskCacheV111.util.PnfsId;
import diskCacheV111.util.RetentionPolicy;
import diskCacheV111.util.SpreadAndWait;
import diskCacheV111.vehicles.HttpProtocolInfo;
import diskCacheV111.vehicles.PoolCheckFileMessage;
import diskCacheV111.vehicles.PoolManagerPoolInformation;

import dmg.cells.nucleus.CellPath;

import org.dcache.alarms.AlarmMarkerFactory;
import org.dcache.alarms.PredefinedAlarm;
import org.dcache.cells.CellStub;
import org.dcache.pool.migration.PoolMigrationCopyFinishedMessage;
import org.dcache.pool.migration.PoolSelectionStrategy;
import org.dcache.pool.migration.Task;
import org.dcache.pool.migration.TaskParameters;
import org.dcache.pool.repository.ReplicaState;
import org.dcache.pool.repository.StickyRecord;
import org.dcache.resilience.data.FileOperation;
import org.dcache.resilience.data.FileOperationMap;
import org.dcache.resilience.data.FileUpdate;
import org.dcache.resilience.data.MessageType;
import org.dcache.resilience.data.PoolInfoMap;
import org.dcache.resilience.data.StorageUnitConstraints;
import org.dcache.resilience.db.NamespaceAccess;
import org.dcache.resilience.util.CacheExceptionUtils;
import org.dcache.resilience.util.DegenerateSelectionStrategy;
import org.dcache.resilience.util.ExceptionMessage;
import org.dcache.resilience.util.LocationSelectionException;
import org.dcache.resilience.util.LocationSelector;
import org.dcache.resilience.util.RemoveLocationExtractor;
import org.dcache.resilience.util.ResilientFileTask;
import org.dcache.resilience.util.StaticSinglePoolList;
import org.dcache.util.CacheExceptionFactory;
import org.dcache.vehicles.FileAttributes;
import org.dcache.vehicles.resilience.RemoveReplicaMessage;

/**
 * <p>Principal resilience logic component.</p>
 *
 * <p>Contains methods for registering a pnfsid for handling and
 *    for updating the number of times it should be processed (based
 *    on the arrival of update messages), for verifying if some
 *    action indeed needs to be taken, for launching a copy/migration
 *    task and for eliminating an excess copy.</p>
 *
 * <p>Class is not marked final for stubbing/mocking purposes.</p>
 */
public class FileOperationHandler {
    private static final Logger LOGGER = LoggerFactory.getLogger(
                    FileOperationHandler.class);
    private static final Logger ACTIVITY_LOGGER =
            LoggerFactory.getLogger("org.dcache.resilience-log");

    private static final String INCONSISTENT_LOCATIONS_ALARM
                    = "Resilience has detected an inconsistency or lag between "
                    + "the namespace replica locations and the actual locations on disk.";

    private static final ImmutableList<StickyRecord> ONLINE_STICKY_RECORD
                    = ImmutableList.of(
                    new StickyRecord("system", StickyRecord.NON_EXPIRING));

    private static final RateLimiter LIMITER = RateLimiter.create(0.001);

    private static void sendOutOfSyncAlarm() {
        /*
         *  Create a new alarm every hour by keying the alarm to
         *  an hourly timestamp.  Otherwise, the alarm counter will
         *  just be updated for each alarm sent.  The rate limiter
         *  will not alarm more than once every 1000 seconds (once every
         *  15 minutes).
         */
        if (LIMITER.tryAcquire()) {
            LOGGER.warn(AlarmMarkerFactory.getMarker(
                            PredefinedAlarm.RESILIENCE_LOC_SYNC_ISSUE,
                            Instant.now().truncatedTo(ChronoUnit.HOURS).toString()),
                         INCONSISTENT_LOCATIONS_ALARM);
        }
    }

    /**
     * <p>For communication with the {@link ResilientFileTask}.</p>
     */
    public enum Type {
        COPY,
        REMOVE,
        VOID
    }

    private final PoolSelectionStrategy taskSelectionStrategy
                    = new DegenerateSelectionStrategy();

    private FileOperationMap fileOpMap;
    private PoolInfoMap      poolInfoMap;
    private NamespaceAccess  namespace;
    private LocationSelector locationSelector;

    private CellStub                  pinManager;
    private CellStub                  pools;
    private ScheduledExecutorService  taskService;
    private ScheduledExecutorService  migrationTaskService;
    private FileTaskCompletionHandler completionHandler;
    private InaccessibleFileHandler   inaccessibleFileHandler;

    private long        launchDelay       =  0L;
    private TimeUnit    launchDelayUnit   = TimeUnit.SECONDS;

    public ScheduledExecutorService getTaskService() {
        return taskService;
    }

    public ScheduledExecutorService getRemoveService() {
        return migrationTaskService;
    }

    public long getLaunchDelay() {
       return launchDelay;
    }

    public TimeUnit getLaunchDelayUnit() {
        return launchDelayUnit;
    }

    public void handleBrokenFileLocation(PnfsId pnfsId, String pool) {
        try {
            FileAttributes attributes
                            = FileUpdate.getAttributes(pnfsId, pool,
                                                       MessageType.CORRUPT_FILE,
                                                       namespace);
            if (attributes == null) {
                LOGGER.trace("{} not ONLINE.", pnfsId);
                return;
            }

            if (!poolInfoMap.isResilientPool(pool)) {
                LOGGER.trace("{} not in resilient group.", pool);
                return;
            }

            Collection<String> locations = attributes.getLocations();

            int actual = locations.size();
            int countable = poolInfoMap.getCountableLocations(locations);

            if (actual <= 1) {
                /*
                 * This is the only copy, or it is not/no longer in the
                 * namespace. In either case, do nothing.
                 */
                LOGGER.error(AlarmMarkerFactory.getMarker(PredefinedAlarm.INACCESSIBLE_FILE,
                                                          pnfsId.toString()),
                             "{}: Repair of broken replicas is not possible, "
                                             + "file currently inaccessible", pnfsId);
                return;
            }

            removeTarget(pnfsId, pool);

            if (countable > 1) {
                FileUpdate update = new FileUpdate(pnfsId, pool,
                                                   MessageType.CLEAR_CACHE_LOCATION,
                                                   false);
                /*
                 * Bypass the message guard check of CDC session.
                 */
                handleLocationUpdate(update);
            } else {
                /*
                 *  No alternate readable source; cannot attempt to make
                 *  any further replicas.
                 */
                LOGGER.error(AlarmMarkerFactory.getMarker(PredefinedAlarm.INACCESSIBLE_FILE,
                                                          pnfsId.toString()),
                             "{}: Repair of broken replicas is not possible, "
                                             + "file currently inaccessible", pnfsId);
            }
        } catch (CacheException e) {
            LOGGER.error("Error during handling of broken file removal ({}, {}): {}",
                         pnfsId, pool, new ExceptionMessage(e));
        }
    }

    /**
     * <p>The entry method for a PnfsId operation from a location update, called
     *      in response to an incoming message.</p>
     *
     * <p>If the entry is already in the current map, its count is incremented.</p>
     *
     * <p>A check is then made to see if the source belongs to a resilient
     *      pool group.  If not, the update is discarded.</p>
     *
     * <p>All attributes of the file that are necessary for resilience
     *      processing are then fetched.  Thereafter a series of preliminary
     *      checks are run for other disqualifying conditions.  If the pnfsId
     *      does qualify, an entry is added to the {@link FileOperationMap}.</p>
     *
     * @return true if a new operation is added to the map.
     */
    public boolean handleLocationUpdate(FileUpdate data)
                    throws CacheException {
        LOGGER.trace("handleLocationUpdate {}", data);

        if (data.pool == null) {
            LOGGER.debug("Update of {} with no location; file has likely "
                                         + "been deleted from namespace.",
                         data.pnfsId);
            return false;
        }

        if (!data.verifyPoolGroup(poolInfoMap)) {
            LOGGER.debug("Handle location update ({}, {}, {}; "
                                         + "pool is not a member "
                                         + "of a resilient group.",
                         data.pnfsId, data.pool, data.getGroup());
            return false;
        }

        /*
         * Prefetch all necessary file attributes, including current locations.
         */
        if (!data.validateAttributes(namespace)) {
            /*
             * Could be the result of a delete from namespace triggering a
             * clear cache location message.
             */
            return false;
        }

        /*
         *  Determine if action needs to be taken (counts).
         */
        if (!data.validateForAction(null, poolInfoMap, locationSelector)) {
            return false;
        }

        LOGGER.trace("handleLocationUpdate, update to be registered: {}", data);
        return fileOpMap.register(data);
    }

    /**
     * <p>The entry method for a PnfsId operation from a pool scan task.</p>
     *
     * <p>If the entry is already in the current map, its count is incremented.</p>
     *
     * <p>All attributes of the file that are necessary for resilience
     *      processing are then fetched.  Preliminary checks run for disqualifying
     *      conditions here include whether this is a storage unit modification,
     *      in which case the task is registered if the file has the storage unit
     *      in question. Otherwise, verification proceeds as in the
     *      {@link #handleLocationUpdate(FileUpdate)} method.</p>
     *
     * @return true if a new operation is added to the map.
     */
    public boolean handleScannedLocation(FileUpdate data, Integer storageUnit)
                    throws CacheException {
        LOGGER.debug("handleScannedLocation {}", data);

        /*
         * Prefetch all necessary file attributes, including current locations.
         */
        if (!data.validateAttributes(namespace)) {
            /*
             * Could be the result of deletion from namespace during the scan.
             */
            return false;
        }

        data.verifyPoolGroup(poolInfoMap);

        /*
         *  Determine if action needs to be taken.
         */
        if (!data.validateForAction(storageUnit, poolInfoMap, locationSelector)) {
            return false;
        }

        LOGGER.debug("handleScannedLocation, update to be registered: {}", data);
        return fileOpMap.register(data);
    }

    /**
     * <p>Wraps the creation of a migration {@link Task}.  The task is given
     *      a static single pool list and a degenerate selection strategy,
     *      since the target has already been selected by this handler.</p>
     */
    public Task handleMakeOneCopy(FileAttributes attributes) {
        PnfsId pnfsId = attributes.getPnfsId();
        FileOperation operation = fileOpMap.getOperation(pnfsId);

        /*
         * Fire-and-forget best effort.
         */
        operation.ensureSticky(poolInfoMap, pools);

        LOGGER.trace("Configuring migration task for {}.", pnfsId);
        StaticSinglePoolList list;

        try {
            list = new StaticSinglePoolList(poolInfoMap.getPoolManagerInfo(operation.getTarget()));
        } catch (NoSuchElementException e) {
            CacheException exception = CacheExceptionUtils.getCacheException(
                            CacheException.NO_POOL_CONFIGURED,
                            "Copy %s, could not get PoolManager info for %s: %s.",
                            pnfsId, poolInfoMap.getPool(operation.getTarget()),
                            e);
            completionHandler.taskFailed(pnfsId, exception);
            return null;
        }

        String source = poolInfoMap.getPool(operation.getSource());

        TaskParameters taskParameters = new TaskParameters(
                        pools,
                        null,   // PnfsManager cell stub not used
                        pinManager,
                        migrationTaskService,
                        taskSelectionStrategy,
                        list,
                        false,  // eager; update should not happen
                        false,  // isMetaOnly; just move the metadata
                        false,  // compute checksum on update; should not happen
                        false,  // force copy even if pool not readable
                        true,   // maintain atime
                        1);

        Task task = new Task(taskParameters, completionHandler, source, pnfsId,
                             ReplicaState.CACHED, ONLINE_STICKY_RECORD,
                             Collections.EMPTY_LIST, attributes,
                             attributes.getAccessTime());
        if (ACTIVITY_LOGGER.isInfoEnabled()) {
            List<String> allPools = list.getPools().stream()
                    .map(PoolManagerPoolInformation::getName)
                    .collect(Collectors.toList());
            ACTIVITY_LOGGER.info("Initiating replication of {} from {} to"
                    + " pools: {}, offline: {}", pnfsId, source, allPools,
                    list.getOfflinePools());
        }
        LOGGER.trace("Created migration task for {}: source {}, list {}.",
                     pnfsId, source, list);

        return task;
    }

    /**
     * @param message returned by pool migration task, needs to be passed
     *                to the migration task.
     */
    public void handleMigrationCopyFinished(
                    PoolMigrationCopyFinishedMessage message) {
        LOGGER.trace("Migration copy finished {}", message);
        try {
            fileOpMap.updateOperation(message);
        } catch (IllegalStateException e) {
            /*
             *  We treat the missing entry benignly, as it is
             *  possible to have a race between removal from forced cancellation
             *  and the arrival of the message from the pool.
             */
            LOGGER.trace("{}", new ExceptionMessage(e));
        }
    }

    /**
     * <p>Calls {@link #removeTarget(PnfsId, String)} and then reports
     *    success or failure to the completion handler.</p>
     */
    public void handleRemoveOneCopy(FileAttributes attributes) {
        PnfsId pnfsId = attributes.getPnfsId();
        FileOperation operation = fileOpMap.getOperation(pnfsId);

        try {
            String target = poolInfoMap.getPool(operation.getTarget());
            LOGGER.trace("handleRemoveOneCopy {}, removing {}.", pnfsId,
                         target);
            removeTarget(pnfsId, target);
        } catch (CacheException e) {
            completionHandler.taskFailed(pnfsId, e);
        }

        completionHandler.taskCompleted(pnfsId);
    }

    /**
     * <p>Called when a pnfsid has been selected from the operation map for
     *      possible processing. Refreshes locations from namespace, and checks
     *      which of those are currently readable.  Sends an alarm if
     *      no operation can occur but should.</p>
     *
     * @return COPY, REMOVE, or VOID if no operation is necessary.
     */
    public Type handleVerification(FileAttributes attributes) {
        PnfsId pnfsId = attributes.getPnfsId();
        FileOperation operation = fileOpMap.getOperation(pnfsId);

        int gindex = operation.getPoolGroup();

        /*
         *  Note that the location-dependent checks are run on this thread
         *  instead of being pre-checked by the FileOperationMap consumer thread
         *  because they require a call to the database.
         */
        try {
            namespace.refreshLocations(attributes);
        } catch (CacheException e) {
            CacheException exception = CacheExceptionUtils.getCacheException(e.getRc(),
                            FileTaskCompletionHandler.VERIFY_FAILURE_MESSAGE,
                            pnfsId, null, e.getCause());
            completionHandler.taskFailed(pnfsId, exception);
            return Type.VOID;
        }

        Collection<String> locations = attributes.getLocations();

        LOGGER.trace("handleVerification {}, locations from namespace: {}",
                     pnfsId, locations);

        /*
         * Somehow, all the cache locations for this file have been removed.
         */
        if (locations.isEmpty()) {
            return inaccessibleFileHandler.handleNoLocationsForFile(operation);
        }

        locations = poolInfoMap.getMemberLocations(gindex, locations);

        LOGGER.trace("handleVerification {}, valid group member locations {}",
                     pnfsId, locations);

        /*
         * If all the locations are pools no longer belonging to the group,
         * the operation should be voided.
         */
        if (locations.isEmpty()) {
            fileOpMap.voidOperation(pnfsId);
            return Type.VOID;
        }

        /*
         *  If we have arrived here, we are expecting there to be an
         *  available source file.  So we need the strictly readable
         *  locations, not just "countable" ones.
         */
        Set<String> namespaceReadable
                        = poolInfoMap.getReadableLocations(locations);

        Set<String> verifiedReadable;

        try {
            verifiedReadable  = verifyLocations(pnfsId,
                                                namespaceReadable,
                                                pools);
        } catch (InterruptedException e) {
            LOGGER.trace("handleVerification, {}, verify pool "
                                         + "locations interrupted; "
                                         + "cancelling operation", pnfsId);
            completionHandler.taskCancelled(pnfsId);
            return Type.VOID;
        }

        LOGGER.trace("handleVerification, {}, namespace readable locations {},"
                                     + "verified readable locations {}", pnfsId,
                     namespaceReadable, verifiedReadable);

        if (namespaceReadable.size() != verifiedReadable.size()) {
            ACTIVITY_LOGGER.info("The namespace is not in sync with the pool "
                                                 + "repositories for {}: "
                                                 + "namespace locations "
                                                 + "that are readable: {}; "
                                                 + "actually found: {}.",
                                 pnfsId, namespaceReadable, verifiedReadable);
            sendOutOfSyncAlarm();
        }

        if (inaccessibleFileHandler.isInaccessible(verifiedReadable, operation)) {
            LOGGER.trace("handleVerification {}, no readable locations found, "
                                         + "checking to see if "
                                         + "file can be staged.", pnfsId);
            return inaccessibleFileHandler.handleInaccessibleFile(operation);
        }

        if (shouldEvictALocation(operation, verifiedReadable)) {
            LOGGER.trace("handleVerification, location should be evicted {}",
                         verifiedReadable);
            return Type.REMOVE;
        }

        LOGGER.trace("handleVerification after eviction check, {}, locations {}",
                        pnfsId, locations);

        return determineTypeFromConstraints(operation,
                                            locations,
                                            verifiedReadable);
    }

    public void setCompletionHandler(
                    FileTaskCompletionHandler completionHandler) {
        this.completionHandler = completionHandler;
    }

    public void setFileOpMap(FileOperationMap fileOpMap) {
        this.fileOpMap = fileOpMap;
    }

    public void setInaccessibleFileHandler(
                    InaccessibleFileHandler inaccessibleFileHandler) {
        this.inaccessibleFileHandler = inaccessibleFileHandler;
    }

    public void setLaunchDelay(long launchDelay) {
        this.launchDelay = launchDelay;
    }

    public void setLaunchDelayUnit(TimeUnit launchDelayUnit) {
        this.launchDelayUnit = launchDelayUnit;
    }

    public void setLocationSelector(LocationSelector locationSelector) {
        this.locationSelector = locationSelector;
    }

    public void setNamespace(NamespaceAccess namespace) {
        this.namespace = namespace;
    }

    public void setPinManagerStub(CellStub pinManager) {
        this.pinManager = pinManager;
    }

    public void setPoolInfoMap(PoolInfoMap poolInfoMap) {
        this.poolInfoMap = poolInfoMap;
    }

    public void setPoolStub(CellStub pools) {
        this.pools = pools;
    }

    public void setMigrationTaskService(ScheduledExecutorService migrationTaskService) {
        this.migrationTaskService = migrationTaskService;
    }

    public void setTaskService(ScheduledExecutorService taskService) {
        this.taskService = taskService;
    }

    /**
     * <p>Checks the readable locations against the requirements.
     *      If previous operations on this pnfsId have already satisfied them,
     *      the operation should be voided.</p>
     *
     * @return the type of operation which should take place, if any.
     */
    private Type determineTypeFromConstraints(FileOperation operation,
                                              Collection<String> locations,
                                              Set<String> readableLocations) {
        PnfsId pnfsId = operation.getPnfsId();
        Integer gindex = operation.getPoolGroup();
        Integer sindex = operation.getStorageUnit();

        LOGGER.trace("determineTypeFromConstraints {}, group {}, unit {}.",
                     pnfsId, gindex, sindex);

        StorageUnitConstraints constraints
                        = poolInfoMap.getStorageUnitConstraints(sindex);

        Set<String> excluded = poolInfoMap.getExcludedLocationNames(locations);
        readableLocations = Sets.difference(readableLocations, excluded);

        int required = constraints.getRequired();
        int missing = required - readableLocations.size();

        /*
         *  First compute the missing files on the basis of just the readable
         *  files.  If this is positive, recompute by adding in all the
         *  excluded locations.  If these satisfy the requirement, void
         *  the operation.  Do no allow removes in this case, since this
         *  would imply decreasing already deficient locations.
         */
        if (missing > 0) {
            missing -= excluded.size();
            if (missing < 0) {
                missing = 0;
            }
        }

        Collection<String> tags = constraints.getOneCopyPer();

        LOGGER.trace("{}, readable locations {}, required {}, missing {}.",
                     pnfsId, readableLocations, constraints.getRequired(),
                     missing);

        Type type;
        String source = null;
        String target = null;

        try {
            /*
             *  Note that if the operation source or target is preset,
             *  the selection is skipped.
             */
            if (missing < 0) {
                type = Type.REMOVE;
                Integer pool = operation.getTarget();
                if (pool == null || !poolInfoMap.isPoolViable(pool, true)) {
                    target = locationSelector.selectRemoveTarget(operation,
                                    readableLocations, tags);
                }
                LOGGER.trace("target to remove: {}", target);
            } else if (missing > 0) {
                type = Type.COPY;
                Integer pool = operation.getSource();
                if (pool == null || !poolInfoMap.isPoolViable(pool, false)) {
                    source = locationSelector.selectCopySource(operation,
                                                               readableLocations);
                }
                LOGGER.trace("source: {}", source);
                pool = operation.getTarget();
                if (pool == null || !poolInfoMap.isPoolViable(pool, true)) {
                    target = locationSelector.selectCopyTarget(operation, gindex,
                                    readableLocations, tags);
                }
                LOGGER.trace("target to copy: {}", target);
            } else {
                LOGGER.trace("Nothing to do, VOID operation for {}", pnfsId);
                fileOpMap.voidOperation(pnfsId);
                return Type.VOID;
            }
        } catch (LocationSelectionException e) {
            CacheException exception = CacheExceptionUtils.getCacheException(
                            CacheException.DEFAULT_ERROR_CODE,
                            FileTaskCompletionHandler.VERIFY_FAILURE_MESSAGE,
                            pnfsId, null, e);
            completionHandler.taskFailed(pnfsId, exception);
            return Type.VOID;
        }

        fileOpMap.updateOperation(pnfsId, source, target);

        return type;
    }

    /**
     * <p>Synchronously removes from the target location the cache entry of the
     *      pnfsid associated with this task.  This is done via a message
     *      sent to a handler for this purpose on the pool itself.</p>
     */
    private void removeTarget(PnfsId pnfsId, String target)
                    throws CacheException {
        RemoveReplicaMessage msg = new RemoveReplicaMessage(target,
                                                            pnfsId);

        LOGGER.trace("Sending RemoveReplicasMessage {}.", msg);
        ACTIVITY_LOGGER.info("Removing {} from {}", pnfsId, target);
        Future<RemoveReplicaMessage> future = pools.send(new CellPath(target), msg);

        try {
            msg = future.get();
            LOGGER.trace("Returned ReplicationRepRmMessage {}.", msg);
        } catch (InterruptedException | ExecutionException e) {
            throw CacheExceptionUtils.getCacheException(
                            CacheException.SELECTED_POOL_FAILED,
                            FileTaskCompletionHandler.FAILED_REMOVE_MESSAGE,
                            pnfsId, target, e);
        }

        CacheException exception = msg.getErrorObject() == null ? null :
                        CacheExceptionFactory.exceptionOf(msg);

        if (exception != null && !CacheExceptionUtils.replicaNotFound(exception)) {
            throw CacheExceptionUtils.getCacheException(
                            CacheException.SELECTED_POOL_FAILED,
                            FileTaskCompletionHandler.FAILED_REMOVE_MESSAGE,
                            pnfsId, target, exception);
        }
    }

    /**
     * <p>Checks for necessary eviction due to pool tag changes or
     *      constraint change.  This call will automatically set
     *      the offending location as the target for a remove operation,
     *      and will increment the operation count so that there will
     *      be a chance to repeat the operation in order to make a new copy.</p>
     */
    private boolean shouldEvictALocation(FileOperation operation,
                                         Collection<String> readableLocations) {
        if (readableLocations.isEmpty()) {
            return false;
        }

        Integer sunit = operation.getStorageUnit();
        if (sunit == null) {
            return false;
        }

        StorageUnitConstraints constraints
                        = poolInfoMap.getStorageUnitConstraints(sunit);
        RemoveLocationExtractor extractor
                        = new RemoveLocationExtractor(
                        constraints.getOneCopyPer(),
                        poolInfoMap);
        String toEvict = extractor.findALocationToEvict(readableLocations);

        if (toEvict != null) {
            operation.setTarget(poolInfoMap.getPoolIndex(toEvict));
            int count = operation.getOpCount();
            operation.setOpCount(++count);
            return true;
        }

        return false;
    }

    /**
     * <p>Called when there are no accessible replicas for the file.</p>
     *
     * <p>If the file's RetentionPolicy is CUSTODIAL, set the count to 1,
     *    to make sure the task completes after this.  Staging is fire-and-forget,
     *    and depends on a new add cache location message being processed
     *    after staging.</p>
     *
     * @return true if file is CUSTODIAL, false otherwise.
     */
    private boolean shouldTryToStage(FileAttributes attributes,
                                     FileOperation operation) {
        if (attributes.getRetentionPolicy() == RetentionPolicy.CUSTODIAL) {
            LOGGER.trace("shouldTryToStage {}, retention policy is CUSTODIAL.",
                         operation.getPnfsId());
            operation.setOpCount(1);
            return true;
        }
        LOGGER.trace("shouldTryToStage {}, retention policy is not CUSTODIAL",
                     operation.getPnfsId());
        return false;
    }

    /*
     *  REVISIT -- this will be expanded into a more general verification procedure
     *
     *  Check the readable pools for the actual existence of the replica.
     */
    private Set<String> verifyLocations(PnfsId pnfsId,
                                        Collection<String> locations,
                                        CellStub stub)
                    throws InterruptedException {
        SpreadAndWait<PoolCheckFileMessage> controller = new SpreadAndWait<>(pools);

        for(String pool: locations) {
            LOGGER.trace("Sending query to {} to verify replica exists.", pool);
            PoolCheckFileMessage request = new PoolCheckFileMessage(pool, pnfsId);
            controller.send(new CellPath(pool), PoolCheckFileMessage.class, request);
        }

        controller.waitForReplies();

        return controller.getReplies().values()
                         .stream()
                         .filter(PoolCheckFileMessage::getHave)
                         .map(PoolCheckFileMessage::getPoolName)
                         .collect(Collectors.toSet());
    }
}