db_raii.cpp

/**
 *    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
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 */

#define MONGO_LOG_DEFAULT_COMPONENT ::mongo::logger::LogComponent::kStorage

#include "mongo/platform/basic.h"

#include "mongo/db/db_raii.h"

#include "mongo/db/catalog/database_holder.h"
#include "mongo/db/concurrency/locker.h"
#include "mongo/db/curop.h"
#include "mongo/db/db_raii_gen.h"
#include "mongo/db/repl/replication_coordinator.h"
#include "mongo/db/s/collection_sharding_state.h"
#include "mongo/util/log.h"

namespace mongo {
namespace {

const boost::optional<int> kDoNotChangeProfilingLevel = boost::none;

}  // namespace

AutoStatsTracker::AutoStatsTracker(OperationContext* opCtx,
                                   const NamespaceString& nss,
                                   Top::LockType lockType,
                                   LogMode logMode,
                                   boost::optional<int> dbProfilingLevel,
                                   Date_t deadline)
    : _opCtx(opCtx), _lockType(lockType), _nss(nss) {
    if (!dbProfilingLevel && logMode == LogMode::kUpdateTopAndCurop) {
        // No profiling level was determined, attempt to read the profiling level from the Database
        // object. Since we are only reading the in-memory profiling level out of the database
        // object (which is configured on a per-node basis and not replicated or persisted), we
        // never need to conflict with secondary batch application.
        ShouldNotConflictWithSecondaryBatchApplicationBlock noConflict(opCtx->lockState());
        AutoGetDb autoDb(_opCtx, _nss.db(), MODE_IS, deadline);
        if (autoDb.getDb()) {
            dbProfilingLevel = autoDb.getDb()->getProfilingLevel();
        }
    }

    stdx::lock_guard<Client> clientLock(*_opCtx->getClient());
    if (logMode == LogMode::kUpdateTopAndCurop) {
        CurOp::get(_opCtx)->enter_inlock(_nss.ns().c_str(), dbProfilingLevel);
    }
}

AutoStatsTracker::~AutoStatsTracker() {
    auto curOp = CurOp::get(_opCtx);
    Top::get(_opCtx->getServiceContext())
        .record(_opCtx,
                _nss.ns(),
                curOp->getLogicalOp(),
                _lockType,
                durationCount<Microseconds>(curOp->elapsedTimeExcludingPauses()),
                curOp->isCommand(),
                curOp->getReadWriteType());
}

AutoGetCollectionForRead::AutoGetCollectionForRead(OperationContext* opCtx,
                                                   const NamespaceStringOrUUID& nsOrUUID,
                                                   AutoGetCollection::ViewMode viewMode,
                                                   Date_t deadline) {
    // Don't take the ParallelBatchWriterMode lock when the server parameter is set and our
    // storage engine supports snapshot reads.
    if (gAllowSecondaryReadsDuringBatchApplication.load() &&
        opCtx->getServiceContext()->getStorageEngine()->supportsReadConcernSnapshot()) {
        _shouldNotConflictWithSecondaryBatchApplicationBlock.emplace(opCtx->lockState());
    }
    const auto collectionLockMode = getLockModeForQuery(opCtx, nsOrUUID.nss());
    _autoColl.emplace(opCtx, nsOrUUID, collectionLockMode, viewMode, deadline);

    // If the read source is explicitly set to kNoTimestamp, we read the most up to date data and do
    // not consider reading at last applied (e.g. FTDC needs that).
    if (opCtx->recoveryUnit()->getTimestampReadSource() == RecoveryUnit::ReadSource::kNoTimestamp)
        return;

    repl::ReplicationCoordinator* const replCoord = repl::ReplicationCoordinator::get(opCtx);
    const auto readConcernLevel = repl::ReadConcernArgs::get(opCtx).getLevel();

    // If the collection doesn't exist or disappears after releasing locks and waiting, there is no
    // need to check for pending catalog changes.
    while (auto coll = _autoColl->getCollection()) {

        auto readSource = opCtx->recoveryUnit()->getTimestampReadSource();
        auto minSnapshot = coll->getMinimumVisibleSnapshot();
        auto mySnapshot = opCtx->recoveryUnit()->getPointInTimeReadTimestamp();

        // If we are reading at a provided timestamp earlier than the latest catalog changes, then
        // we must return an error.
        if (readSource == RecoveryUnit::ReadSource::kProvided && minSnapshot &&
            (*mySnapshot < *minSnapshot)) {
            uasserted(ErrorCodes::SnapshotUnavailable,
                      str::stream()
                          << "Unable to read from a snapshot due to pending collection catalog "
                             "changes; please retry the operation. Snapshot timestamp is "
                          << mySnapshot->toString() << ". Collection minimum is "
                          << minSnapshot->toString());
        }

        // During batch application on secondaries, there is a potential to read inconsistent states
        // that would normally be protected by the PBWM lock. In order to serve secondary reads
        // during this period, we default to not acquiring the lock (by setting
        // _shouldNotConflictWithSecondaryBatchApplicationBlock). On primaries, we always read at a
        // consistent time, so not taking the PBWM lock is not a problem. On secondaries, we have to
        // guarantee we read at a consistent state, so we must read at the last applied timestamp,
        // which is set after each complete batch.
        //
        // If an attempt to read at the last applied timestamp is unsuccessful because there are
        // pending catalog changes that occur after the last applied timestamp, we release our locks
        // and try again with the PBWM lock (by unsetting
        // _shouldNotConflictWithSecondaryBatchApplicationBlock).

        const NamespaceString nss = coll->ns();

        bool readAtLastAppliedTimestamp =
            _shouldReadAtLastAppliedTimestamp(opCtx, nss, readConcernLevel);

        if (readAtLastAppliedTimestamp) {
            opCtx->recoveryUnit()->setTimestampReadSource(RecoveryUnit::ReadSource::kLastApplied);
            readSource = opCtx->recoveryUnit()->getTimestampReadSource();
        }

        // This timestamp could be earlier than the timestamp seen when the transaction is opened
        // because it is set asynchonously. This is not problematic because holding the collection
        // lock guarantees no metadata changes will occur in that time.
        auto lastAppliedTimestamp = readAtLastAppliedTimestamp
            ? boost::optional<Timestamp>(replCoord->getMyLastAppliedOpTime().getTimestamp())
            : boost::none;

        if (!_conflictingCatalogChanges(opCtx, minSnapshot, lastAppliedTimestamp)) {
            return;
        }

        invariant(lastAppliedTimestamp ||
                  // The kMajorityCommitted and kNoOverlap read sources already read from timestamps
                  // that are safe with respect to concurrent secondary batch application.
                  readSource == RecoveryUnit::ReadSource::kMajorityCommitted ||
                  readSource == RecoveryUnit::ReadSource::kNoOverlap);
        invariant(readConcernLevel != repl::ReadConcernLevel::kSnapshotReadConcern);

        // Yield locks in order to do the blocking call below.
        _autoColl = boost::none;

        // If there are pending catalog changes, we should conflict with any in-progress batches (by
        // taking the PBWM lock) and choose not to read from the last applied timestamp by unsetting
        // _shouldNotConflictWithSecondaryBatchApplicationBlock. Index builds on secondaries can
        // complete at timestamps later than the lastAppliedTimestamp during initial sync. After
        // initial sync finishes, if we waited instead of retrying, readers would block indefinitely
        // waiting for the lastAppliedTimestamp to move forward. Instead we force the reader take
        // the PBWM lock and retry.
        if (lastAppliedTimestamp) {
            LOG(0) << "tried reading at last-applied time: " << *lastAppliedTimestamp
                   << " on ns: " << nss.ns() << ", but future catalog changes are pending at time "
                   << *minSnapshot << ". Trying again without reading at last-applied time.";
            // Destructing the block sets _shouldConflictWithSecondaryBatchApplication back to the
            // previous value. If the previous value is false (because there is another
            // shouldNotConflictWithSecondaryBatchApplicationBlock outside of this function), this
            // does not take the PBWM lock.
            _shouldNotConflictWithSecondaryBatchApplicationBlock = boost::none;
            invariant(opCtx->lockState()->shouldConflictWithSecondaryBatchApplication());

            // Cannot change ReadSource while a RecoveryUnit is active, which may result from
            // calling getPointInTimeReadTimestamp().
            opCtx->recoveryUnit()->abandonSnapshot();
            opCtx->recoveryUnit()->setTimestampReadSource(RecoveryUnit::ReadSource::kUnset);
        }

        // If there are pending catalog changes when using a no-overlap read source, we choose to
        // take the PBWM lock to conflict with any in-progress batches. This prevents us from idly
        // spinning in this loop trying to get a new read timestamp ahead of the minimum visible
        // snapshot. This helps us guarantee liveness (i.e. we can eventually get a suitable read
        // timestamp) but should not be necessary for correctness.
        if (readSource == RecoveryUnit::ReadSource::kNoOverlap) {
            invariant(!lastAppliedTimestamp);  // no-overlap read source selects its own timestamp.
            _shouldNotConflictWithSecondaryBatchApplicationBlock = boost::none;
            invariant(opCtx->lockState()->shouldConflictWithSecondaryBatchApplication());

            // Abandon our snapshot but don't change our read source, so that we can select a new
            // read timestamp on the next loop iteration.
            opCtx->recoveryUnit()->abandonSnapshot();
        }

        if (readSource == RecoveryUnit::ReadSource::kMajorityCommitted) {
            replCoord->waitUntilSnapshotCommitted(opCtx, *minSnapshot);
            uassertStatusOK(opCtx->recoveryUnit()->obtainMajorityCommittedSnapshot());
        }

        {
            stdx::lock_guard<Client> lk(*opCtx->getClient());
            CurOp::get(opCtx)->yielded();
        }

        _autoColl.emplace(opCtx, nsOrUUID, collectionLockMode, viewMode, deadline);
    }
}

bool AutoGetCollectionForRead::_shouldReadAtLastAppliedTimestamp(
    OperationContext* opCtx,
    const NamespaceString& nss,
    repl::ReadConcernLevel readConcernLevel) const {

    // If this block is unset, then the operation did not opt-out of the PBWM lock, implying that it
    // cannot read at lastApplied. It's important to note that it is possible for this to be set,
    // but still be holding the PBWM lock, explained below.
    if (!_shouldNotConflictWithSecondaryBatchApplicationBlock) {
        return false;
    }

    // If we are already holding the PBWM lock, do not read at last-applied. This is because once an
    // operation reads without a timestamp (effectively seeing all writes), it is no longer safe to
    // start reading at a timestamp, as writes or catalog operations may appear to vanish.
    // This may occur when multiple collection locks are held concurrently, which is often the case
    // when DBDirectClient is used.
    if (opCtx->lockState()->isLockHeldForMode(resourceIdParallelBatchWriterMode, MODE_IS)) {
        LOG(1) << "not reading at last-applied because the PBWM lock is held";
        return false;
    }

    // Majority and snapshot readConcern levels should not read from lastApplied; these read
    // concerns already have a designated timestamp to read from.
    if (readConcernLevel != repl::ReadConcernLevel::kLocalReadConcern &&
        readConcernLevel != repl::ReadConcernLevel::kAvailableReadConcern) {
        return false;
    }

    // If we are in a replication state (like secondary or primary catch-up) where we are not
    // accepting writes, we should read at lastApplied. If this node can accept writes, then no
    // conflicting replication batches are being applied and we can read from the default snapshot.
    if (repl::ReplicationCoordinator::get(opCtx)->canAcceptWritesForDatabase(opCtx, "admin")) {
        return false;
    }

    // Non-replicated collections do not need to read at lastApplied, as those collections are not
    // written by the replication system.  However, the oplog is special, as it *is* written by the
    // replication system.
    if (!nss.isReplicated() && !nss.isOplog()) {
        return false;
    }

    return true;
}

bool AutoGetCollectionForRead::_conflictingCatalogChanges(
    OperationContext* opCtx,
    boost::optional<Timestamp> minSnapshot,
    boost::optional<Timestamp> lastAppliedTimestamp) const {
    // This is the timestamp of the most recent catalog changes to this collection. If this is
    // greater than any point in time read timestamps, we should either wait or return an error.
    if (!minSnapshot) {
        return false;
    }

    // If we are reading from the lastAppliedTimestamp and it is up-to-date with any catalog
    // changes, we can return.
    if (lastAppliedTimestamp &&
        (lastAppliedTimestamp->isNull() || *lastAppliedTimestamp >= *minSnapshot)) {
        return false;
    }

    // This can be set when readConcern is "snapshot" or "majority".
    auto mySnapshot = opCtx->recoveryUnit()->getPointInTimeReadTimestamp();

    // If we do not have a point in time to conflict with minSnapshot, return.
    if (!mySnapshot && !lastAppliedTimestamp) {
        return false;
    }

    // Return if there are no conflicting catalog changes with mySnapshot.
    if (mySnapshot && *mySnapshot >= *minSnapshot) {
        return false;
    }

    return true;
}

AutoGetCollectionForReadCommand::AutoGetCollectionForReadCommand(
    OperationContext* opCtx,
    const NamespaceStringOrUUID& nsOrUUID,
    AutoGetCollection::ViewMode viewMode,
    Date_t deadline,
    AutoStatsTracker::LogMode logMode)
    : _autoCollForRead(opCtx, nsOrUUID, viewMode, deadline),
      _statsTracker(opCtx,
                    _autoCollForRead.getNss(),
                    Top::LockType::ReadLocked,
                    logMode,
                    _autoCollForRead.getDb() ? _autoCollForRead.getDb()->getProfilingLevel()
                                             : kDoNotChangeProfilingLevel,
                    deadline) {

    // Perform the check early so the query planner would be able to extract the correct
    // shard key. Also make sure that version is compatible if query planner decides to
    // use an empty plan.
    invariant(!_autoCollForRead.getView() || !_autoCollForRead.getCollection());
    auto css = CollectionShardingState::get(opCtx, _autoCollForRead.getNss());
    css->checkShardVersionOrThrow(opCtx, _autoCollForRead.getCollection());
}

OldClientContext::OldClientContext(OperationContext* opCtx, const std::string& ns, bool doVersion)
    : _opCtx(opCtx), _db(DatabaseHolder::get(opCtx)->getDb(opCtx, ns)) {
    if (!_db) {
        const auto dbName = nsToDatabaseSubstring(ns);
        invariant(_opCtx->lockState()->isDbLockedForMode(dbName, MODE_X));
        _db = DatabaseHolder::get(opCtx)->openDb(_opCtx, dbName, &_justCreated);
        invariant(_db);
    }

    auto const currentOp = CurOp::get(_opCtx);

    if (doVersion) {
        switch (currentOp->getNetworkOp()) {
            case dbGetMore:  // getMore is special and should be handled elsewhere
            case dbUpdate:   // update & delete check shard version as part of the write executor
            case dbDelete:   // path, so no need to check them here as well
                break;
            default:
                CollectionShardingState::get(_opCtx, NamespaceString(ns))
                    ->checkShardVersionOrThrow(_opCtx,
                                               _db->getCollection(opCtx, NamespaceString(ns)));
                break;
        }
    }

    stdx::lock_guard<Client> lk(*_opCtx->getClient());
    currentOp->enter_inlock(ns.c_str(), _db->getProfilingLevel());
}

OldClientContext::~OldClientContext() {
    // If in an interrupt, don't record any stats.
    // It is possible to have no lock after saving the lock state and being interrupted while
    // waiting to restore.
    if (_opCtx->getKillStatus() != ErrorCodes::OK)
        return;

    invariant(_opCtx->lockState()->isLocked());
    auto currentOp = CurOp::get(_opCtx);
    Top::get(_opCtx->getClient()->getServiceContext())
        .record(_opCtx,
                currentOp->getNS(),
                currentOp->getLogicalOp(),
                _opCtx->lockState()->isWriteLocked() ? Top::LockType::WriteLocked
                                                     : Top::LockType::ReadLocked,
                _timer.micros(),
                currentOp->isCommand(),
                currentOp->getReadWriteType());
}

LockMode getLockModeForQuery(OperationContext* opCtx, const boost::optional<NamespaceString>& nss) {
    invariant(opCtx);

    // Use IX locks for multi-statement transactions; otherwise, use IS locks.
    if (opCtx->inMultiDocumentTransaction()) {
        uassert(51071,
                "Cannot query system.views within a transaction",
                !nss || !nss->isSystemDotViews());
        return MODE_IX;
    }
    return MODE_IS;
}

}  // namespace mongo