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/**
* 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
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#include "mongo/platform/basic.h"
#include "mongo/db/query/plan_yield_policy.h"
#include "mongo/db/catalog/collection.h"
#include "mongo/db/concurrency/exception_util.h"
#include "mongo/db/operation_context.h"
#include "mongo/util/assert_util.h"
#include "mongo/util/scopeguard.h"
#include "mongo/util/time_support.h"
namespace mongo {
PlanYieldPolicy::PlanYieldPolicy(YieldPolicy policy,
ClockSource* cs,
int yieldIterations,
Milliseconds yieldPeriod,
const Yieldable* yieldable,
std::unique_ptr<const YieldPolicyCallbacks> callbacks)
: _policy(policy),
_yieldable(yieldable),
_callbacks(std::move(callbacks)),
_elapsedTracker(cs, yieldIterations, yieldPeriod) {}
bool PlanYieldPolicy::shouldYieldOrInterrupt(OperationContext* opCtx) {
if (_policy == YieldPolicy::INTERRUPT_ONLY) {
return _elapsedTracker.intervalHasElapsed();
}
if (!canAutoYield())
return false;
invariant(!opCtx->lockState()->inAWriteUnitOfWork());
if (_forceYield)
return true;
return _elapsedTracker.intervalHasElapsed();
}
void PlanYieldPolicy::resetTimer() {
_elapsedTracker.resetLastTime();
}
Status PlanYieldPolicy::yieldOrInterrupt(OperationContext* opCtx,
std::function<void()> whileYieldingFn) {
invariant(opCtx);
if (_policy == YieldPolicy::INTERRUPT_ONLY) {
ON_BLOCK_EXIT([this]() { resetTimer(); });
invariant(opCtx);
if (_callbacks) {
_callbacks->preCheckInterruptOnly(opCtx);
}
return opCtx->checkForInterruptNoAssert();
}
invariant(!opCtx->lockState()->inAWriteUnitOfWork());
// After we finish yielding (or in any early return), call resetTimer() to prevent yielding
// again right away. We delay the resetTimer() call so that the clock doesn't start ticking
// until after we return from the yield.
ON_BLOCK_EXIT([this]() { resetTimer(); });
_forceYield = false;
for (int attempt = 1; true; attempt++) {
try {
// Saving and restoring can modify '_yieldable', so we make a copy before we start.
const Yieldable* yieldable = _yieldable;
try {
saveState(opCtx);
} catch (const WriteConflictException&) {
// Saving the state of an execution plan must never throw WCE.
MONGO_UNREACHABLE;
}
boost::optional<ScopeGuard<std::function<void()>>> exitGuard;
if (useExperimentalCommitTxnBehavior()) {
// All data pointed to by cursors must remain valid across the yield. Setting this
// flag for the duration of yield will force any calls to abandonSnapshot() to
// commit the transaction, rather than abort it, in order to leave the cursors
// valid.
opCtx->recoveryUnit()->setAbandonSnapshotMode(
RecoveryUnit::AbandonSnapshotMode::kCommit);
exitGuard.emplace([&] {
invariant(opCtx->recoveryUnit()->abandonSnapshotMode() ==
RecoveryUnit::AbandonSnapshotMode::kCommit);
opCtx->recoveryUnit()->setAbandonSnapshotMode(
RecoveryUnit::AbandonSnapshotMode::kAbort);
});
}
if (getPolicy() == PlanYieldPolicy::YieldPolicy::WRITE_CONFLICT_RETRY_ONLY) {
// This yield policy doesn't release locks, but it does relinquish our storage
// snapshot.
invariant(!opCtx->isLockFreeReadsOp());
opCtx->recoveryUnit()->abandonSnapshot();
} else {
invariant(yieldable);
performYield(opCtx, *yieldable, whileYieldingFn);
}
restoreState(opCtx, yieldable);
return Status::OK();
} catch (const WriteConflictException&) {
if (_callbacks) {
_callbacks->handledWriteConflict(opCtx);
}
logWriteConflictAndBackoff(attempt, "query yield", ""_sd);
// Retry the yielding process.
} catch (...) {
// Errors other than write conflicts don't get retried, and should instead result in
// the PlanExecutor dying. We propagate all such errors as status codes.
return exceptionToStatus();
}
}
MONGO_UNREACHABLE;
}
void PlanYieldPolicy::performYield(OperationContext* opCtx,
const Yieldable& yieldable,
std::function<void()> whileYieldingFn) {
// Things have to happen here in a specific order:
// * Release 'yieldable'.
// * Release lock mgr locks.
// * Check for interrupt if the yield policy requires.
// * Abondon the query's current storage engine snapshot.
// * Reacquire lock mgr locks.
// * Restore 'yieldable'.
Locker* locker = opCtx->lockState();
if (locker->isGlobalLockedRecursively()) {
// No purpose in yielding if the locks are recursively held and cannot be released.
return;
}
// Since the locks are not recursively held, this is a top level operation and we can safely
// clear the 'yieldable' state before unlocking and then re-establish it after re-locking.
yieldable.yield();
Locker::LockSnapshot snapshot;
auto unlocked = locker->saveLockStateAndUnlock(&snapshot);
// After all steps to relinquish locks and save the execution plan have been taken, check
// for interrupt. This is the main interrupt check during query execution. Yield points and
// interrupt points are one and the same.
if (getPolicy() == PlanYieldPolicy::YieldPolicy::YIELD_AUTO) {
opCtx->checkForInterrupt(); // throws
}
if (!unlocked) {
// Nothing was unlocked. Recursively held locks are not the only reason locks cannot be
// released. Restore the 'yieldable' state before returning.
yieldable.restore();
return;
}
// Top-level locks are freed, release any potential low-level (storage engine-specific
// locks). If we are yielding, we are at a safe place to do so.
opCtx->recoveryUnit()->abandonSnapshot();
if (_callbacks) {
_callbacks->duringYield(opCtx);
}
if (whileYieldingFn) {
whileYieldingFn();
}
locker->restoreLockState(opCtx, snapshot);
// A yield has occurred, but there still may not be a 'yieldable' if the PlanExecutor
// has a 'locks internally' lock policy.
// Yieldable restore may set a new read source if necessary.
yieldable.restore();
}
} // namespace mongo
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