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/**
* Copyright (C) 2022-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include "mongo/base/string_data.h"
#include "mongo/db/curop.h"
#include "mongo/db/operation_context.h"
#include "mongo/util/assert_util.h"
#include "mongo/util/fail_point.h"
namespace mongo {
extern FailPoint skipWriteConflictRetries;
/**
* Will log a message if sensible and will do an exponential backoff to make sure
* we don't hammer the same doc over and over.
* @param attempt - what attempt is this, 1 based
* @param operation - e.g. "update"
*/
void logWriteConflictAndBackoff(int attempt,
StringData operation,
StringData reason,
StringData ns);
void handleTemporarilyUnavailableException(OperationContext* opCtx,
int attempts,
StringData opStr,
StringData ns,
const TemporarilyUnavailableException& e);
/**
* Handle a TemporarilyUnavailableException inside a multi-document transaction.
*/
void handleTemporarilyUnavailableExceptionInTransaction(OperationContext* opCtx,
StringData opStr,
StringData ns,
const TemporarilyUnavailableException& e);
void handleTransactionTooLargeForCacheException(OperationContext* opCtx,
int* writeConflictAttempts,
StringData opStr,
StringData ns,
const TransactionTooLargeForCacheException& e);
namespace error_details {
/**
* A faster alternative to `iasserted`, designed to throw exceptions for unexceptional events on the
* critical execution path (e.g., `WriteConflict`).
*/
template <ErrorCodes::Error ec>
[[noreturn]] void throwExceptionFor(std::string reason) {
throw ExceptionFor<ec>({ec, std::move(reason)});
}
} // namespace error_details
/**
* A `WriteConflictException` is thrown if during a write, two or more operations conflict with each
* other. For example if two operations get the same version of a document, and then both try to
* modify that document, this exception will get thrown by one of them.
*/
[[noreturn]] inline void throwWriteConflictException(StringData context) {
error_details::throwExceptionFor<ErrorCodes::WriteConflict>(
"Caused by :: {} :: Please retry your operation or multi-document transaction."_format(
context));
}
/**
* A `TemporarilyUnavailableException` is thrown if an operation aborts due to the server being
* temporarily unavailable, e.g. due to excessive load. For user-originating operations, this will
* be retried internally by the `writeConflictRetry` helper a finite number of times before
* eventually being returned.
*/
[[noreturn]] inline void throwTemporarilyUnavailableException(std::string context) {
error_details::throwExceptionFor<ErrorCodes::TemporarilyUnavailable>(std::move(context));
}
/**
* A `TransactionTooLargeForCache` is thrown if it has been determined that it is unlikely to
* ever complete the operation because the configured cache is insufficient to hold all the
* transaction state. This helps to avoid retrying, maybe indefinitely, a transaction which would
* never be able to complete.
*/
[[noreturn]] inline void throwTransactionTooLargeForCache(std::string context) {
error_details::throwExceptionFor<ErrorCodes::TransactionTooLargeForCache>(std::move(context));
}
/**
* Runs the argument function f as many times as needed for f to complete or throw an exception
* other than WriteConflictException or TemporarilyUnavailableException. For each time f throws
* one of these exceptions, logs the error, waits a spell, cleans up, and then tries f again.
* Imposes no upper limit on the number of times to re-try f after a WriteConflictException, so any
* required timeout behavior must be enforced within f. When retrying a
* TemporarilyUnavailableException, f is called a finite number of times before we eventually let
* the error escape.
*
* If we are already in a WriteUnitOfWork, we assume that we are being called within a
* WriteConflictException retry loop up the call stack. Hence, this retry loop is reduced to an
* invocation of the argument function f without any exception handling and retry logic.
*/
template <typename F>
auto writeConflictRetry(OperationContext* opCtx, StringData opStr, StringData ns, F&& f) {
invariant(opCtx);
invariant(opCtx->lockState());
invariant(opCtx->recoveryUnit());
// This failpoint disables exception handling for write conflicts. Only allow this exception to
// escape user operations. Do not allow exceptions to escape internal threads, which may rely on
// this exception handler to avoid crashing.
bool userSkipWriteConflictRetry = MONGO_unlikely(skipWriteConflictRetries.shouldFail()) &&
opCtx->getClient()->isFromUserConnection();
if (opCtx->lockState()->inAWriteUnitOfWork() || userSkipWriteConflictRetry) {
try {
return f();
} catch (TemporarilyUnavailableException const& e) {
if (opCtx->inMultiDocumentTransaction()) {
handleTemporarilyUnavailableExceptionInTransaction(opCtx, opStr, ns, e);
}
throw;
}
}
int writeConflictAttempts = 0;
int attemptsTempUnavailable = 0;
while (true) {
try {
return f();
} catch (WriteConflictException const& e) {
CurOp::get(opCtx)->debug().additiveMetrics.incrementWriteConflicts(1);
logWriteConflictAndBackoff(writeConflictAttempts, opStr, e.reason(), ns);
++writeConflictAttempts;
opCtx->recoveryUnit()->abandonSnapshot();
} catch (TemporarilyUnavailableException const& e) {
handleTemporarilyUnavailableException(opCtx, ++attemptsTempUnavailable, opStr, ns, e);
} catch (TransactionTooLargeForCacheException const& e) {
handleTransactionTooLargeForCacheException(opCtx, &writeConflictAttempts, opStr, ns, e);
}
}
}
} // namespace mongo
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