2017-07-23 12:54:33 +00:00
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// Copyright 2016 The Cockroach Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
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// implied. See the License for the specific language governing
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// permissions and limitations under the License.
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// Package crdb provides helpers for using CockroachDB in client
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// applications.
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package crdb
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import (
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2017-09-05 22:06:47 +00:00
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"context"
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"database/sql"
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2018-10-03 16:55:26 +00:00
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"github.com/jackc/pgx"
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"github.com/lib/pq"
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)
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// ExecuteTx runs fn inside a transaction and retries it as needed.
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// On non-retryable failures, the transaction is aborted and rolled
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// back; on success, the transaction is committed.
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// There are cases where the state of a transaction is inherently ambiguous: if
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// we err on RELEASE with a communication error it's unclear if the transaction
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// has been committed or not (similar to erroring on COMMIT in other databases).
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// In that case, we return AmbiguousCommitError.
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// There are cases when restarting a transaction fails: we err on ROLLBACK
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// to the SAVEPOINT. In that case, we return a TxnRestartError.
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//
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// For more information about CockroachDB's transaction model see
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// https://cockroachlabs.com/docs/stable/transactions.html.
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//
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// NOTE: the supplied fn closure should not have external side
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// effects beyond changes to the database.
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//
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// fn must take care when wrapping errors returned from the database driver with
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// additional context. For example, if the UPDATE statement fails in the
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// following snippet, the original retryable error will be masked by the call to
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// fmt.Errorf, and the transaction will not be automatically retried.
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//
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// crdb.ExecuteTx(ctx, db, txopts, func (tx *sql.Tx) error {
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// if err := tx.ExecContext(ctx, "UPDATE..."); err != nil {
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// return fmt.Errorf("updating record: %s", err)
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// }
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// return nil
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// })
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//
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// Instead, add context by returning an error that implements the ErrorCauser
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// interface. Either create a custom error type that implements ErrorCauser or
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// use a helper function that does so automatically, like pkg/errors.Wrap:
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//
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// import "github.com/pkg/errors"
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//
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// crdb.ExecuteTx(ctx, db, txopts, func (tx *sql.Tx) error {
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// if err := tx.ExecContext(ctx, "UPDATE..."); err != nil {
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// return errors.Wrap(err, "updating record")
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// }
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// return nil
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// })
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//
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func ExecuteTx(
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ctx context.Context, db *sql.DB, txopts *sql.TxOptions, fn func(*sql.Tx) error,
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) error {
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// Start a transaction.
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tx, err := db.BeginTx(ctx, txopts)
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if err != nil {
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return err
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}
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return ExecuteInTx(ctx, tx, func() error { return fn(tx) })
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}
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// Tx is used to permit clients to implement custom transaction logic.
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type Tx interface {
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ExecContext(context.Context, string, ...interface{}) (sql.Result, error)
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Commit() error
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Rollback() error
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}
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// ExecuteInTx runs fn inside tx which should already have begun.
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// *WARNING*: Do not execute any statements on the supplied tx before calling this function.
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// ExecuteInTx will only retry statements that are performed within the supplied
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// closure (fn). Any statements performed on the tx before ExecuteInTx is invoked will *not*
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// be re-run if the transaction needs to be retried.
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//
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// fn is subject to the same restrictions as the fn passed to ExecuteTx.
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func ExecuteInTx(ctx context.Context, tx Tx, fn func() error) (err error) {
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defer func() {
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if err == nil {
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// Ignore commit errors. The tx has already been committed by RELEASE.
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_ = tx.Commit()
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} else {
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// We always need to execute a Rollback() so sql.DB releases the
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// connection.
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_ = tx.Rollback()
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}
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}()
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// Specify that we intend to retry this txn in case of CockroachDB retryable
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// errors.
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if _, err = tx.ExecContext(ctx, "SAVEPOINT cockroach_restart"); err != nil {
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return err
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}
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for {
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released := false
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err = fn()
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if err == nil {
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// RELEASE acts like COMMIT in CockroachDB. We use it since it gives us an
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// opportunity to react to retryable errors, whereas tx.Commit() doesn't.
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released = true
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if _, err = tx.ExecContext(ctx, "RELEASE SAVEPOINT cockroach_restart"); err == nil {
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return nil
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}
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}
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// We got an error; let's see if it's a retryable one and, if so, restart.
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// We look for either:
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// - the standard PG errcode SerializationFailureError:40001 or
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// - the Cockroach extension errcode RetriableError:CR000. This extension
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// has been removed server-side, but support for it has been left here for
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// now to maintain backwards compatibility.
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code := errCode(err)
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if retryable := (code == "CR000" || code == "40001"); !retryable {
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if released {
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err = newAmbiguousCommitError(err)
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}
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return err
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}
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if _, retryErr := tx.ExecContext(ctx, "ROLLBACK TO SAVEPOINT cockroach_restart"); retryErr != nil {
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return newTxnRestartError(retryErr, err)
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}
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}
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}
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func errCode(err error) string {
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switch t := errorCause(err).(type) {
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case *pq.Error:
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return string(t.Code)
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case pgx.PgError:
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return t.Code
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default:
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return ""
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}
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}
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