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cancel_checker.go
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/
cancel_checker.go
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// Copyright 2017 The Cockroach Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
// implied. See the License for the specific language governing
// permissions and limitations under the License.
package sqlbase
import (
"golang.org/x/net/context"
)
// Interval of Check() calls to wait between checks for context cancellation.
const cancelCheckInterval int64 = 1000
// CancelChecker is a helper object for repeatedly checking whether the associated context
// has been cancelled or not. Encapsulates all logic for waiting for cancelCheckInterval
// rows before actually checking for cancellation. The cancellation check
// has a significant time overhead, so it's not checked in every iteration.
type CancelChecker struct {
// Reference to associated context to check.
ctx context.Context
// Number of times Check() has been called since last context cancellation check.
callsSinceLastCheck int64
// Last returned cancellation value.
isCancelled bool
}
// NewCancelChecker returns a new CancelChecker.
func NewCancelChecker(ctx context.Context) *CancelChecker {
return &CancelChecker{
ctx: ctx,
}
}
// Check returns an error if the associated query has been cancelled.
func (c *CancelChecker) Check() error {
if !c.isCancelled && c.callsSinceLastCheck%cancelCheckInterval == 0 {
select {
case <-c.ctx.Done():
c.isCancelled = true
default:
c.isCancelled = false
}
}
c.callsSinceLastCheck++
if c.isCancelled {
return NewQueryCanceledError()
}
return nil
}