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// Copyright 2015 Google Inc. All Rights Reserved.
//
// 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 syncutil
import (
"sync"
"golang.org/x/net/context"
)
// A collection of concurrently-executing operations, each of which may fail.
//
// Operations are defined by functions that accept a context and return an
// error. If any operation returns a non-nil error, all concurrent and future
// operations will have their contexts cancelled . When Join() is called on a
// bundle with one or more operations that returned an error, it always returns
// the first error (i.e. that which led to the cancellation of others).
//
// Bundles can be used to set up pipelines of concurrent actors sending data to
// each other, conveniently cancelling the pipeline if anything fails. A
// typical use looks like the following:
//
// // Run a pipeline that consists of one goroutine listing object names,
// // while N goroutines concurrently delete the listed objects one by one.
// // If any listing or deletion operation fails, cancel the whole pipeline
// // and return the error.
// func deleteAllObjects(ctx context.Context, N int) error {
// bundle := syncutil.NewBundle(ctx)
//
// // List objects into a channel. Assuming that listObjects responds to
// // cancellation of its context, it will not get stuck blocking forever
// // on a write into objectNames if the deleters return early in error
// // before draining the channel.
// objectNames := make(chan string)
// bundle.Add(func(ctx context.Context) error {
// defer close(objectNames)
// return listObjects(ctx, objectNames)
// })
//
// // Run N deletion workers.
// for i := 0; i < N; i++ {
// bundle.Add(func(ctx context.Context) error {
// for name := range objectNames {
// if err := deleteObject(ctx, name); err != nil {
// return err
// }
// }
// })
// }
//
// // Wait for the whole pipeline to finish, and return its status.
// return bundle.Join()
// }
//
// Bundles must be created using NewBundle. Bundle methods must not be called
// concurrently.
type Bundle struct {
context context.Context
cancel context.CancelFunc
waitGroup sync.WaitGroup
errorOnce sync.Once
firstError error
}
// Add a new operation to the bundle. The operation will be invoked with a
// context that will be cancelled if any other operation fails or has already
// failed.
func (b *Bundle) Add(f func(context.Context) error) {
b.waitGroup.Add(1)
// Run the function in the background.
go func() {
defer b.waitGroup.Done()
err := f(b.context)
if err == nil {
return
}
// On first error, cancel the context and save the error.
b.errorOnce.Do(func() {
b.firstError = err
b.cancel()
})
}()
}
// Wait for all previously-added operations to complete. Return nil if all
// operations succeeded. Otherwise return the first error.
//
// Add must not be called concurrently with or after Join.
func (b *Bundle) Join() error {
b.waitGroup.Wait()
// context.WithCancel requires that we arrange for this to be called
// eventually in order to avoid leaking resources. Since everything is done,
// to do so now is harmless.
b.cancel()
return b.firstError
}
// Create a bundle whose operations are fed a context inheriting from the given
// parent context, which must be non-nil. The bundle must eventually be joined
// with Join.
func NewBundle(parent context.Context) *Bundle {
b := &Bundle{}
b.context, b.cancel = context.WithCancel(parent)
return b
}
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