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// Copyright 2013 Canonical Ltd.
// Licensed under the LGPLv3, see LICENCE file for details.
// The parallel package provides utilities for running tasks
// concurrently.
package parallel
import (
// Run represents a number of functions running concurrently.
type Run struct {
mu sync.Mutex
results chan Errors
max int
running int
work chan func() error
// Errors holds any errors encountered during the parallel run.
type Errors []error
func (errs Errors) Error() string {
switch len(errs) {
case 0:
return "no error"
case 1:
return errs[0].Error()
return fmt.Sprintf("%s (and %d more)", errs[0].Error(), len(errs)-1)
// NewRun returns a new parallel instance. It provides a way of running
// functions concurrently while limiting the maximum number running at
// once to max.
func NewRun(max int) *Run {
if max < 1 {
panic("parameter max must be >= 1")
return &Run{
max: max,
results: make(chan Errors),
work: make(chan func() error),
// Do requests that r run f concurrently. If there are already the maximum
// number of functions running concurrently, it will block until one of them
// has completed. Do may itself be called concurrently, but may not be called
// concurrently with Wait.
func (r *Run) Do(f func() error) {
select {
case <- f:
if r.running < r.max {
go r.runner()
} <- f
// Wait marks the parallel instance as complete and waits for all the functions
// to complete. If any errors were encountered, it returns an Errors value
// describing all the errors in arbitrary order.
func (r *Run) Wait() error {
var errs Errors
for i := 0; i < r.running; i++ {
errs = append(errs, <-r.results...)
if len(errs) == 0 {
return nil
// TODO(rog) sort errors by original order of Do request?
return errs
func (r *Run) runner() {
var errs Errors
for f := range {
if err := f(); err != nil {
errs = append(errs, err)
r.results <- errs
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