Pkg.go.dev is a new destination for Go discovery & docs. Check it out at pkg.go.dev/golang.org/x/sync/errgroup and share your feedback.

package errgroup

import "golang.org/x/sync/errgroup"

Package errgroup provides synchronization, error propagation, and Context cancelation for groups of goroutines working on subtasks of a common task.

errgroup.go

❖ type Group struct { // contains filtered or unexported fields }

A Group is a collection of goroutines working on subtasks that are part of the same overall task.

A zero Group is valid and does not cancel on error.

Example (JustErrors) JustErrors illustrates the use of a Group in place of a sync.WaitGroup to simplify goroutine counting and error handling. This example is derived from the sync.WaitGroup example at https://golang.org/pkg/sync/#example_WaitGroup. Code: g := new(errgroup.Group) var urls = []string{ "http://www.golang.org/", "http://www.google.com/", "http://www.somestupidname.com/", } for _, url := range urls { // Launch a goroutine to fetch the URL. url := url // https://golang.org/doc/faq#closures_and_goroutines g.Go(func() error { // Fetch the URL. resp, err := http.Get(url) if err == nil { resp.Body.Close() } return err }) } // Wait for all HTTP fetches to complete. if err := g.Wait(); err == nil { fmt.Println("Successfully fetched all URLs.") } Example (Parallel) Parallel illustrates the use of a Group for synchronizing a simple parallel task: the "Google Search 2.0" function from https://talks.golang.org/2012/concurrency.slide#46, augmented with a Context and error-handling. Code: Google := func(ctx context.Context, query string) ([]Result, error) { g, ctx := errgroup.WithContext(ctx) searches := []Search{Web, Image, Video} results := make([]Result, len(searches)) for i, search := range searches { i, search := i, search // https://golang.org/doc/faq#closures_and_goroutines g.Go(func() error { result, err := search(ctx, query) if err == nil { results[i] = result } return err }) } if err := g.Wait(); err != nil { return nil, err } return results, nil } results, err := Google(context.Background(), "golang") if err != nil { fmt.Fprintln(os.Stderr, err) return } for _, result := range results { fmt.Println(result) } Output: web result for "golang" image result for "golang" video result for "golang" Example (Pipeline) Pipeline demonstrates the use of a Group to implement a multi-stage pipeline: a version of the MD5All function with bounded parallelism from https://blog.golang.org/pipelines. Code: package main import ( "context" "crypto/md5" "fmt" "io/ioutil" "log" "os" "path/filepath" "golang.org/x/sync/errgroup" ) // Pipeline demonstrates the use of a Group to implement a multi-stage // pipeline: a version of the MD5All function with bounded parallelism from // https://blog.golang.org/pipelines. func main() { m, err := MD5All(context.Background(), ".") if err != nil { log.Fatal(err) } for k, sum := range m { fmt.Printf("%s:\t%x

", k, sum) } } type result struct { path string sum [md5.Size]byte } // MD5All reads all the files in the file tree rooted at root and returns a map // from file path to the MD5 sum of the file's contents. If the directory walk // fails or any read operation fails, MD5All returns an error. func MD5All(ctx context.Context, root string) (map[string][md5.Size]byte, error) { // ctx is canceled when g.Wait() returns. When this version of MD5All returns // - even in case of error! - we know that all of the goroutines have finished // and the memory they were using can be garbage-collected. g, ctx := errgroup.WithContext(ctx) paths := make(chan string) g.Go(func() error { defer close(paths) return filepath.Walk(root, func(path string, info os.FileInfo, err error) error { if err != nil { return err } if !info.Mode().IsRegular() { return nil } select { case paths <- path: case <-ctx.Done(): return ctx.Err() } return nil }) }) // Start a fixed number of goroutines to read and digest files. c := make(chan result) const numDigesters = 20 for i := 0; i < numDigesters; i++ { g.Go(func() error { for path := range paths { data, err := ioutil.ReadFile(path) if err != nil { return err } select { case c <- result{path, md5.Sum(data)}: case <-ctx.Done(): return ctx.Err() } } return nil }) } go func() { g.Wait() close(c) }() m := make(map[string][md5.Size]byte) for r := range c { m[r.path] = r.sum } // Check whether any of the goroutines failed. Since g is accumulating the // errors, we don't need to send them (or check for them) in the individual // results sent on the channel. if err := g.Wait(); err != nil { return nil, err } return m, nil }

WithContext returns a new Group and an associated Context derived from ctx.

The derived Context is canceled the first time a function passed to Go returns a non-nil error or the first time Wait returns, whichever occurs first.

Go calls the given function in a new goroutine.

The first call to return a non-nil error cancels the group; its error will be returned by Wait.

Wait blocks until all function calls from the Go method have returned, then returns the first non-nil error (if any) from them.