Errors from goroutines in Go

A goroutine cannot return, so the error travels on a channel.

var ErrFetch = errors.New("fetch failed")

type result struct {
    url string
    err error
}

func fetchAll(urls []string) error {
    results := make(chan result, len(urls))
    var wg sync.WaitGroup

    for _, url := range urls {
        wg.Go(func() {
            var err error
            if strings.HasPrefix(url, "bad") {
                err = fmt.Errorf("fetch %s: %w", url, ErrFetch)
            }
            results <- result{url: url, err: err}
        })
    }
    wg.Wait()
    close(results)

    var problems error
    for r := range results {
        problems = errors.Join(problems, r.err)
    }
    return problems
}

How it works

1. Every worker sends its result, error included.
2. The collector decides whether one failure stops everything.
3. Never just log and drop it inside the goroutine.

Keywords and builtins used here

• chan
• close
• error
• for
• func
• if
• len
• make
• range
• return
• string
• struct
• type
• var

The run, in numbers

Lines

29

Characters to type

501

Tokens

136

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 74 seconds.

Type this snippet

Step 1 of 1 in Errors across goroutines, step 14 of 15 in Errors.

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