errors: Using errors.Join on a joinError should expand the existing joined error

[ItalyPaleAle]

What version of Go are you using (go version)?

$ go version
go version go1.20.4 linux/amd64

Does this issue reproduce with the latest release?

Yes

Description

Consider this code (Playground)

var err error
err = errors.Join(err, errors.New("err1"))
err = errors.Join(err, errors.New("err2"))
err = errors.Join(err, nil)

errs := err.(interface{ Unwrap() []error }).Unwrap()
fmt.Println(len(errs), errs)

// Output:
// 1 [err1
// err2]

This happens because when calling errors.Join, it does not check if one of the errors is a joinError already, so it creates a new one every time.

1. After err = errors.Join(err, errors.New("err1")), err is a joinError with 1 item inside (and that's expected)
2. After err = errors.Join(err, errors.New("err2")), err is a joinError in which item 1 is a joinError itself, and item 2 is the error.
3. After err = errors.Join(err, nil), err is a joinError that contains 2 joinError's (and the first one contains a joinError itself)

We found this bug while trying to implement a pattern where we collect all errors from a channel. We have N goroutines running in parallel, and we wanted to use this pattern to both collect errors (which could be nil) and wait for all goroutines to complete:

errCh := make(chan error, 3)
errCh<-errors.New("err1")
errCh<-errors.New("err2")
errCh<-nil

var err error
for i := 0; i < 3; i++ {
	err = errors.Join(err, <-errCh)
}

In the case above, calling Unwrap() []error on the result lead to unexpected results because of errors being wrapped multiple times in joinError objects

[seankhliao]

We explicitly decided against this in #53435 (comment)

[ItalyPaleAle]

@seankhliao thanks for the link, although I'm a bit bummed the code above (which looks cleaner than the alternative of collecting all errors in a slice first) can't be used :(

Would it be possible to have this included in the docs so it's clear the example above is not the intended use?

[ianlancetaylor]

@ItalyPaleAle I'm not clear on how the docs should be changed. Do you want to send a patch or at least outline the doc change here? Thanks.

[ItalyPaleAle]

@ianlancetaylor I have opened #60215. It's my first contribution to Go itself so I hope I did it right :)

[gopherbot]

Change https://go.dev/cl/495076 mentions this issue: errors: clarify that Join doesn't extend already-joined errors