errors: unwrap joinError to avoid deeply-nested joins

[sethvargo]

This PR is a quick proposal to unwrap joinError (and only joinError)
when calling errors.Join. While I don't think we should unwrap
everything, this is an internal structure that has only internally
semantic meaning. Therefore, I think it makes sense to special-case
joinError and unwrapped the contained errors on joinError.

Pros:

• More closely mirrors how other error joining packages in the ecosystem
  behave.

• Even though Unwrap() []error isn't a public API, this will ensure
  that the method behaves as end-users expect for errors.Join,
  returning the list of joined errors.

Cons:

• Introduces type assertion and could result in a performance
  regression, since we're not pre-computing the slice length. This could
  be addressed if there's interest in pursuing this. I also think there's
  a reasonable argument to be made about the benefits realized from this
  change when printing the error.

• Users could be depending on the broken behavior, but that seems
  unlikely since joinError is an internal API.

I could not find any corresponding issues, but "errors" and "unwrap" are
not particularly unique keywords, so apologies if I missed them.

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Seth Vargo:

Patch Set 2:

(1 comment)

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[sethvargo]

This might be one of those situations where optimizing for the common use case makes sense. Given "most" error joining is of the form merr = errors.Join(merr, err), we can optimize for the single-entry error:

switch len(errs) {
  case 0:
    return nil
  case 1:
    return errs[0]
  case 2:
    if errs[0] == nil {
      return errs[1]
    } else if errs[1] == nil {
      return errs[0]
    } else {
      return &joinError{
        errs: errs, 
      }
    }
  default:
    // as below
}

[gopherbot]

Message from Ian Lance Taylor:

Patch Set 3:

(1 comment)

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Message from Seth Vargo:

Patch Set 3:

(1 comment)

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Message from Damien Neil:

Patch Set 3: Hold+1

(1 comment)

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