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errors: unwrap joinError to avoid deeply-nested joins #65360

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errors: unwrap joinError to avoid deeply-nested joins #65360

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35 changes: 22 additions & 13 deletions src/errors/join.go
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Original file line number Diff line number Diff line change
Expand Up @@ -17,24 +17,33 @@ import (
//
// A non-nil error returned by Join implements the Unwrap() []error method.
func Join(errs ...error) error {
n := 0
for _, err := range errs {
if err != nil {
n++
allErrs := make([]error, 0, len(errs))
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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
}

for _, e := range errs {
// Ignore nil errors.
if e == nil {
continue
}

// Specifically handle nested join errors from the standard library. This
// avoids deeply-nesting values which can be unexpected when unwrapping
// errors.
joinErr, ok := e.(*joinError)
if !ok {
allErrs = append(allErrs, e)
continue
}

allErrs = append(allErrs, joinErr.errs...)
}
if n == 0 {

// Ensure we return nil if all contained errors were nil.
if len(allErrs) == 0 {
return nil
}
e := &joinError{
errs: make([]error, 0, n),
}
for _, err := range errs {
if err != nil {
e.errs = append(e.errs, err)
}

return &joinError{
errs: allErrs,
}
return e
}

type joinError struct {
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3 changes: 3 additions & 0 deletions src/errors/join_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -37,6 +37,9 @@ func TestJoin(t *testing.T) {
}, {
errs: []error{err1, nil, err2},
want: []error{err1, err2},
}, {
errs: []error{errors.Join(err1, err2), err1, err2},
want: []error{err1, err2, err1, err2},
}} {
got := errors.Join(test.errs...).(interface{ Unwrap() []error }).Unwrap()
if !reflect.DeepEqual(got, test.want) {
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