commit 8deb4d8f8484161d95e127ebafbf8ad88f90f736
author Prashant Varanasi <github@prashantv.com> Mon Apr 10 15:02:29 2017 -0700
committer GitHub <noreply@github.com> Mon Apr 10 15:02:29 2017 -0700
tree f96f2d5d78b4a5b75f5b153f5d1b29692064462c
parent 737b41aa3bf31c25d11dc84d5275af6bfe2ef4d2

Avoid allocating a new slice on every Append (#9)

Currently, appending a non-nil error to a multierror always copies the underlying slice, since there may be multiple goroutines appending to the same multierr,
```go
merr := multierr.Append(errors.New("initial"), errors.New("multierr"))
go func() {
  err1 = multierr.Append(merr, errors.New("1"))
  [..]
}()
go func() {
  err2 = multierr.Append(merr, errors.New("2"))
  [..]
}()
```

However, the common use case is a standard for loop:
```go
var merr error
for _, v := range values {
  merr = multierr.Append(merr, processValue(v))
}
return merr
```

Since there is only a single resulting slice, we don't need to create a full copy on every `Append`. Instead, we track whether we have modified the underlying slice from `Append` already using an atomic boolean, and only create a copy if the underlying slice has been modified.