A Go language package with an abstraction for errors which represent multiple child errors.
MultiError is an interface which includes error, and adds two additional methods:
Add(error) MultiError
Errors() []errorThink of Add like append, it can add another error into those being wrapped by a MultiError instance.
Errors gives back the list of contained errors. But the MultiError implementation offered by this package also implements As and Is, so for most things you might want to do with the contained errors, reach for errors.Is or errors.As first.
The interface behind errors.Unwrap is also supported, where iteratively progressing through the unwrap chain goes through the unwrap chains of each contained error, proceeding in depth-first order. To support this change to the results of Unwrap there is a thin wrapper type that is produced at each step, but it also passes through Error, Is, and As, and also supports passing through stack traces from popular errors libraries via interfaces expected by a few popular error aggregation services.
Because the most common source of multiple errors arising is concurrent operations which can each produce errors, the multierror package also provides a concurrency group structure which supports starting goroutines and collecting the errors they produce.
The zero value of Group is usable immediately, and its method (*Group) Go(func() error) kicks off goroutines. The errors returned by the given functions will be grouped into a MultiError which is conveniently given back by the method (*Group) Wait() MultiError. The group also manages a sync.WaitGroup so the Wait method will block until all goroutines have completed.
As a convenience, Group also supports a method (*Group) Error() MultiError which produces the complete MultiError again. This is also safe for concurrent use, so you could use this method to pull a MultiError wrapping all the errors so far when only a subset of goroutines have completed.
Finally, by default a Group will capture panics by the goroutines it starts, turn them into errors, and include them in the MultiError. To change this behavior call (*Group) RecoverPanics(bool) and pass it false.