Merge pull request #35 from hashicorp/f-go113-errors

Support Go 1.13 errors.As/Is/Unwrap functionality

multierror.go

@@ -1,6 +1,7 @@
 package multierror
 
 import (
+	"errors"
 	"fmt"
 )
 
@@ -49,3 +50,69 @@ func (e *Error) GoString() string {
 func (e *Error) WrappedErrors() []error {
 	return e.Errors
 }
+
+// Unwrap returns an error from Error (or nil if there are no errors).
+// This error returned will further support Unwrap to get the next error,
+// etc. The order will match the order of Errors in the multierror.Error
+// at the time of calling.
+//
+// The resulting error supports errors.As/Is/Unwrap so you can continue
+// to use the stdlib errors package to introspect further.
+//
+// This will perform a shallow copy of the errors slice. Any errors appended
+// to this error after calling Unwrap will not be available until a new
+// Unwrap is called on the multierror.Error.
+func (e *Error) Unwrap() error {
+	// If we have no errors then we do nothing
+	if e == nil || len(e.Errors) == 0 {
+		return nil
+	}
+
+	// If we have exactly one error, we can just return that directly.
+	if len(e.Errors) == 1 {
+		return e.Errors[0]
+	}
+
+	// Shallow copy the slice
+	errs := make([]error, len(e.Errors))
+	copy(errs, e.Errors)
+	return chain(errs)
+}
+
+// chain implements the interfaces necessary for errors.Is/As/Unwrap to
+// work in a deterministic way with multierror. A chain tracks a list of
+// errors while accounting for the current represented error. This lets
+// Is/As be meaningful.
+//
+// Unwrap returns the next error. In the cleanest form, Unwrap would return
+// the wrapped error here but we can't do that if we want to properly
+// get access to all the errors. Instead, users are recommended to use
+// Is/As to get the correct error type out.
+//
+// Precondition: []error is non-empty (len > 0)
+type chain []error
+
+// Error implements the error interface
+func (e chain) Error() string {
+	return e[0].Error()
+}
+
+// Unwrap implements errors.Unwrap by returning the next error in the
+// chain or nil if there are no more errors.
+func (e chain) Unwrap() error {
+	if len(e) == 1 {
+		return nil
+	}
+
+	return e[1:]
+}
+
+// As implements errors.As by attempting to map to the current value.
+func (e chain) As(target interface{}) bool {
+	return errors.As(e[0], target)
+}
+
+// Is implements errors.Is by comparing the current value directly.
+func (e chain) Is(target error) bool {
+	return errors.Is(e[0], target)
+}

multierror_test.go

@@ -2,6 +2,7 @@ package multierror
 
 import (
 	"errors"
+	"fmt"
 	"reflect"
 	"testing"
 )
@@ -69,3 +70,134 @@ func TestErrorWrappedErrors(t *testing.T) {
 		t.Fatalf("bad: %s", multi.WrappedErrors())
 	}
 }
+
+func TestErrorUnwrap(t *testing.T) {
+	t.Run("with errors", func(t *testing.T) {
+		err := &Error{Errors: []error{
+			errors.New("foo"),
+			errors.New("bar"),
+			errors.New("baz"),
+		}}
+
+		var current error = err
+		for i := 0; i < len(err.Errors); i++ {
+			current = errors.Unwrap(current)
+			if !errors.Is(current, err.Errors[i]) {
+				t.Fatal("should be next value")
+			}
+		}
+
+		if errors.Unwrap(current) != nil {
+			t.Fatal("should be nil at the end")
+		}
+	})
+
+	t.Run("with no errors", func(t *testing.T) {
+		err := &Error{Errors: nil}
+		if errors.Unwrap(err) != nil {
+			t.Fatal("should be nil")
+		}
+	})
+
+	t.Run("with nil multierror", func(t *testing.T) {
+		var err *Error
+		if errors.Unwrap(err) != nil {
+			t.Fatal("should be nil")
+		}
+	})
+}
+
+func TestErrorIs(t *testing.T) {
+	errBar := errors.New("bar")
+
+	t.Run("with errBar", func(t *testing.T) {
+		err := &Error{Errors: []error{
+			errors.New("foo"),
+			errBar,
+			errors.New("baz"),
+		}}
+
+		if !errors.Is(err, errBar) {
+			t.Fatal("should be true")
+		}
+	})
+
+	t.Run("with errBar wrapped by fmt.Errorf", func(t *testing.T) {
+		err := &Error{Errors: []error{
+			errors.New("foo"),
+			fmt.Errorf("errorf: %w", errBar),
+			errors.New("baz"),
+		}}
+
+		if !errors.Is(err, errBar) {
+			t.Fatal("should be true")
+		}
+	})
+
+	t.Run("without errBar", func(t *testing.T) {
+		err := &Error{Errors: []error{
+			errors.New("foo"),
+			errors.New("baz"),
+		}}
+
+		if errors.Is(err, errBar) {
+			t.Fatal("should be false")
+		}
+	})
+}
+
+func TestErrorAs(t *testing.T) {
+	match := &nestedError{}
+
+	t.Run("with the value", func(t *testing.T) {
+		err := &Error{Errors: []error{
+			errors.New("foo"),
+			match,
+			errors.New("baz"),
+		}}
+
+		var target *nestedError
+		if !errors.As(err, &target) {
+			t.Fatal("should be true")
+		}
+		if target == nil {
+			t.Fatal("target should not be nil")
+		}
+	})
+
+	t.Run("with the value wrapped by fmt.Errorf", func(t *testing.T) {
+		err := &Error{Errors: []error{
+			errors.New("foo"),
+			fmt.Errorf("errorf: %w", match),
+			errors.New("baz"),
+		}}
+
+		var target *nestedError
+		if !errors.As(err, &target) {
+			t.Fatal("should be true")
+		}
+		if target == nil {
+			t.Fatal("target should not be nil")
+		}
+	})
+
+	t.Run("without the value", func(t *testing.T) {
+		err := &Error{Errors: []error{
+			errors.New("foo"),
+			errors.New("baz"),
+		}}
+
+		var target *nestedError
+		if errors.As(err, &target) {
+			t.Fatal("should be false")
+		}
+		if target != nil {
+			t.Fatal("target should be nil")
+		}
+	})
+}
+
+// nestedError implements error and is used for tests.
+type nestedError struct{}
+
+func (*nestedError) Error() string { return "" }