union: Implement multiple writable remotes

backend/union/errors.go

@@ -0,0 +1,68 @@
+package union
+
+import (
+	"bytes"
+	"fmt"
+)
+
+// The Errors type wraps a slice of errors
+type Errors []error
+
+// Map returns a copy of the error slice with all its errors modified
+// according to the mapping function. If mapping returns nil,
+// the error is dropped from the error slice with no replacement.
+func (e Errors) Map(mapping func(error) error) Errors {
+	s := make([]error, len(e))
+	i := 0
+	for _, err := range e {
+		nerr := mapping(err)
+		if nerr == nil {
+			continue
+		}
+		s[i] = nerr
+		i++
+	}
+	return Errors(s[:i])
+}
+
+// FilterNil returns the Errors without nil
+func (e Errors) FilterNil() Errors {
+	ne := e.Map(func(err error) error {
+		return err
+	})
+	return ne
+}
+
+// Err returns a error interface that filtered nil,
+// or nil if no non-nil Error is presented.
+func (e Errors) Err() error {
+	ne := e.FilterNil()
+	if len(ne) == 0 {
+		return nil
+	}
+	return ne
+}
+
+// Error returns a concatenated string of the contained errors
+func (e Errors) Error() string {
+	var buf bytes.Buffer
+
+	if len(e) == 0 {
+		buf.WriteString("no error")
+	}
+	if len(e) == 1 {
+		buf.WriteString("1 error: ")
+	} else {
+		fmt.Fprintf(&buf, "%d errors: ", len(e))
+	}
+
+	for i, err := range e {
+		if i != 0 {
+			buf.WriteString("; ")
+		}
+
+		buf.WriteString(err.Error())
+	}
+
+	return buf.String()
+}