Add collapsible error type

Signed-off-by: Derek McGowan <derek@mcg.dev>

internal/types/collapsible.go

@@ -0,0 +1,57 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package types
+
+import "fmt"
+
+// CollapsibleError indicates the error should be collapsed
+type CollapsibleError interface {
+	CollapseError()
+}
+
+// CollapsedError returns a new error with the collapsed
+// error returned on unwrapped or when formatted with "%+v"
+func CollapsedError(err error, collapsed ...error) error {
+	return collapsedError{err, collapsed}
+}
+
+type collapsedError struct {
+	error
+	collapsed []error
+}
+
+func (c collapsedError) Unwrap() []error {
+	return append([]error{c.error}, c.collapsed...)
+}
+
+func (c collapsedError) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		if s.Flag('+') {
+			fmt.Fprintf(s, "%+v", c.error)
+			for _, err := range c.collapsed {
+				fmt.Fprintf(s, "\n%+v", err)
+			}
+			return
+		}
+		fallthrough
+	case 's':
+		fmt.Fprint(s, c.Error())
+	case 'q':
+		fmt.Fprintf(s, "%q", c.Error())
+	}
+}