xerr/Errorv += Append*(), Err() + tests

- various .Append*() are functions to add errors to error vector.
- .Err() returns error in canonical form accumulated in error vector -
  e.g. nil when error vector is empty.

xerr/xerr.go

@@ -31,3 +31,36 @@ func (errv Errorv) Error() string {
 	}
 	return msg
 }
+
+// append err to error vector
+func (errv *Errorv) Append(err error) {
+	*errv = append(*errv, err)
+}
+
+// append err to error vector, but only if err != nil
+func (errv *Errorv) Appendif(err error) {
+	if err == nil {
+		return
+	}
+	errv.Append(err)
+}
+
+// append formatted error string
+func (errv *Errorv) Appendf(format string, a ...interface{}) {
+	errv.Append(fmt.Errorf(format, a...))
+}
+
+// Err returns error in canonical form accumulated in error vector
+// - nil	if len(errv)==0
+// - errv[0]	if len(errv)==1		// XXX is this good idea?
+// - errv	otherwise
+func (errv Errorv) Err() error {
+	switch len(errv) {
+	case 0:
+		return nil
+	case 1:
+		return errv[0]
+	default:
+		return errv
+	}
+}

xerr/xerr_test.go

+// Copyright (C) 2016  Nexedi SA and Contributors.
+//                     Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+
+package xerr
+
+import (
+	"errors"
+	"reflect"
+	"testing"
+)
+
+func TestErrorv(t *testing.T) {
+	var errv Errorv
+	check := func(aserr error, errmsg string) {
+		err := errv.Err()
+		// cannot use err != aserr as Errorv is not comparable (it is a slice)
+		if !reflect.DeepEqual(err, aserr) {
+			t.Fatalf("%#v: Err() -> %#v  ; want %#v", errv, err, aserr)
+		}
+		msg := errv.Error()
+		if msg != errmsg {
+			t.Fatalf("%#v: Error() -> %q  ; want %q", errv, msg, errmsg)
+		}
+	}
+
+	err1 := errors.New("err1")
+	err2 := errors.New("err2")
+
+	errv.Append(err1)
+	check(err1, "err1")
+
+	errv.Appendif(nil)
+	check(err1, "err1")
+
+	errv.Appendif(err2)
+	check(errv,
+`2 errors:
+	- err1
+	- err2
+`)
+
+	errv.Appendf("err3 %q", "hello world")
+	check(errv,
+`3 errors:
+	- err1
+	- err2
+	- err3 "hello world"
+`)
+}