util: Add errwrap package

Simplify working with errors across our code base. Instead of constantly
importing the necessary error helpers, assemble them all into one
package and import and use that as needed.

test/test-govet.sh

@@ -50,6 +50,10 @@ function naked-error() {
 }
 
 function consistent-imports() {
+	if [ "$1" = './util/errwrap/errwrap.go' ]; then
+		return 0
+	fi
+
 	if grep $'\t"github.com/pkg/errors"' "$1"; then	# import as errwrap
 		return 1
 	fi

util/errwrap/errwrap.go

@@ -0,0 +1,46 @@
+// Mgmt
+// Copyright (C) 2013-2018+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+// Package errwrap contains some error helpers.
+package errwrap
+
+import (
+	"github.com/hashicorp/go-multierror"
+	"github.com/pkg/errors"
+)
+
+// Wrapf adds a new error onto an existing chain of errors. If the new error to
+// be added is nil, then the old error is returned unchanged.
+func Wrapf(err error, format string, args ...interface{}) error {
+	return errors.Wrapf(err, format, args...)
+}
+
+// Append can be used to safely append an error onto an existing one. If you
+// pass in a nil error to append, the existing error will be returned unchanged.
+// If the existing error is already nil, then the new error will be returned
+// unchanged. This makes it easy to use Append as a safe `reterr += err`, when
+// you don't know if either is nil or not.
+func Append(reterr, err error) error {
+	if reterr == nil { // keep it simple, pass it through
+		return err // which might even be nil
+	}
+	if err == nil { // no error, so don't do anything
+		return reterr
+	}
+	// both are real errors
+	return multierror.Append(reterr, err)
+}

util/errwrap/errwrap_test.go

@@ -0,0 +1,51 @@
+// Mgmt
+// Copyright (C) 2013-2018+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+// +build !root
+
+package errwrap
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestWrapfErr1(t *testing.T) {
+	if err := Wrapf(nil, "whatever: %d", 42); err != nil {
+		t.Errorf("expected nil result")
+	}
+}
+
+func TestAppendErr1(t *testing.T) {
+	if err := Append(nil, nil); err != nil {
+		t.Errorf("expected nil result")
+	}
+}
+
+func TestAppendErr2(t *testing.T) {
+	reterr := fmt.Errorf("reterr")
+	if err := Append(reterr, nil); err != reterr {
+		t.Errorf("expected reterr")
+	}
+}
+
+func TestAppendErr3(t *testing.T) {
+	err := fmt.Errorf("err")
+	if reterr := Append(nil, err); reterr != err {
+		t.Errorf("expected err")
+	}
+}