Adding a go version compatibility package.

Currently enables Errors.Is functionality as ErrorsIs.
Using this functionality in errors.go and unit tests.

Signed-off-by: Matthias Hanel <mh@synadia.com>

server/errors.go

@@ -16,6 +16,7 @@ package server
 import (
 	"errors"
 	"fmt"
+	"github.com/nats-io/nats-server/v2/server/gobackcomp"
 )
 
 var (
@@ -246,3 +247,7 @@ func UnpackIfErrorCtx(err error) string {
 	}
 	return err.Error()
 }
+
+func IsErr(err, target error) bool {
+	return gobackcomp.ErrorsIs(err, target)
+}

server/errors_test.go

@@ -14,7 +14,6 @@
 package server
 
 import (
-	"errors"
 	"strings"
 	"testing"
 )
@@ -29,7 +28,7 @@ func TestErrCtx(t *testing.T) {
 	if e == ErrWrongGateway {
 		t.Fatalf("%v and %v can't be compared this way", e, ErrWrongGateway)
 	}
-	if !errors.Is(e, ErrWrongGateway) {
+	if !IsErr(e, ErrWrongGateway) {
 		t.Fatalf("%s and %s ", e, ErrWrongGateway)
 	}
 	if UnpackIfErrorCtx(ErrWrongGateway) != ErrWrongGateway.Error() {
@@ -56,7 +55,7 @@ func TestErrCtxWrapped(t *testing.T) {
 	if e == ErrWrongGateway {
 		t.Fatalf("%v and %v can't be compared this way", e, ErrWrongGateway)
 	}
-	if !errors.Is(e, ErrWrongGateway) {
+	if !IsErr(e, ErrWrongGateway) {
 		t.Fatalf("%s and %s ", e, ErrWrongGateway)
 	}
 	if UnpackIfErrorCtx(ErrWrongGateway) != ErrWrongGateway.Error() {

server/gobackcomp/go113.go

@@ -0,0 +1,23 @@
+// Copyright 2020 The NATS 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.
+
+// +build go1.13
+
+package gobackcomp
+
+import "errors"
+
+// implements: go 1.13 errors.Is(err, target error) bool
+func ErrorsIs(err, target error) bool {
+	return errors.Is(err, target)
+}

server/gobackcomp/go113_prev.go

@@ -0,0 +1,53 @@
+// Copyright 2020 The NATS 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.
+
+// +build !go1.13
+
+package gobackcomp
+
+// implements: go 1.13 errors.Unwrap(err error) error
+func errorsUnwrap(err error) error {
+	u, ok := err.(interface {
+		Unwrap() error
+	})
+	if !ok {
+		return nil
+	}
+	return u.Unwrap()
+}
+
+// implements: go 1.13 errors.Is(err, target error) bool
+func ErrorsIs(err, target error) bool {
+	// this is an outright copy of go 1.13 errors.Is(err, target error) bool
+	// removed isComparable
+	if target == nil {
+		return err == target
+	}
+
+	for {
+		if err == target {
+			return true
+		}
+		if x, ok := err.(interface{ Is(error) bool }); ok && x.Is(target) {
+			return true
+		}
+		// TODO: consider supporing target.Is(err). This would allow
+		// user-definable predicates, but also may allow for coping with sloppy
+		// APIs, thereby making it easier to get away with them.
+		if err = errorsUnwrap(err); err == nil {
+			return false
+		}
+	}
+
+	return false
+}

server/parser_test.go

@@ -15,7 +15,6 @@ package server
 
 import (
 	"bytes"
-	"errors"
 	"testing"
 )
 
@@ -583,7 +582,7 @@ func TestMaxControlLine(t *testing.T) {
 
 	pub := []byte("PUB foo.bar 11\r")
 	err := c.parse(pub)
-	if !errors.Is(err, ErrMaxControlLine) {
+	if !IsErr(err, ErrMaxControlLine) {
 		t.Fatalf("Expected an error parsing longer than expected control line")
 	}
 }