feat: add support for Go 1.13's errors.Is/As functions (#461)

ldap.Error wraps an underlying error, but doesn't interoperate
with errors.Is or errors.As since it doesn't expose an Unwrap method.
This change makes it easier to detect things like an LDAP error
caused due to a network timeout.

Co-authored-by: Christopher Puschmann <cp@lumen.sh>

v3/error.go

@@ -192,6 +192,8 @@ func (e *Error) Error() string {
 	return fmt.Sprintf("LDAP Result Code %d %q: %s", e.ResultCode, LDAPResultCodeMap[e.ResultCode], e.Err.Error())
 }
 
+func (e *Error) Unwrap() error { return e.Err }
+
 // GetLDAPError creates an Error out of a BER packet representing a LDAPResult
 // The return is an error object. It can be casted to a Error structure.
 // This function returns nil if resultCode in the LDAPResult sequence is success(0).

v3/error_test.go

@@ -2,6 +2,7 @@ package ldap
 
 import (
 	"errors"
+	"io"
 	"net"
 	"strings"
 	"testing"
@@ -103,6 +104,24 @@ func TestGetLDAPErrorInvalidResponse(t *testing.T) {
 	}
 }
 
+func TestErrorIs(t *testing.T) {
+	err := NewError(ErrorNetwork, io.EOF)
+	if !errors.Is(err, io.EOF) {
+		t.Errorf("Expected an io.EOF error: %v", err)
+	}
+}
+
+func TestErrorAs(t *testing.T) {
+	var netErr net.InvalidAddrError = "invalid addr"
+	err := NewError(ErrorNetwork, netErr)
+
+	var target net.InvalidAddrError
+	ok := errors.As(err, &target)
+	if !ok {
+		t.Error("Expected an InvalidAddrError")
+	}
+}
+
 // TestGetLDAPErrorSuccess tests parsing of a result with no error (resultCode == 0).
 func TestGetLDAPErrorSuccess(t *testing.T) {
 	bindResponse := ber.Encode(ber.ClassApplication, ber.TypeConstructed, ApplicationBindResponse, nil, "Bind Response")