support Is comparison on MySQLError (go-sql-driver#1210)

* support Is comparison on MySQLError

* add myself to authors

* skip error tests for go 1.12

* remove test build tag

Author: chriskirkland

AUTHORS

@@ -23,6 +23,7 @@ Asta Xie <xiemengjun at gmail.com>
 Bulat Gaifullin <gaifullinbf at gmail.com>
 Caine Jette <jette at alum.mit.edu>
 Carlos Nieto <jose.carlos at menteslibres.net>
+Chris Kirkland <chriskirkland at github.com>
 Chris Moos <chris at tech9computers.com>
 Craig Wilson <craiggwilson at gmail.com>
 Daniel Montoya <dsmontoyam at gmail.com>

errors.go

@@ -63,3 +63,10 @@ type MySQLError struct {
 func (me *MySQLError) Error() string {
 	return fmt.Sprintf("Error %d: %s", me.Number, me.Message)
 }
+
+func (me *MySQLError) Is(err error) bool {
+	if merr, ok := err.(*MySQLError); ok {
+		return merr.Number == me.Number
+	}
+	return false
+}

errors_test.go

@@ -10,6 +10,7 @@ package mysql
 
 import (
 	"bytes"
+	"errors"
 	"log"
 	"testing"
 )
@@ -40,3 +41,21 @@ func TestErrorsStrictIgnoreNotes(t *testing.T) {
 		dbt.mustExec("DROP TABLE IF EXISTS does_not_exist")
 	})
 }
+
+func TestMySQLErrIs(t *testing.T) {
+	infraErr := &MySQLError{1234, "the server is on fire"}
+	otherInfraErr := &MySQLError{1234, "the datacenter is flooded"}
+	if !errors.Is(infraErr, otherInfraErr) {
+		t.Errorf("expected errors to be the same: %+v %+v", infraErr, otherInfraErr)
+	}
+
+	differentCodeErr := &MySQLError{5678, "the server is on fire"}
+	if errors.Is(infraErr, differentCodeErr) {
+		t.Fatalf("expected errors to be different: %+v %+v", infraErr, differentCodeErr)
+	}
+
+	nonMysqlErr := errors.New("not a mysql error")
+	if errors.Is(infraErr, nonMysqlErr) {
+		t.Fatalf("expected errors to be different: %+v %+v", infraErr, nonMysqlErr)
+	}
+}