do not query max_allowed_packet by default (#680)

README.md

@@ -232,10 +232,10 @@ Please keep in mind, that param values must be [url.QueryEscape](https://golang.
 ##### `maxAllowedPacket`
 ```
 Type:          decimal number
-Default:       0
+Default:       4194304
 ```
 
-Max packet size allowed in bytes. Use `maxAllowedPacket=0` to automatically fetch the `max_allowed_packet` variable from server.
+Max packet size allowed in bytes. The default value is 4 MiB and should be adjusted to match the server settings. `maxAllowedPacket=0` can be used to automatically fetch the `max_allowed_packet` variable from server *on every connection*.
 
 ##### `multiStatements`
 

const.go

@@ -9,7 +9,8 @@
 package mysql
 
 const (
-	minProtocolVersion byte = 10
+	defaultMaxAllowedPacket = 4 << 20 // 4 MiB
+	minProtocolVersion      = 10
 	maxPacketSize           = 1<<24 - 1
 	timeFormat              = "2006-01-02 15:04:05.999999"
 )

driver_test.go

@@ -964,7 +964,7 @@ func TestUint64(t *testing.T) {
 }
 
 func TestLongData(t *testing.T) {
-	runTests(t, dsn, func(dbt *DBTest) {
+	runTests(t, dsn+"&maxAllowedPacket=0", func(dbt *DBTest) {
 		var maxAllowedPacketSize int
 		err := dbt.db.QueryRow("select @@max_allowed_packet").Scan(&maxAllowedPacketSize)
 		if err != nil {

dsn.go

@@ -64,6 +64,7 @@ func NewConfig() *Config {
 	return &Config{
 		Collation:            defaultCollation,
 		Loc:                  time.UTC,
+		MaxAllowedPacket:     defaultMaxAllowedPacket,
 		AllowNativePasswords: true,
 	}
 }
@@ -274,7 +275,7 @@ func (cfg *Config) FormatDSN() string {
 		buf.WriteString(cfg.WriteTimeout.String())
 	}
 
-	if cfg.MaxAllowedPacket > 0 {
+	if cfg.MaxAllowedPacket != defaultMaxAllowedPacket {
 		if hasParam {
 			buf.WriteString("&maxAllowedPacket=")
 		} else {

dsn_test.go

@@ -22,55 +22,55 @@ var testDSNs = []struct {
 	out *Config
 }{{
 	"username:password@protocol(address)/dbname?param=value",
-	&Config{User: "username", Passwd: "password", Net: "protocol", Addr: "address", DBName: "dbname", Params: map[string]string{"param": "value"}, Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true},
+	&Config{User: "username", Passwd: "password", Net: "protocol", Addr: "address", DBName: "dbname", Params: map[string]string{"param": "value"}, Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true},
 }, {
 	"username:password@protocol(address)/dbname?param=value&columnsWithAlias=true",
-	&Config{User: "username", Passwd: "password", Net: "protocol", Addr: "address", DBName: "dbname", Params: map[string]string{"param": "value"}, Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true, ColumnsWithAlias: true},
+	&Config{User: "username", Passwd: "password", Net: "protocol", Addr: "address", DBName: "dbname", Params: map[string]string{"param": "value"}, Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true, ColumnsWithAlias: true},
 }, {
 	"username:password@protocol(address)/dbname?param=value&columnsWithAlias=true&multiStatements=true",
-	&Config{User: "username", Passwd: "password", Net: "protocol", Addr: "address", DBName: "dbname", Params: map[string]string{"param": "value"}, Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true, ColumnsWithAlias: true, MultiStatements: true},
+	&Config{User: "username", Passwd: "password", Net: "protocol", Addr: "address", DBName: "dbname", Params: map[string]string{"param": "value"}, Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true, ColumnsWithAlias: true, MultiStatements: true},
 }, {
 	"user@unix(/path/to/socket)/dbname?charset=utf8",
-	&Config{User: "user", Net: "unix", Addr: "/path/to/socket", DBName: "dbname", Params: map[string]string{"charset": "utf8"}, Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true},
+	&Config{User: "user", Net: "unix", Addr: "/path/to/socket", DBName: "dbname", Params: map[string]string{"charset": "utf8"}, Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true},
 }, {
 	"user:password@tcp(localhost:5555)/dbname?charset=utf8&tls=true",
-	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "localhost:5555", DBName: "dbname", Params: map[string]string{"charset": "utf8"}, Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true, TLSConfig: "true"},
+	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "localhost:5555", DBName: "dbname", Params: map[string]string{"charset": "utf8"}, Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true, TLSConfig: "true"},
 }, {
 	"user:password@tcp(localhost:5555)/dbname?charset=utf8mb4,utf8&tls=skip-verify",
-	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "localhost:5555", DBName: "dbname", Params: map[string]string{"charset": "utf8mb4,utf8"}, Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true, TLSConfig: "skip-verify"},
+	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "localhost:5555", DBName: "dbname", Params: map[string]string{"charset": "utf8mb4,utf8"}, Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true, TLSConfig: "skip-verify"},
 }, {
 	"user:password@/dbname?loc=UTC&timeout=30s&readTimeout=1s&writeTimeout=1s&allowAllFiles=1&clientFoundRows=true&allowOldPasswords=TRUE&collation=utf8mb4_unicode_ci&maxAllowedPacket=16777216",
 	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "127.0.0.1:3306", DBName: "dbname", Collation: "utf8mb4_unicode_ci", Loc: time.UTC, AllowNativePasswords: true, Timeout: 30 * time.Second, ReadTimeout: time.Second, WriteTimeout: time.Second, AllowAllFiles: true, AllowOldPasswords: true, ClientFoundRows: true, MaxAllowedPacket: 16777216},
 }, {
-	"user:password@/dbname?allowNativePasswords=false",
-	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "127.0.0.1:3306", DBName: "dbname", Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: false},
+	"user:password@/dbname?allowNativePasswords=false&maxAllowedPacket=0",
+	&Config{User: "user", Passwd: "password", Net: "tcp", Addr: "127.0.0.1:3306", DBName: "dbname", Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: 0, AllowNativePasswords: false},
 }, {
 	"user:p@ss(word)@tcp([de:ad:be:ef::ca:fe]:80)/dbname?loc=Local",
-	&Config{User: "user", Passwd: "p@ss(word)", Net: "tcp", Addr: "[de:ad:be:ef::ca:fe]:80", DBName: "dbname", Collation: "utf8_general_ci", Loc: time.Local, AllowNativePasswords: true},
+	&Config{User: "user", Passwd: "p@ss(word)", Net: "tcp", Addr: "[de:ad:be:ef::ca:fe]:80", DBName: "dbname", Collation: "utf8_general_ci", Loc: time.Local, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true},
 }, {
 	"/dbname",
-	&Config{Net: "tcp", Addr: "127.0.0.1:3306", DBName: "dbname", Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true},
+	&Config{Net: "tcp", Addr: "127.0.0.1:3306", DBName: "dbname", Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true},
 }, {
 	"@/",
-	&Config{Net: "tcp", Addr: "127.0.0.1:3306", Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true},
+	&Config{Net: "tcp", Addr: "127.0.0.1:3306", Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true},
 }, {
 	"/",
-	&Config{Net: "tcp", Addr: "127.0.0.1:3306", Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true},
+	&Config{Net: "tcp", Addr: "127.0.0.1:3306", Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true},
 }, {
 	"",
-	&Config{Net: "tcp", Addr: "127.0.0.1:3306", Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true},
+	&Config{Net: "tcp", Addr: "127.0.0.1:3306", Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true},
 }, {
 	"user:p@/ssword@/",
-	&Config{User: "user", Passwd: "p@/ssword", Net: "tcp", Addr: "127.0.0.1:3306", Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true},
+	&Config{User: "user", Passwd: "p@/ssword", Net: "tcp", Addr: "127.0.0.1:3306", Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true},
 }, {
 	"unix/?arg=%2Fsome%2Fpath.ext",
-	&Config{Net: "unix", Addr: "/tmp/mysql.sock", Params: map[string]string{"arg": "/some/path.ext"}, Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true},
+	&Config{Net: "unix", Addr: "/tmp/mysql.sock", Params: map[string]string{"arg": "/some/path.ext"}, Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true},
 }, {
 	"tcp(127.0.0.1)/dbname",
-	&Config{Net: "tcp", Addr: "127.0.0.1:3306", DBName: "dbname", Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true},
+	&Config{Net: "tcp", Addr: "127.0.0.1:3306", DBName: "dbname", Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true},
 }, {
 	"tcp(de:ad:be:ef::ca:fe)/dbname",
-	&Config{Net: "tcp", Addr: "[de:ad:be:ef::ca:fe]:3306", DBName: "dbname", Collation: "utf8_general_ci", Loc: time.UTC, AllowNativePasswords: true},
+	&Config{Net: "tcp", Addr: "[de:ad:be:ef::ca:fe]:3306", DBName: "dbname", Collation: "utf8_general_ci", Loc: time.UTC, MaxAllowedPacket: defaultMaxAllowedPacket, AllowNativePasswords: true},
 },
 }
 
@@ -233,16 +233,14 @@ func TestDSNUnsafeCollation(t *testing.T) {
 
 func TestParamsAreSorted(t *testing.T) {
 	expected := "/dbname?interpolateParams=true&foobar=baz&quux=loo"
-	dsn := &Config{
-		DBName:               "dbname",
-		InterpolateParams:    true,
-		AllowNativePasswords: true,
-		Params: map[string]string{
-			"quux":   "loo",
-			"foobar": "baz",
-		},
+	cfg := NewConfig()
+	cfg.DBName = "dbname"
+	cfg.InterpolateParams = true
+	cfg.Params = map[string]string{
+		"quux":   "loo",
+		"foobar": "baz",
 	}
-	actual := dsn.FormatDSN()
+	actual := cfg.FormatDSN()
 	if actual != expected {
 		t.Errorf("generic Config.Params were not sorted: want %#v, got %#v", expected, actual)
 	}