remove unnecessary logs (go-sql-driver#1599)

Logging ErrInvalidConn when the connection already closed doesn't
provide any help to users.

Additonally, database/sql now uses Validator() to check connection
liveness before calling query methods.
So stop using `mc.log(ErrInvalidConn)` idiom.

This PR includes some cleanup and documentation relating to
`mc.markBadConn()`.

connection.go

@@ -111,14 +111,13 @@ func (mc *mysqlConn) handleParams() (err error) {
 	return
 }
 
+// markBadConn replaces errBadConnNoWrite with driver.ErrBadConn.
+// This function is used to return driver.ErrBadConn only when safe to retry.
 func (mc *mysqlConn) markBadConn(err error) error {
-	if mc == nil {
-		return err
-	}
-	if err != errBadConnNoWrite {
-		return err
+	if err == errBadConnNoWrite {
+		return driver.ErrBadConn
 	}
-	return driver.ErrBadConn
+	return err
 }
 
 func (mc *mysqlConn) Begin() (driver.Tx, error) {
@@ -127,7 +126,6 @@ func (mc *mysqlConn) Begin() (driver.Tx, error) {
 
 func (mc *mysqlConn) begin(readOnly bool) (driver.Tx, error) {
 	if mc.closed.Load() {
-		mc.log(ErrInvalidConn)
 		return nil, driver.ErrBadConn
 	}
 	var q string
@@ -189,7 +187,6 @@ func (mc *mysqlConn) error() error {
 
 func (mc *mysqlConn) Prepare(query string) (driver.Stmt, error) {
 	if mc.closed.Load() {
-		mc.log(ErrInvalidConn)
 		return nil, driver.ErrBadConn
 	}
 	// Send command
@@ -324,7 +321,6 @@ func (mc *mysqlConn) interpolateParams(query string, args []driver.Value) (strin
 
 func (mc *mysqlConn) Exec(query string, args []driver.Value) (driver.Result, error) {
 	if mc.closed.Load() {
-		mc.log(ErrInvalidConn)
 		return nil, driver.ErrBadConn
 	}
 	if len(args) != 0 {
@@ -384,7 +380,6 @@ func (mc *mysqlConn) query(query string, args []driver.Value) (*textRows, error)
 	handleOk := mc.clearResult()
 
 	if mc.closed.Load() {
-		mc.log(ErrInvalidConn)
 		return nil, driver.ErrBadConn
 	}
 	if len(args) != 0 {
@@ -408,7 +403,7 @@ func (mc *mysqlConn) query(query string, args []driver.Value) (*textRows, error)
 	var resLen int
 	resLen, err = handleOk.readResultSetHeaderPacket()
 	if err != nil {
-		return nil, mc.markBadConn(err)
+		return nil, err
 	}
 
 	rows := new(textRows)
@@ -482,7 +477,6 @@ func (mc *mysqlConn) finish() {
 // Ping implements driver.Pinger interface
 func (mc *mysqlConn) Ping(ctx context.Context) (err error) {
 	if mc.closed.Load() {
-		mc.log(ErrInvalidConn)
 		return driver.ErrBadConn
 	}
 
@@ -704,3 +698,6 @@ func (mc *mysqlConn) ResetSession(ctx context.Context) error {
 func (mc *mysqlConn) IsValid() bool {
 	return !mc.closed.Load()
 }
+
+var _ driver.SessionResetter = &mysqlConn{}
+var _ driver.Validator = &mysqlConn{}

errors.go

@@ -32,7 +32,7 @@ var (
 
 	// errBadConnNoWrite is used for connection errors where nothing was sent to the database yet.
 	// If this happens first in a function starting a database interaction, it should be replaced by driver.ErrBadConn
-	// to trigger a resend.
+	// to trigger a resend. Use mc.markBadConn(err) to do this.
 	// See https://github.com/go-sql-driver/mysql/pull/302
 	errBadConnNoWrite = errors.New("bad connection")
 )

statement.go

@@ -51,7 +51,6 @@ func (stmt *mysqlStmt) CheckNamedValue(nv *driver.NamedValue) (err error) {
 
 func (stmt *mysqlStmt) Exec(args []driver.Value) (driver.Result, error) {
 	if stmt.mc.closed.Load() {
-		stmt.mc.log(ErrInvalidConn)
 		return nil, driver.ErrBadConn
 	}
 	// Send command
@@ -95,7 +94,6 @@ func (stmt *mysqlStmt) Query(args []driver.Value) (driver.Rows, error) {
 
 func (stmt *mysqlStmt) query(args []driver.Value) (*binaryRows, error) {
 	if stmt.mc.closed.Load() {
-		stmt.mc.log(ErrInvalidConn)
 		return nil, driver.ErrBadConn
 	}
 	// Send command