add rejectReadOnly option (to fix AWS Aurora failover)

AUTHORS

@@ -59,4 +59,5 @@ Zhenye Xie <xiezhenye at gmail.com>
 
 Barracuda Networks, Inc.
 Google Inc.
+Keybase Inc.
 Stripe Inc.

README.md

@@ -267,6 +267,29 @@ Default:        0
 
 I/O read timeout. The value must be a decimal number with a unit suffix (*"ms"*, *"s"*, *"m"*, *"h"*), such as *"30s"*, *"0.5m"* or *"1m30s"*.
 
+##### `rejectReadOnly`
+
+```
+Type:           bool
+Valid Values:   true, false
+Default:        false
+```
+
+
+RejectreadOnly causes mysql driver to reject read-only connections. This is
+specifically for AWS Aurora: During a failover, there seems to be a race
+condition on Aurora, where we get connected to the [old master before
+failover], i.e. the [new read-only slave after failover].
+
+Note that this should be a fairly rare case, as automatic failover normally
+happens when master is down, and the race condition shouldn't happen unless it
+comes back up online as soon as the failover is kicked off. But it's pretty
+easy to reproduce using a manual failover. In case this happens, we should
+reconnect to the Aurora cluster by returning a driver.ErrBadConnection.
+
+tl;dr: Set this if you are using Aurora.
+
+
 ##### `strict`
 
 ```

dsn.go

@@ -53,6 +53,7 @@ type Config struct {
 	InterpolateParams       bool // Interpolate placeholders into query string
 	MultiStatements         bool // Allow multiple statements in one query
 	ParseTime               bool // Parse time values to time.Time
+	RejectReadOnly          bool // Reject read-only connections; see README.md
 	Strict                  bool // Return warnings as errors
 }
 
@@ -195,6 +196,15 @@ func (cfg *Config) FormatDSN() string {
 		buf.WriteString(cfg.ReadTimeout.String())
 	}
 
+	if cfg.RejectReadOnly {
+		if hasParam {
+			buf.WriteString("&rejectReadOnly=true")
+		} else {
+			hasParam = true
+			buf.WriteString("?rejectReadOnly=true")
+		}
+	}
+
 	if cfg.Strict {
 		if hasParam {
 			buf.WriteString("&strict=true")
@@ -472,6 +482,14 @@ func parseDSNParams(cfg *Config, params string) (err error) {
 				return
 			}
 
+		// Reject read-only connections
+		case "rejectReadOnly":
+			var isBool bool
+			cfg.RejectReadOnly, isBool = readBool(value)
+			if !isBool {
+				return errors.New("invalid bool value: " + value)
+			}
+
 		// Strict mode
 		case "strict":
 			var isBool bool

packets.go

@@ -551,6 +551,21 @@ func (mc *mysqlConn) handleErrorPacket(data []byte) error {
 	// Error Number [16 bit uint]
 	errno := binary.LittleEndian.Uint16(data[1:3])
 
+	// 1792: ER_CANT_EXECUTE_IN_READ_ONLY_TRANSACTION
+	if errno == 1792 && mc.cfg.RejectReadOnly {
+		// Oops; we are connected to a read-only connection, and won't be able
+		// to issue any write statements. Since RejectReadOnly is configured,
+		// we throw away this connection hoping this one would have write
+		// permission. This is specifically for an AWS Aurora behavior during
+		// failover. See README.md for more.
+		//
+		// We explicitly close the connection before returning
+		// driver.ErrBadConn to ensure that `database/sql` purges this
+		// connection and initiates a new one for next statement next time.
+		mc.Close()
+		return driver.ErrBadConn
+	}
+
 	pos := 3
 
 	// SQL State [optional: # + 5bytes string]