Add BeforeConnect callback to configuration object

This can be used to alter the connection options for each connection, right before it's established

Author: ItalyPaleAle

connector.go

@@ -66,12 +66,22 @@ func newConnector(cfg *Config) (*connector, error) {
 func (c *connector) Connect(ctx context.Context) (driver.Conn, error) {
 	var err error
 
+	// Invoke BeforeConnect if present, with a copy of the configuration
+	cfg := c.cfg
+	if c.cfg.BeforeConnect != nil {
+		cfg = c.cfg.Clone()
+		err = c.cfg.BeforeConnect(ctx, cfg)
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	// New mysqlConn
 	mc := &mysqlConn{
 		maxAllowedPacket: maxPacketSize,
 		maxWriteSize:     maxPacketSize - 1,
 		closech:          make(chan struct{}),
-		cfg:              c.cfg,
+		cfg:              cfg,
 		connector:        c,
 	}
 	mc.parseTime = mc.cfg.ParseTime

driver_test.go

@@ -1965,6 +1965,40 @@ func TestCustomDial(t *testing.T) {
 	}
 }
 
+func TestBeforeConnect(t *testing.T) {
+	if !available {
+		t.Skipf("MySQL server not running on %s", netAddr)
+	}
+
+	// dbname is set in the BeforeConnect handle
+	cfg, err := ParseDSN(fmt.Sprintf("%s:%s@%s/%s?timeout=30s", user, pass, netAddr, "_"))
+	if err != nil {
+		t.Fatalf("error parsing DSN: %v", err)
+	}
+
+	cfg.BeforeConnect = func(ctx context.Context, c *Config) error {
+		c.DBName = dbname
+		return nil
+	}
+
+	connector, err := NewConnector(cfg)
+	if err != nil {
+		t.Fatalf("error creating connector: %v", err)
+	}
+
+	db := sql.OpenDB(connector)
+	defer db.Close()
+
+	var connectedDb string
+	err = db.QueryRow("SELECT DATABASE();").Scan(&connectedDb)
+	if err != nil {
+		t.Fatalf("error executing query: %v", err)
+	}
+	if connectedDb != dbname {
+		t.Fatalf("expected to connect to DB %s, but connected to %s instead", dbname, connectedDb)
+	}
+}
+
 func TestSQLInjection(t *testing.T) {
 	createTest := func(arg string) func(dbt *DBTest) {
 		return func(dbt *DBTest) {

dsn.go

@@ -10,6 +10,7 @@ package mysql
 
 import (
 	"bytes"
+	"context"
 	"crypto/rsa"
 	"crypto/tls"
 	"errors"
@@ -65,6 +66,8 @@ type Config struct {
 	MultiStatements          bool // Allow multiple statements in one query
 	ParseTime                bool // Parse time values to time.Time
 	RejectReadOnly           bool // Reject read-only connections
+
+	BeforeConnect func(context.Context, *Config) error // Invoked before a connection is established
 }
 
 // NewConfig creates a new Config and sets default values.