Add BeforeConnect callback to configuration object

AUTHORS

@@ -132,6 +132,7 @@ GitHub Inc.
 Google Inc.
 InfoSum Ltd.
 Keybase Inc.
+Microsoft Corp.
 Multiplay Ltd.
 Percona LLC
 PingCAP Inc.

connector.go

@@ -66,12 +66,22 @@ func newConnector(cfg *Config) *connector {
 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

@@ -2044,6 +2044,40 @@ 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.Apply(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"
@@ -71,8 +72,9 @@ type Config struct {
 
 	// unexported fields. new options should be come here
 
-	pubKey       *rsa.PublicKey // Server public key
-	timeTruncate time.Duration  // Truncate time.Time values to the specified duration
+	beforeConnect func(context.Context, *Config) error // Invoked before a connection is established
+	pubKey        *rsa.PublicKey                       // Server public key
+	timeTruncate  time.Duration                        // Truncate time.Time values to the specified duration
 }
 
 // Functional Options Pattern
@@ -112,6 +114,14 @@ func TimeTruncate(d time.Duration) Option {
 	}
 }
 
+// BeforeConnect sets the function to be invoked before a connection is established.
+func BeforeConnect(fn func(context.Context, *Config) error) Option {
+	return func(cfg *Config) error {
+		cfg.beforeConnect = fn
+		return nil
+	}
+}
+
 func (cfg *Config) Clone() *Config {
 	cp := *cfg
 	if cp.TLS != nil {