Add fallback TLS ConnConfig option

This is in preparation for supporting libpq style SSL options.

conn.go

@@ -30,6 +30,8 @@ type ConnConfig struct {
 	User              string // default: OS user name
 	Password          string
 	TLSConfig         *tls.Config // config for TLS connection -- nil disables TLS
+	UseFallbackTLS    bool        // Try FallbackTLSConfig if connecting with TLSConfig fails. Used for preferring TLS, but allowing unencrypted, or vice-versa
+	FallbackTLSConfig *tls.Config // config for fallback TLS connection (only used if UseFallBackTLS is true)-- nil disables TLS
 	Logger            Logger
 	Dial              DialFunc
 }
@@ -140,11 +142,25 @@ func Connect(config ConnConfig) (c *Conn, err error) {
 	if c.config.Dial == nil {
 		c.config.Dial = (&net.Dialer{KeepAlive: 5 * time.Minute}).Dial
 	}
+
+	err = c.connect(config, network, address, config.TLSConfig)
+	if err != nil && config.UseFallbackTLS {
+		err = c.connect(config, network, address, config.FallbackTLSConfig)
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	return c, nil
+}
+
+func (c *Conn) connect(config ConnConfig, network, address string, tlsConfig *tls.Config) (err error) {
 	c.logger.Info(fmt.Sprintf("Dialing PostgreSQL server at %s address: %s", network, address))
 	c.conn, err = c.config.Dial(network, address)
 	if err != nil {
 		c.logger.Error(fmt.Sprintf("Connection failed: %v", err))
-		return nil, err
+		return err
 	}
 	defer func() {
 		if c != nil && err != nil {
@@ -159,11 +175,11 @@ func Connect(config ConnConfig) (c *Conn, err error) {
 	c.alive = true
 	c.lastActivityTime = time.Now()
 
-	if config.TLSConfig != nil {
+	if tlsConfig != nil {
 		c.logger.Debug("Starting TLS handshake")
-		if err = c.startTLS(); err != nil {
+		if err := c.startTLS(tlsConfig); err != nil {
 			c.logger.Error(fmt.Sprintf("TLS failed: %v", err))
-			return
+			return err
 		}
 	}
 
@@ -176,7 +192,7 @@ func Connect(config ConnConfig) (c *Conn, err error) {
 		msg.options["database"] = c.config.Database
 	}
 	if err = c.txStartupMessage(msg); err != nil {
-		return
+		return err
 	}
 
 	for {
@@ -184,7 +200,7 @@ func Connect(config ConnConfig) (c *Conn, err error) {
 		var r *msgReader
 		t, r, err = c.rxMsg()
 		if err != nil {
-			return nil, err
+			return err
 		}
 
 		switch t {
@@ -192,7 +208,7 @@ func Connect(config ConnConfig) (c *Conn, err error) {
 			c.rxBackendKeyData(r)
 		case authenticationX:
 			if err = c.rxAuthenticationX(r); err != nil {
-				return nil, err
+				return err
 			}
 		case readyForQuery:
 			c.rxReadyForQuery(r)
@@ -203,13 +219,13 @@ func Connect(config ConnConfig) (c *Conn, err error) {
 
 			err = c.loadPgTypes()
 			if err != nil {
-				return nil, err
+				return err
 			}
 
-			return c, nil
+			return nil
 		default:
 			if err = c.processContextFreeMsg(t, r); err != nil {
-				return nil, err
+				return err
 			}
 		}
 	}
@@ -905,7 +921,7 @@ func (c *Conn) rxNotificationResponse(r *msgReader) {
 	c.notifications = append(c.notifications, n)
 }
 
-func (c *Conn) startTLS() (err error) {
+func (c *Conn) startTLS(tlsConfig *tls.Config) (err error) {
 	err = binary.Write(c.conn, binary.BigEndian, []int32{8, 80877103})
 	if err != nil {
 		return
@@ -920,7 +936,7 @@ func (c *Conn) startTLS() (err error) {
 		return ErrTLSRefused
 	}
 
-	c.conn = tls.Client(c.conn, c.config.TLSConfig)
+	c.conn = tls.Client(c.conn, tlsConfig)
 
 	return nil
 }

conn_test.go

@@ -1,6 +1,7 @@
 package pgx_test
 
 import (
+	"crypto/tls"
 	"fmt"
 	"github.com/jackc/pgx"
 	"net"
@@ -184,6 +185,35 @@ func TestConnectWithMD5Password(t *testing.T) {
 	}
 }
 
+func TestConnectWithTLSFallback(t *testing.T) {
+	t.Parallel()
+
+	if tlsConnConfig == nil {
+		return
+	}
+
+	connConfig := *tlsConnConfig
+	connConfig.TLSConfig = &tls.Config{ServerName: "bogus.local"} // bogus ServerName should ensure certificate validation failure
+
+	conn, err := pgx.Connect(connConfig)
+	if err == nil {
+		t.Fatal("Expected failed connection, but succeeded")
+	}
+
+	connConfig.UseFallbackTLS = true
+	connConfig.FallbackTLSConfig = &tls.Config{InsecureSkipVerify: true}
+
+	conn, err = pgx.Connect(connConfig)
+	if err != nil {
+		t.Fatal("Unable to establish connection: " + err.Error())
+	}
+
+	err = conn.Close()
+	if err != nil {
+		t.Fatal("Unable to close connection")
+	}
+}
+
 func TestConnectWithConnectionRefused(t *testing.T) {
 	t.Parallel()