Set SNI for TSL connections

This allows an SNI-aware proxy to route connections. Patch adds a new
connection option (`sslsni`) for opting out of the SNI, to have the same
behavior as `libpq` does. See more in `sslsni` sections at
<https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PARAMKEYWORDS>.

conn.go

@@ -1127,7 +1127,7 @@ func isDriverSetting(key string) bool {
 		return true
 	case "password":
 		return true
-	case "sslmode", "sslcert", "sslkey", "sslrootcert", "sslinline":
+	case "sslmode", "sslcert", "sslkey", "sslrootcert", "sslinline", "sslsni":
 		return true
 	case "fallback_application_name":
 		return true
@@ -2020,6 +2020,8 @@ func parseEnviron(env []string) (out map[string]string) {
 			accrue("sslkey")
 		case "PGSSLROOTCERT":
 			accrue("sslrootcert")
+		case "PGSSLSNI":
+			accrue("sslsni")
 		case "PGREQUIRESSL", "PGSSLCRL":
 			unsupported()
 		case "PGREQUIREPEER":

ssl.go

@@ -50,6 +50,15 @@ func ssl(o values) (func(net.Conn) (net.Conn, error), error) {
 		return nil, fmterrorf(`unsupported sslmode %q; only "require" (default), "verify-full", "verify-ca", and "disable" supported`, mode)
 	}
 
+	// Set Server Name Indication (SNI), if enabled by connection parameters.
+	// By default SNI is on, any value != "1" disables SNI.
+	if sslsni := o["sslsni"]; sslsni == "" || sslsni == "1" {
+		// Per RFC 6066, do not set SNI if the host is a literal IP address (IPv4
+		// or IPv6), but this check is coded already crypto.tls.hostnameInSNI, so
+		// just always set ServerName and let crypto/tls do the filtering.
+		tlsConf.ServerName = o["host"]
+	}
+
 	err := sslClientCertificates(&tlsConf, o)
 	if err != nil {
 		return nil, err

ssl_test.go

@@ -3,12 +3,19 @@ package pq
 // This file contains SSL tests
 
 import (
+	"bytes"
 	_ "crypto/sha256"
+	"crypto/tls"
 	"crypto/x509"
 	"database/sql"
+	"fmt"
+	"io"
+	"net"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
+	"time"
 )
 
 func maybeSkipSSLTests(t *testing.T) {
@@ -280,3 +287,135 @@ func TestSSLClientCertificates(t *testing.T) {
 		}
 	}
 }
+
+// Check that clint sends SNI data when `sslsni` is not disabled
+func TestSNISupport(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name         string
+		conn_param   string
+		hostname     string
+		expected_sni string
+	}{
+		{
+			name:         "SNI is set by default",
+			conn_param:   "",
+			hostname:     "localhost",
+			expected_sni: "localhost",
+		},
+		{
+			name:         "SNI is passed when asked for",
+			conn_param:   "sslsni=1",
+			hostname:     "localhost",
+			expected_sni: "localhost",
+		},
+		{
+			name:         "SNI is not passed when disabled",
+			conn_param:   "sslsni=0",
+			hostname:     "localhost",
+			expected_sni: "",
+		},
+		{
+			name:         "SNI is not set for IPv4",
+			conn_param:   "",
+			hostname:     "127.0.0.1",
+			expected_sni: "",
+		},
+	}
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			// Start mock postgres server on OS-provided port
+			listener, err := net.Listen("tcp", "127.0.0.1:")
+			if err != nil {
+				t.Fatal(err)
+			}
+			serverErrChan := make(chan error, 1)
+			serverSNINameChan := make(chan string, 1)
+			go mockPostgresSSL(listener, serverErrChan, serverSNINameChan)
+
+			defer listener.Close()
+			defer close(serverErrChan)
+			defer close(serverSNINameChan)
+
+			// Try to establish a connection with the mock server. Connection will error out after TLS
+			// clientHello, but it is enough to catch SNI data on the server side
+			port := strings.Split(listener.Addr().String(), ":")[1]
+			connStr := fmt.Sprintf("sslmode=require host=%s port=%s %s", tt.hostname, port, tt.conn_param)
+
+			// We are okay to skip this error as we are polling serverErrChan and we'll get an error
+			// or timeout from the server side in case of problems here.
+			db, _ := sql.Open("postgres", connStr)
+			_, _ = db.Exec("SELECT 1")
+
+			// Check SNI data
+			select {
+			case sniHost := <-serverSNINameChan:
+				if sniHost != tt.expected_sni {
+					t.Fatalf("Expected SNI to be 'localhost', got '%+v' instead", sniHost)
+				}
+			case err = <-serverErrChan:
+				t.Fatalf("mock server failed with error: %+v", err)
+			case <-time.After(time.Second):
+				t.Fatal("exceeded connection timeout without erroring out")
+			}
+		})
+	}
+}
+
+// Make a postgres mock server to test TLS SNI
+//
+// Accepts postgres StartupMessage and handles TLS clientHello, then closes a connection.
+// While reading clientHello catch passed SNI data and report it to nameChan.
+func mockPostgresSSL(listener net.Listener, errChan chan error, nameChan chan string) {
+	var sniHost string
+
+	conn, err := listener.Accept()
+	if err != nil {
+		errChan <- err
+		return
+	}
+	defer conn.Close()
+
+	err = conn.SetDeadline(time.Now().Add(time.Second))
+	if err != nil {
+		errChan <- err
+		return
+	}
+
+	// Receive StartupMessage with SSL Request
+	startupMessage := make([]byte, 8)
+	if _, err := io.ReadFull(conn, startupMessage); err != nil {
+		errChan <- err
+		return
+	}
+	// StartupMessage: first four bytes -- total len = 8, last four bytes SslRequestNumber
+	if !bytes.Equal(startupMessage, []byte{0, 0, 0, 0x8, 0x4, 0xd2, 0x16, 0x2f}) {
+		errChan <- fmt.Errorf("unexpected startup message: %#v", startupMessage)
+		return
+	}
+
+	// Respond with SSLOk
+	_, err = conn.Write([]byte("S"))
+	if err != nil {
+		errChan <- err
+		return
+	}
+
+	// Set up TLS context to catch clientHello. It will always error out during handshake
+	// as no certificate is set.
+	srv := tls.Server(conn, &tls.Config{
+		GetConfigForClient: func(argHello *tls.ClientHelloInfo) (*tls.Config, error) {
+			sniHost = argHello.ServerName
+			return nil, nil
+		},
+	})
+	defer srv.Close()
+
+	// Do the TLS handshake ignoring errors
+	_ = srv.Handshake()
+
+	nameChan <- sniHost
+}