Add pgconn.CheckConn

pgconn/pgconn.go

@@ -65,7 +65,7 @@ type NotificationHandler func(*PgConn, *Notification)
 
 // PgConn is a low-level PostgreSQL connection handle. It is not safe for concurrent usage.
 type PgConn struct {
-	conn              net.Conn          // the underlying TCP or unix domain socket connection
+	conn              nbconn.Conn       // the non-blocking wrapper for the underlying TCP or unix domain socket connection
 	pid               uint32            // backend pid
 	secretKey         uint32            // key to use to send a cancel query message to the server
 	parameterStatuses map[string]string // parameters that have been reported by the server
@@ -230,22 +230,22 @@ func connect(ctx context.Context, config *Config, fallbackConfig *FallbackConfig
 		}
 		return nil, &connectError{config: config, msg: "dial error", err: err}
 	}
-	netConn = nbconn.NewNetConn(netConn, false)
+	nbNetConn := nbconn.NewNetConn(netConn, false)
 
-	pgConn.conn = netConn
-	pgConn.contextWatcher = newContextWatcher(netConn)
+	pgConn.conn = nbNetConn
+	pgConn.contextWatcher = newContextWatcher(nbNetConn)
 	pgConn.contextWatcher.Watch(ctx)
 
 	if fallbackConfig.TLSConfig != nil {
-		tlsConn, err := startTLS(netConn.(*nbconn.NetConn), fallbackConfig.TLSConfig)
+		nbTLSConn, err := startTLS(nbNetConn, fallbackConfig.TLSConfig)
 		pgConn.contextWatcher.Unwatch() // Always unwatch `netConn` after TLS.
 		if err != nil {
 			netConn.Close()
 			return nil, &connectError{config: config, msg: "tls error", err: err}
 		}
 
-		pgConn.conn = tlsConn
-		pgConn.contextWatcher = newContextWatcher(tlsConn)
+		pgConn.conn = nbTLSConn
+		pgConn.contextWatcher = newContextWatcher(nbTLSConn)
 		pgConn.contextWatcher.Watch(ctx)
 	}
 
@@ -353,7 +353,7 @@ func newContextWatcher(conn net.Conn) *ctxwatch.ContextWatcher {
 	)
 }
 
-func startTLS(conn *nbconn.NetConn, tlsConfig *tls.Config) (net.Conn, error) {
+func startTLS(conn *nbconn.NetConn, tlsConfig *tls.Config) (*nbconn.TLSConn, error) {
 	err := binary.Write(conn, binary.BigEndian, []int32{8, 80877103})
 	if err != nil {
 		return nil, err
@@ -1596,6 +1596,18 @@ func (pgConn *PgConn) EscapeString(s string) (string, error) {
 	return strings.Replace(s, "'", "''", -1), nil
 }
 
+// CheckConn checks the underlying connection without writing any bytes. This is currently implemented by reading and
+// buffering until the read would block or an error occurs. This can be used to check if the server has closed the
+// connection. If this is done immediately before sending a query it reduces the chances a query will be sent that fails
+// without the client knowing whether the server received it or not.
+func (pgConn *PgConn) CheckConn() error {
+	err := pgConn.conn.BufferReadUntilBlock()
+	if err != nil && !errors.Is(err, nbconn.ErrWouldBlock) {
+		return err
+	}
+	return nil
+}
+
 // makeCommandTag makes a CommandTag. It does not retain a reference to buf or buf's underlying memory.
 func (pgConn *PgConn) makeCommandTag(buf []byte) CommandTag {
 	ct := make([]byte, len(buf))
@@ -1608,7 +1620,7 @@ func (pgConn *PgConn) makeCommandTag(buf []byte) CommandTag {
 // Due to the necessary exposure of internal implementation details, it is not covered by the semantic versioning
 // compatibility.
 type HijackedConn struct {
-	Conn              net.Conn          // the underlying TCP or unix domain socket connection
+	Conn              nbconn.Conn       // the non-blocking wrapper of the underlying TCP or unix domain socket connection
 	PID               uint32            // backend pid
 	SecretKey         uint32            // key to use to send a cancel query message to the server
 	ParameterStatuses map[string]string // parameters that have been reported by the server

pgconn/pgconn_test.go

@@ -2059,6 +2059,34 @@ func TestConnLargeResponseWhileWritingDoesNotDeadlock(t *testing.T) {
 	ensureConnValid(t, pgConn)
 }
 
+func TestConnCheckConn(t *testing.T) {
+	t.Parallel()
+
+	c1, err := pgconn.Connect(context.Background(), os.Getenv("PGX_TEST_TCP_CONN_STRING"))
+	require.NoError(t, err)
+	defer c1.Close(context.Background())
+
+	if c1.ParameterStatus("crdb_version") != "" {
+		t.Skip("Server does not support pg_terminate_backend() (https://github.com/cockroachdb/cockroach/issues/35897)")
+	}
+
+	err = c1.CheckConn()
+	require.NoError(t, err)
+
+	c2, err := pgconn.Connect(context.Background(), os.Getenv("PGX_TEST_TCP_CONN_STRING"))
+	require.NoError(t, err)
+	defer c2.Close(context.Background())
+
+	_, err = c2.Exec(context.Background(), fmt.Sprintf("select pg_terminate_backend(%d)", c1.PID())).ReadAll()
+	require.NoError(t, err)
+
+	// Give a little time for the signal to actually kill the backend.
+	time.Sleep(500 * time.Millisecond)
+
+	err = c1.CheckConn()
+	require.Error(t, err)
+}
+
 func Example() {
 	pgConn, err := pgconn.Connect(context.Background(), os.Getenv("PGX_TEST_CONN_STRING"))
 	if err != nil {