MDEV-8014 MariaDB client can hang in an infinite loop

On EOF vio_read returns 0, it's not an error so the errno
is not reset. If the previous error was EINTR the client
will loop forever. See also man recv.

sql/net_serv.cc

@@ -917,7 +917,7 @@ my_real_read(NET *net, size_t *complen,
 		    my_progname,vio_errno(net->vio));
 	  }
 #ifndef MYSQL_SERVER
-	  if (vio_errno(net->vio) == SOCKET_EINTR)
+	  if (length != 0 && vio_errno(net->vio) == SOCKET_EINTR)
 	  {
 	    DBUG_PRINT("warning",("Interrupted read. Retrying..."));
 	    continue;

vio/viosocket.c

@@ -137,6 +137,9 @@ int vio_socket_io_wait(Vio *vio, enum enum_vio_io_event event)
 #define VIO_DONTWAIT 0
 #endif
 
+/*
+  returns number of bytes read or -1 in case of an error
+*/
 size_t vio_read(Vio *vio, uchar *buf, size_t size)
 {
   ssize_t ret;