bpo-35310: Clear select() lists before returning upon EINTR (GH-10877)

select() calls are retried on EINTR (per PEP 475).  However, if a
timeout was provided and the deadline has passed after running the
signal handlers, rlist, wlist and xlist should be cleared since select(2)
left them unmodified.

1 files changed, 4 insertions, 0 deletions

diff --git a/Modules/selectmodule.c b/Modules/selectmodule.c
index 5a6b13466c..fe69cd58dc 100644
--- a/Modules/selectmodule.c
+++ b/Modules/selectmodule.c
@@ -335,6 +335,10 @@ select_select_impl(PyObject *module, PyObject *rlist, PyObject *wlist,
         if (tvp) {
             timeout = deadline - _PyTime_GetMonotonicClock();
             if (timeout < 0) {
+                /* bpo-35310: lists were unmodified -- clear them explicitly */
+                FD_ZERO(&ifdset);
+                FD_ZERO(&ofdset);
+                FD_ZERO(&efdset);
                 n = 0;
                 break;
             }