From 7f52415a6d4841d77d3b7853e83b25a22e0048dc Mon Sep 17 00:00:00 2001
From: Oran Avraham <252748+oranav@users.noreply.github.com>
Date: Wed, 5 Dec 2018 22:36:03 +0200
Subject: [PATCH] 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.
---
 .../next/Library/2018-12-03-19-45-00.bpo-35310.9k28gR.rst     | 4 ++++
 Modules/selectmodule.c                                        | 4 ++++
 2 files changed, 8 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2018-12-03-19-45-00.bpo-35310.9k28gR.rst

diff --git a/Misc/NEWS.d/next/Library/2018-12-03-19-45-00.bpo-35310.9k28gR.rst b/Misc/NEWS.d/next/Library/2018-12-03-19-45-00.bpo-35310.9k28gR.rst
new file mode 100644
index 00000000000..1ab2e168c86
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-12-03-19-45-00.bpo-35310.9k28gR.rst
@@ -0,0 +1,4 @@
+Fix a bug in :func:`select.select` where, in some cases, the file descriptor
+sequences were returned unmodified after a signal interruption, even though the
+file descriptors might not be ready yet.  :func:`select.select` will now always
+return empty lists if a timeout has occurred.  Patch by Oran Avraham.
diff --git a/Modules/selectmodule.c b/Modules/selectmodule.c
index 5a6b13466c8..fe69cd58dc6 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;
             }