From badf2f540cd96e8ac3b558380d16c1f656e523f7 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.
(cherry picked from commit 7f52415a6d4841d77d3b7853e83b25a22e0048dc)

Co-authored-by: Oran Avraham <252748+oranav@users.noreply.github.com>
---
 .../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 00000000000000..1ab2e168c86a27
--- /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 f02f5ae4e10c8e..4b9965724915d7 100644
--- a/Modules/selectmodule.c
+++ b/Modules/selectmodule.c
@@ -279,6 +279,10 @@ select_select(PyObject *self, PyObject *args)
         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;
             }