From b9aacc1427e712dd3a4fbdcbf41fa42f4027ff0f 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 63266af4dce959..88679e81449592 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;
             }