From ea80ae04e2ec68c7e289048d3224a24b3c3fb107 Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Fri, 3 Nov 2017 12:23:12 -0700
Subject: [PATCH] bpo-30057: Fix potential missed signal in signal.signal().
 (GH-4258) (#4261)

Bug report and patch by Jeroen Demeyer.
(cherry picked from commit f6f90ff079a22b79a58d47b6117cc8a8c7d366f3)
---
 Misc/ACKS                                                    | 1 +
 .../next/Library/2017-11-03-19-11-43.bpo-30057.NCaijI.rst    | 1 +
 Modules/signalmodule.c                                       | 5 ++++-
 3 files changed, 6 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2017-11-03-19-11-43.bpo-30057.NCaijI.rst

diff --git a/Misc/ACKS b/Misc/ACKS
index 667f447bc5..5f21c4bce0 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -358,6 +358,7 @@ Vincent Delft
 Arnaud Delobelle
 Konrad Delong
 Erik Demaine
+Jeroen Demeyer
 Martin Dengler
 John Dennis
 L. Peter Deutsch
diff --git a/Misc/NEWS.d/next/Library/2017-11-03-19-11-43.bpo-30057.NCaijI.rst b/Misc/NEWS.d/next/Library/2017-11-03-19-11-43.bpo-30057.NCaijI.rst
new file mode 100644
index 0000000000..5740eb2500
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2017-11-03-19-11-43.bpo-30057.NCaijI.rst
@@ -0,0 +1 @@
+Fix potential missed signal in signal.signal().
diff --git a/Modules/signalmodule.c b/Modules/signalmodule.c
index 0e6099fd36..0b9e8f1c23 100644
--- a/Modules/signalmodule.c
+++ b/Modules/signalmodule.c
@@ -462,12 +462,15 @@ signal_signal_impl(PyObject *module, int signalnum, PyObject *handler)
     }
     else
         func = signal_handler;
+    /* Check for pending signals before changing signal handler */
+    if (PyErr_CheckSignals()) {
+        return NULL;
+    }
     if (PyOS_setsig(signalnum, func) == SIG_ERR) {
         PyErr_SetFromErrno(PyExc_OSError);
         return NULL;
     }
     old_handler = Handlers[signalnum].func;
-    _Py_atomic_store_relaxed(&Handlers[signalnum].tripped, 0);
     Py_INCREF(handler);
     Handlers[signalnum].func = handler;
     if (old_handler != NULL)
-- 
2.40.0