From ce76e2051ce668ff4623263215bf4b38cfca46ca Mon Sep 17 00:00:00 2001
From: Guus Sliepen <guus@meshlink.io>
Date: Thu, 11 Jun 2020 21:52:00 +0200
Subject: [PATCH] Use atomic operations to check whether to write to the signal
 pipe.

We need to do an atomic test-and-set operation to check whether we can
avoid writing to the signal pipe. Use C11 atomics to do this in a portable
way (hopefully).
---
 configure.ac | 2 ++
 src/event.c  | 8 ++++----
 src/event.h  | 2 +-
 src/have.h   | 1 +
 4 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/configure.ac b/configure.ac
index cfef2a11..f0617f32 100644
--- a/configure.ac
+++ b/configure.ac
@@ -54,6 +54,8 @@ if test -d /sw/lib ; then
   LIBS="$LIBS -L/sw/lib"
 fi
 
+AX_CHECK_COMPILE_FLAG([-std=c11], [CPPFLAGS="-std=c11"])
+
 dnl Compiler hardening flags
 dnl No -fstack-protector-all because it doesn't work on all platforms or architectures.
 
diff --git a/src/event.c b/src/event.c
index 5e9763e2..43e4e040 100644
--- a/src/event.c
+++ b/src/event.c
@@ -199,7 +199,7 @@ static void signalio_handler(event_loop_t *loop, void *data, int flags) {
 	});
 
 	if(sig) {
-		sig->set = false;
+		atomic_flag_clear(&sig->set);
 		sig->cb(loop, sig->data);
 	}
 }
@@ -224,12 +224,11 @@ static void pipe_exit(event_loop_t *loop) {
 }
 
 void signal_trigger(event_loop_t *loop, signal_t *sig) {
-	if(sig->set) {
+	if(atomic_flag_test_and_set(&sig->set)) {
 		return;
 	}
 
 	uint8_t signum = sig->signum;
-	sig->set = true;
 	write(loop->pipefd[1], &signum, 1);
 	return;
 }
@@ -240,9 +239,10 @@ void signal_add(event_loop_t *loop, signal_t *sig, signal_cb_t cb, void *data, u
 	sig->cb = cb;
 	sig->data = data;
 	sig->signum = signum;
-	sig->set = false;
 	sig->node.data = sig;
 
+	atomic_flag_clear(&sig->set);
+
 	if(loop->pipefd[0] == -1) {
 		pipe_init(loop);
 	}
diff --git a/src/event.h b/src/event.h
index a87685da..ce6701ac 100644
--- a/src/event.h
+++ b/src/event.h
@@ -52,7 +52,7 @@ typedef struct timeout_t {
 typedef struct signal_t {
 	struct splay_node_t node;
 	int signum;
-	bool set;
+	volatile atomic_flag set;
 	signal_cb_t cb;
 	void *data;
 } signal_t;
diff --git a/src/have.h b/src/have.h
index f64e15d0..c0686a54 100644
--- a/src/have.h
+++ b/src/have.h
@@ -32,6 +32,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdarg.h>
+#include <stdatomic.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <inttypes.h>
-- 
2.39.5