[PATCH] preserve errno when inside sig handler for self-pipe

[Eric Wong <e@80x24.org>]

We must not clobber errno of the main thread inside signal
handler in case write fails.

This bug only affects systems without epoll_pwait where the
self-pipe is required, so it does not affect modern GNU/Linux
systems; but does affect FreeBSD systems and anybody else
relying on kqueue.

Thanks to Steven Stewart-Gallus for a Ruby bug report which
inspired this fix: https://bugs.ruby-lang.org/issues/10866

Cc: Mykola Golub <trociny@FreeBSD.org>
Cc: Steven Stewart-Gallus <sstewartgallus00@mylangara.bc.ca>
---
 Steven: just wanted you to know your original bug report
 went a long way :)

 selfwake.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/selfwake.c b/selfwake.c
index 1a6908b..06e9d62 100644
--- a/selfwake.c
+++ b/selfwake.c
@@ -42,8 +42,7 @@ void mog_selfwake_wait(struct mog_fd *mfd)
 	mog_selfwake_drain(mfd);
 }
 
-/* this is async-signal safe (except in the case of bugs) */
-void mog_selfwake_trigger(struct mog_fd *mfd)
+static void selfwake_trigger(struct mog_fd *mfd)
 {
 	ssize_t w;
 
@@ -64,6 +63,12 @@ retry:
 	abort();
 }
 
+/* this is async-signal safe (except in the case of our usage bugs) */
+void mog_selfwake_trigger(struct mog_fd *mfd)
+{
+	PRESERVE_ERRNO(selfwake_trigger(mfd));
+}
+
 void mog_selfwake_drain(struct mog_fd *mfd)
 {
 	ssize_t r;
-- 
EW