From e3b228f098c89bb4e382bb091a7888f3cc807454 Mon Sep 17 00:00:00 2001
From: Corinna Vinschen <corinna@vinschen.de>
Date: Fri, 30 Apr 2021 20:56:31 +0200
Subject: [PATCH] never block when peeking

---
 winsup/cygwin/posix_ipc.cc | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/posix_ipc.cc b/winsup/cygwin/posix_ipc.cc
index 1f13c2298..1e5e40144 100644
--- a/winsup/cygwin/posix_ipc.cc
+++ b/winsup/cygwin/posix_ipc.cc
@@ -966,7 +966,8 @@ _mq_receive (mqd_t mqd, char *ptr, size_t maxlen, unsigned int *priop,
 	}
       if (attr->mq_curmsgs == 0)	/* queue is empty */
 	{
-	  if (mqinfo->mqi_flags & O_NONBLOCK)
+	  /* Don't block if O_NONBLOCK is set or when peeking */
+	  if ((mqinfo->mqi_flags & O_NONBLOCK) || (flags & _MQ_PEEK_PACKET))
 	    {
 	      set_errno (EAGAIN);
 	      __leave;