selftests/coredump: handle edge-triggered epoll correctly

by putting the file descriptor into non-blocking mode.

Link: https://patch.msgid.link/20251028-work-coredump-signal-v1-16-ca449b7b7aa0@kernel.org
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

tools/testing/selftests/coredump/coredump_test_helpers.c

@@ -291,6 +291,14 @@ void process_coredump_worker(int fd_coredump, int fd_peer_pidfd, int fd_core_fil
 	int epfd = -1;
 	int exit_code = EXIT_FAILURE;
 	struct epoll_event ev;
+	int flags;
+
+	/* Set socket to non-blocking mode for edge-triggered epoll */
+	flags = fcntl(fd_coredump, F_GETFL, 0);
+	if (flags < 0)
+		goto out;
+	if (fcntl(fd_coredump, F_SETFL, flags | O_NONBLOCK) < 0)
+		goto out;
 
 	epfd = epoll_create1(0);
 	if (epfd < 0)