doc: Add program to document and test assumptions about SO_REUSEADDR

For the approach we intend to use for handling UDP flows, we have some
pretty specific requirements about how SO_REUSEADDR works with UDP sockets.
Specifically SO_REUSEADDR allows multiple sockets with overlapping bind()s,
and therefore there can be multiple sockets which are eligible to receive
the same datagram.  Which one will actually receive it is important to us.

Add a test program which verifies things work the way we expect, which
documents what those expectations are in the process.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 47 insertions, 0 deletions

diff --git a/doc/platform-requirements/common.h b/doc/platform-requirements/common.h
new file mode 100644
index 0000000..8844b1e
--- /dev/null
+++ b/doc/platform-requirements/common.h
@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+/* common.h
+ *
+ * Useful shared functions
+ *
+ * Copyright Red Hat
+ * Author: David Gibson <david@gibson.dropbear.id.au>
+ */
+#ifndef REUSEADDR_COMMON_H
+#define REUSEADDR_COMMON_H
+
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+static inline void die(const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	(void)vfprintf(stderr, fmt, ap);
+	va_end(ap);
+	exit(EXIT_FAILURE);
+}
+
+#if __BYTE_ORDER == __BIG_ENDIAN
+#define htons_constant(x)       (x)
+#define htonl_constant(x)       (x)
+#else
+#define htons_constant(x)       (__bswap_constant_16(x))
+#define htonl_constant(x)       (__bswap_constant_32(x))
+#endif
+
+#define SOCKADDR_INIT(addr, port)					\
+	{								\
+		.sin_family = AF_INET,					\
+		.sin_addr = { .s_addr = htonl_constant(addr) },		\
+		.sin_port = htons_constant(port),			\
+	}
+
+int sock_reuseaddr(void);
+void send_token(int s, long token);
+bool recv_token(int s, long token);
+
+#endif /* REUSEADDR_COMMON_H */