doc: Test behaviour of zero length datagram recv()s

Add a test program verifying that we're able to discard datagrams from a
socket without needing a big discard buffer, by using a zero length recv().

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

doc/platform-requirements/.gitignore

@@ -1 +1,2 @@
 /reuseaddr-priority
+/recv-zero

doc/platform-requirements/Makefile

@@ -3,8 +3,8 @@
 # Copyright Red Hat
 # Author: David Gibson <david@gibson.dropbear.id.au>
 
-TARGETS = reuseaddr-priority
+TARGETS = reuseaddr-priority recv-zero
-SRCS = reuseaddr-priority.c
+SRCS = reuseaddr-priority.c recv-zero.c
 CFLAGS = -Wall
 
 all: cppcheck clang-tidy $(TARGETS:%=check-%)
@@ -16,7 +16,7 @@ check-%: %
 
 cppcheck:
 	cppcheck --std=c11 --error-exitcode=1 --enable=all --force \
-		--check-level=exhaustive \
+		--check-level=exhaustive --inline-suppr \
 		--inconclusive --library=posix --quiet \
 		--suppress=missingIncludeSystem \
 		$(SRCS)

doc/platform-requirements/recv-zero.c

@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+/* recv-zero.c
+ *
+ * Verify that we're able to discard datagrams by recv()ing into a zero-length
+ * buffer.
+ *
+ * Copyright Red Hat
+ * Author: David Gibson <david@gibson.dropbear.id.au>
+ */
+
+#include <arpa/inet.h>
+#include <errno.h>
+#include <net/if.h>
+#include <netinet/in.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "common.h"
+
+#define DSTPORT	13257U
+
+/* 127.0.0.1:DSTPORT */
+static const struct sockaddr_in lo_dst = SOCKADDR_INIT(INADDR_LOOPBACK, DSTPORT);
+
+static void test_discard(void)
+{
+	long token1, token2;
+	int recv_s, send_s;
+	ssize_t rc;
+
+	token1 = random();
+	token2 = random();
+
+	recv_s = sock_reuseaddr();
+	if (bind(recv_s, (struct sockaddr *)&lo_dst, sizeof(lo_dst)) < 0)
+		die("bind(): %s\n", strerror(errno));
+
+	send_s = sock_reuseaddr();
+	if (connect(send_s, (struct sockaddr *)&lo_dst, sizeof(lo_dst)) < 0)
+		die("connect(): %s\n", strerror(errno));
+
+	send_token(send_s, token1);
+	send_token(send_s, token2);
+
+	/* cppcheck-suppress nullPointer */
+	rc = recv(recv_s, NULL, 0, MSG_DONTWAIT);
+	if (rc < 0)
+		die("discarding recv(): %s\n", strerror(errno));
+	
+	recv_token(recv_s, token2);
+
+	/* cppcheck-suppress nullPointer */
+	rc = recv(recv_s, NULL, 0, MSG_DONTWAIT);
+	if (rc < 0 && errno != EAGAIN)
+		die("redundant discarding recv(): %s\n", strerror(errno));
+	if (rc >= 0)
+		die("Unexpected receive: rc=%zd\n", rc);
+}
+
+int main(int argc, char *argv[])
+{
+	(void)argc;
+	(void)argv;
+
+	test_discard();
+
+	printf("Discarding datagrams with a 0-length recv() seems to work\n");
+
+	exit(0);
+}