selftests/bpf: Reduce more flakyness in sockmap_listen

This patch adds similar retry logic to more places where read() is used, to
reduce flakyness in slow CI environment.
Signed-off-by: Yucong Sun <fallentree@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825184745.2680830-1-fallentree@fb.com

tools/testing/selftests/bpf/prog_tests/sockmap_listen.c

@@ -949,6 +949,7 @@ static void redir_to_connected(int family, int sotype, int sock_mapfd,
 	int err, n;
 	u32 key;
 	char b;
+	int retries = 100;
 
 	zero_verdict_count(verd_mapfd);
 
@@ -1001,10 +1002,15 @@ static void redir_to_connected(int family, int sotype, int sock_mapfd,
 		goto close_peer1;
 	if (pass != 1)
 		FAIL("%s: want pass count 1, have %d", log_prefix, pass);
-
+again:
 	n = read(c0, &b, 1);
-	if (n < 0)
+	if (n < 0) {
+		if (errno == EAGAIN && retries--) {
+			usleep(1000);
+			goto again;
+		}
 		FAIL_ERRNO("%s: read", log_prefix);
+	}
 	if (n == 0)
 		FAIL("%s: incomplete read", log_prefix);
 
@@ -1926,6 +1932,7 @@ static void unix_inet_redir_to_connected(int family, int type, int sock_mapfd,
 	int sfd[2];
 	u32 key;
 	char b;
+	int retries = 100;
 
 	zero_verdict_count(verd_mapfd);
 
@@ -1956,9 +1963,15 @@ static void unix_inet_redir_to_connected(int family, int type, int sock_mapfd,
 	if (pass != 1)
 		FAIL("%s: want pass count 1, have %d", log_prefix, pass);
 
+again:
 	n = read(mode == REDIR_INGRESS ? p0 : c0, &b, 1);
-	if (n < 0)
+	if (n < 0) {
+		if (errno == EAGAIN && retries--) {
+			usleep(1000);
+			goto again;
+		}
 		FAIL_ERRNO("%s: read", log_prefix);
+	}
 	if (n == 0)
 		FAIL("%s: incomplete read", log_prefix);