[nrf fromtree] tests: coap_client: Add testcase for Ack failure

Add testcase where sending Ack to incomming Confirmable message fails.
This should be reported to application as now the server is unaware
that transmission have succeeded, so we cannot thread it as success.

Signed-off-by: Seppo Takalo <[email protected]>
(cherry picked from commit fc51fa4)

Author: SeppoTakalo

tests/net/lib/coap_client/src/main.c

@@ -747,6 +747,41 @@ ZTEST(coap_client, test_separate_response_lost)
 	zassert_equal(last_response_code, -ETIMEDOUT, "");
 }
 
+ZTEST(coap_client, test_separate_response_ack_fail)
+{
+	int ret = 0;
+	struct k_sem sem;
+	struct sockaddr address = {0};
+	struct coap_client_request client_request = {
+		.method = COAP_METHOD_GET,
+		.confirmable = true,
+		.path = test_path,
+		.fmt = COAP_CONTENT_FORMAT_TEXT_PLAIN,
+		.cb = coap_callback,
+		.payload = short_payload,
+		.len = strlen(short_payload),
+		.user_data = &sem,
+	};
+	zassert_ok(k_sem_init(&sem, 0, 1));
+
+	int (*sendto_fakes[])(int, void *, size_t, int, const struct sockaddr *, socklen_t) = {
+		z_impl_zsock_sendto_custom_fake,
+		z_impl_zsock_sendto_custom_fake_err,
+	};
+
+	SET_CUSTOM_FAKE_SEQ(z_impl_zsock_sendto, sendto_fakes, ARRAY_SIZE(sendto_fakes));
+	z_impl_zsock_recvfrom_fake.custom_fake = z_impl_zsock_recvfrom_custom_fake_empty_ack;
+
+	k_sleep(K_MSEC(1));
+
+	LOG_INF("Send request");
+	ret = coap_client_req(&client, 0, &address, &client_request, NULL);
+	zassert_true(ret >= 0, "Sending request failed, %d", ret);
+
+	zassert_ok(k_sem_take(&sem, K_MSEC(COAP_SEPARATE_TIMEOUT)));
+	zassert_equal(last_response_code, -ENETDOWN, "");
+}
+
 ZTEST(coap_client, test_multiple_requests)
 {
 	int ret = 0;