modem: pipe: Don't return EPERM on closed pipe

When working on CMUX power saving, it is typical
that we end up closing the pipe before the last
RX_READY event is handled from workqueue, so we end up
receiving -EPERM which is not really a fatal error.

Pipes recover when they are re-opened. So drop this error
and return zero instead, like modem_pipe_open() and close() does.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>

Author: SeppoTakalo

subsys/modem/modem_pipe.c

@@ -136,7 +136,7 @@ void modem_pipe_attach(struct modem_pipe *pipe, modem_pipe_api_callback callback
 int modem_pipe_transmit(struct modem_pipe *pipe, const uint8_t *buf, size_t size)
 {
 	if (!pipe_test_events(pipe, PIPE_EVENT_OPENED_BIT)) {
-		return -EPERM;
+		return 0;
 	}
 
 	pipe_clear_events(pipe, PIPE_EVENT_TRANSMIT_IDLE_BIT);
@@ -146,7 +146,7 @@ int modem_pipe_transmit(struct modem_pipe *pipe, const uint8_t *buf, size_t size
 int modem_pipe_receive(struct modem_pipe *pipe, uint8_t *buf, size_t size)
 {
 	if (!pipe_test_events(pipe, PIPE_EVENT_OPENED_BIT)) {
-		return -EPERM;
+		return 0;
 	}
 
 	pipe_clear_events(pipe, PIPE_EVENT_RECEIVE_READY_BIT);

tests/subsys/modem/modem_pipe/src/main.c

@@ -327,6 +327,14 @@ static void test_pipe_notify_receive_ready(void)
 		      "Unexpected state %u", (uint32_t)atomic_get(&test_state));
 }
 
+static void test_pipe_receive_closed(void)
+{
+	/* Try to receive from a closed pipe - should return 0 */
+	zassert_equal(modem_pipe_receive(test_pipe, test_buffer, test_buffer_size), 0,
+		      "Reading from closed pipe should return 0");
+	zassert_false(test_backend.receive_called, "receive should not be called on closed pipe");
+}
+
 ZTEST(modem_pipe, test_async_open_close)
 {
 	test_pipe_open();
@@ -397,5 +405,16 @@ ZTEST(modem_pipe, test_attach)
 	test_pipe_attach_receive_not_ready_transmit_idle();
 }
 
+ZTEST(modem_pipe, test_receive_closed)
+{
+	test_pipe_open();
+	test_reset();
+	test_pipe_async_transmit();
+	test_reset();
+	test_pipe_close();
+	/* Test reading from a closed pipe should return 0 */
+	test_pipe_receive_closed();
+}
+
 ZTEST_SUITE(modem_pipe, NULL, modem_backend_fake_setup, modem_backend_fake_before,
 	    modem_backend_fake_after, NULL);