bricks: implement mp_hal_stdout_tx_flush()

This allows waiting for stdout to be sent over the air before exiting
the MicroPython runtime. This seems to help with (but not completely
fix) issues like [1]. It is also a step towards standardizing the stdout
buffer behavior as discussed in [2]. It only affects BLE hubs for now.

[1]: pybricks/support#835
[2]: pybricks/support#305

Author: dlech

CHANGELOG.md

@@ -4,6 +4,9 @@
 
 ## [Unreleased]
 
+### Changed
+- Buffered stdout is flushed before ending user program.
+
 ### Fixed
 - Fixed SPIKE/MINDSTORMS hubs advertising after disconnect while user program
   is still running ([support#849]).

bricks/_common/micropython.c

@@ -292,6 +292,9 @@ void pbsys_main_run_program(pbsys_main_program_t *program) {
     }
     #endif // PYBRICKS_OPT_COMPILER
 
+    // Ensure all buffered output has been sent over the air.
+    mp_hal_stdout_tx_flush();
+
     // Clean up non-MicroPython resources used by the pybricks package.
     pb_package_pybricks_deinit();
 

bricks/_common/mpconfigport.h

@@ -92,6 +92,7 @@
 #define MICROPY_PY_SYS_MODULES                  (0)
 #define MICROPY_PY_SYS_STDFILES                 (PYBRICKS_OPT_EXTRA_MOD)
 #define MICROPY_PY_SYS_STDIO_BUFFER             (PYBRICKS_OPT_EXTRA_MOD)
+#define MICROPY_PY_SYS_STDIO_FLUSH              (PYBRICKS_OPT_EXTRA_MOD)
 #define MICROPY_PY_URANDOM_EXTRA_FUNCS          (PYBRICKS_OPT_EXTRA_MOD)
 #define MICROPY_PY_URANDOM_SEED_INIT_FUNC       ({ extern uint32_t pbdrv_clock_get_us(void); pbdrv_clock_get_us(); })
 #define MICROPY_PY_UTIME_MP_HAL                 (0)

bricks/_common_stm32/mphalport.c

@@ -97,6 +97,10 @@ void mp_hal_stdout_tx_strn(const char *str, mp_uint_t len) {
     }
 }
 
+void mp_hal_stdout_tx_flush(void) {
+    // currently not buffered
+}
+
 #else // !PYBRICKS_HUB_DEBUG
 
 uintptr_t mp_hal_stdio_poll(uintptr_t poll_flags) {
@@ -144,4 +148,10 @@ void mp_hal_stdout_tx_strn(const char *str, mp_uint_t len) {
     }
 }
 
+void mp_hal_stdout_tx_flush(void) {
+    while (!pbsys_bluetooth_tx_is_idle()) {
+        MICROPY_EVENT_POLL_HOOK
+    }
+}
+
 #endif // PYBRICKS_HUB_DEBUG

bricks/ev3dev/ev3dev_mphal.c

@@ -125,6 +125,10 @@ void mp_hal_stdout_tx_str(const char *str) {
     mp_hal_stdout_tx_strn(str, strlen(str));
 }
 
+void mp_hal_stdout_tx_flush(void) {
+    // currently not buffered
+}
+
 mp_uint_t mp_hal_ticks_ms(void) {
     return pbdrv_clock_get_ms();
 }

bricks/ev3rt/mphalport.c

@@ -81,3 +81,7 @@ void mp_hal_stdout_tx_strn(const char *str, mp_uint_t len) {
         serial_wri_dat(EV3_SERIAL_BT, str, len);
     }
 }
+
+void mp_hal_stdout_tx_flush(void) {
+    // currently not buffered
+}

bricks/nxt/mphalport.c

@@ -97,3 +97,7 @@ void mp_hal_stdout_tx_strn(const char *str, mp_uint_t len) {
         MICROPY_EVENT_POLL_HOOK;
     }
 }
+
+void mp_hal_stdout_tx_flush(void) {
+    // currently not buffered
+}

lib/pbio/include/pbsys/bluetooth.h

@@ -31,6 +31,7 @@ void pbsys_bluetooth_rx_flush(void);
 uint32_t pbsys_bluetooth_rx_get_available(void);
 pbio_error_t pbsys_bluetooth_rx(uint8_t *data, uint32_t *size);
 pbio_error_t pbsys_bluetooth_tx(const uint8_t *data, uint32_t *size);
+bool pbsys_bluetooth_tx_is_idle(void);
 
 #else // PBSYS_CONFIG_BLUETOOTH
 
@@ -45,6 +46,9 @@ static inline pbio_error_t pbsys_bluetooth_rx(uint8_t *data, uint32_t *size) {
 static inline pbio_error_t pbsys_bluetooth_tx(const uint8_t *data, uint32_t *size) {
     return PBIO_ERROR_NOT_SUPPORTED;
 }
+static inline bool pbsys_bluetooth_tx_is_idle(void) {
+    return false;
+}
 
 #endif // PBSYS_CONFIG_BLUETOOTH
 

lib/pbio/sys/bluetooth.c

@@ -149,6 +149,21 @@ pbio_error_t pbsys_bluetooth_tx(const uint8_t *data, uint32_t *size) {
     return PBIO_SUCCESS;
 }
 
+/**
+ * Tests if the Tx queue is empty and all data has been sent over the air.
+ *
+ * If there is no connection, this will always return @c true.
+ *
+ * @returns @c true if the condition is met, otherwise @c false.
+ */
+bool pbsys_bluetooth_tx_is_idle(void) {
+    if (!pbdrv_bluetooth_is_connected(PBDRV_BLUETOOTH_CONNECTION_UART)) {
+        return true;
+    }
+
+    return !send_busy && lwrb_get_full(&uart_tx_ring) == 0;
+}
+
 static pbio_pybricks_error_t handle_receive(pbdrv_bluetooth_connection_t connection, const uint8_t *data, uint32_t size) {
     if (connection == PBDRV_BLUETOOTH_CONNECTION_PYBRICKS) {
         return pbsys_command(data, size);