extmod/network_ppp: Allow stream=None to suspend PPP.

This allows the stream to be set to `None`, which essentially stops all PPP
communication without disconnecting the session.

This allows replacing the stream on-the-fly to suspend it, for example to
send AT commands to a modem without completely disconnecting and
re-establishing the PPP connection:

    uart = ppp.config('stream')
    ppp.config(stream=None)
    uart.write(b'+++')
    # do some AT commands
    uart.write(b'ATO\r\n')
    ppp.config(stream=uart)

Any attempted communication by PPP while the stream is not connected will
register as simple packet loss to the LwIP stack because we return 0 for
any write calls, and protocols like TCP will then automatically handle
retrying.

Signed-off-by: Daniël van de Giessen <daniel@dvdgiessen.nl>

ports/esp32/network_ppp.c

@@ -85,7 +85,9 @@ static void ppp_status_cb(ppp_pcb *pcb, int err_code, void *ctx) {
 }
 
 static mp_obj_t ppp_make_new(mp_obj_t stream) {
-    mp_get_stream_raise(stream, MP_STREAM_OP_READ | MP_STREAM_OP_WRITE);
+    if (stream != mp_const_none) {
+        mp_get_stream_raise(stream, MP_STREAM_OP_READ | MP_STREAM_OP_WRITE);
+    }
 
     ppp_if_obj_t *self = mp_obj_malloc_with_finaliser(ppp_if_obj_t, &ppp_if_type);
     self->stream = stream;
@@ -100,8 +102,14 @@ MP_DEFINE_CONST_FUN_OBJ_1(esp_network_ppp_make_new_obj, ppp_make_new);
 
 static u32_t ppp_output_callback(ppp_pcb *pcb, u8_t *data, u32_t len, void *ctx) {
     ppp_if_obj_t *self = ctx;
+
+    mp_obj_t stream = self->stream;
+    if (stream == mp_const_none) {
+        return 0;
+    }
+
     int err;
-    return mp_stream_rw(self->stream, data, len, &err, MP_STREAM_RW_WRITE);
+    return mp_stream_rw(stream, data, len, &err, MP_STREAM_RW_WRITE);
 }
 
 static void pppos_client_task(void *self_in) {
@@ -110,10 +118,15 @@ static void pppos_client_task(void *self_in) {
 
     int len = 0;
     while (ulTaskNotifyTake(pdTRUE, len <= 0) == 0) {
-        int err;
-        len = mp_stream_rw(self->stream, buf, sizeof(buf), &err, 0);
-        if (len > 0) {
-            pppos_input_tcpip(self->pcb, (u8_t *)buf, len);
+        mp_obj_t stream = self->stream;
+        if (stream == mp_const_none) {
+            len = 0;
+        } else {
+            int err;
+            len = mp_stream_rw(stream, buf, sizeof(buf), &err, 0);
+            if (len > 0) {
+                pppos_input_tcpip(self->pcb, (u8_t *)buf, len);
+            }
         }
     }
 
@@ -324,7 +337,9 @@ static mp_obj_t ppp_config(size_t n_args, const mp_obj_t *args, mp_map_t *kwargs
             if (mp_map_slot_is_filled(kwargs, i)) {
                 switch (mp_obj_str_get_qstr(kwargs->table[i].key)) {
                     case MP_QSTR_stream: {
-                        mp_get_stream_raise(kwargs->table[i].value, MP_STREAM_OP_READ | MP_STREAM_OP_WRITE);
+                        if (kwargs->table[i].value != mp_const_none) {
+                            mp_get_stream_raise(kwargs->table[i].value, MP_STREAM_OP_READ | MP_STREAM_OP_WRITE);
+                        }
                         self->stream = kwargs->table[i].value;
                         break;
                     }